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Third Edition

CHAPTER 15

Expressions

Much of the work in a program is done by evaluating expressions, either for their side effects, such as assignments to variables, or for their values, which can be used as arguments or operands in larger expressions, or to affect the execution sequence in statements, or both.

This chapter specifies the meanings of expressions and the rules for their evaluation.

15.1 Evaluation, Denotation, and Result

When an expression in a program is evaluated (executed), the result denotes one of three things:

Evaluation of an expression can also produce side effects, because expressions may contain embedded assignments, increment operators, decrement operators, and method invocations.

An expression denotes nothing if and only if it is a method invocation (§15.12) that invokes a method that does not return a value, that is, a method declared void (§8.4). Such an expression can be used only as an expression statement (§14.8), because every other context in which an expression can appear requires the expression to denote something. An expression statement that is a method invocation may also invoke a method that produces a result; in this case the value returned by the method is quietly discarded.

Value set conversion (§5.1.13) is applied to the result of every expression that produces a value.

Each expression occurs in either:

15.2 Variables as Values

If an expression denotes a variable, and a value is required for use in further evaluation, then the value of that variable is used. In this context, if the expression denotes a variable or a value, we may speak simply of the value of the expression.

If the value of a variable of type float or double is used in this manner, then value set conversion (§5.1.13) is applied to the value of the variable.

15.3 Type of an Expression

If an expression denotes a variable or a value, then the expression has a type known at compile time. The rules for determining the type of an expression are explained separately below for each kind of expression.

The value of an expression is assignment compatible (§5.2) with the type of the expression, unless heap pollution (§4.12.2.1) occurs. Likewise the value stored in a variable is always compatible with the type of the variable, unless heap pollution occurs. In other words, the value of an expression whose type is T is always suitable for assignment to a variable of type T.

Note that an expression whose type is a class type F that is declared final is guaranteed to have a value that is either a null reference or an object whose class is F itself, because final types have no subclasses.

15.4 FP-strict Expressions

If the type of an expression is float or double, then there is a question as to what value set (§4.2.3) the value of the expression is drawn from. This is governed by the rules of value set conversion (§5.1.13); these rules in turn depend on whether or not the expression is FP-strict.

Every compile-time constant expression (§15.28) is FP-strict. If an expression is not a compile-time constant expression, then consider all the class declarations, interface declarations, and method declarations that contain the expression. If any such declaration bears the strictfp modifier, then the expression is FP-strict.

If a class, interface, or method, X, is declared strictfp, then X and any class, interface, method, constructor, instance initializer, static initializer or variable initializer within X is said to be FP-strict. Note that an annotation (§9.7) element value (§9.6) is always FP-strict, because it is always a compile-time constant (§15.28).

It follows that an expression is not FP-strict if and only if it is not a compile-time constant expression and it does not appear within any declaration that has the strictfp modifier.

Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

15.5 Expressions and Run-Time Checks

If the type of an expression is a primitive type, then the value of the expression is of that same primitive type. But if the type of an expression is a reference type, then the class of the referenced object, or even whether the value is a reference to an object rather than null, is not necessarily known at compile time. There are a few places in the Java programming language where the actual class of a referenced object affects program execution in a manner that cannot be deduced from the type of the expression. They are as follows:

Situations where the class of an object is not statically known may lead to run-time type errors.

In addition, there are situations where the statically known type may not be accurate at run-time. Such situations can arise in a program that gives rise to unchecked warnings. Such warnings are given in response to operations that cannot be statically guaranteed to be safe, and cannot immediately be subjected to dynamic checking because they involve non-reifiable (§4.7) types. As a result, dynamic checks later in the course of program execution may detect inconsistencies and result in run-time type errors.

A run-time type error can occur only in these situations:

15.6 Normal and Abrupt Completion of Evaluation

Every expression has a normal mode of evaluation in which certain computational steps are carried out. The following sections describe the normal mode of evaluation for each kind of expression. If all the steps are carried out without an exception being thrown, the expression is said to complete normally.

If, however, evaluation of an expression throws an exception, then the expression is said to complete abruptly. An abrupt completion always has an associated reason, which is always a throw with a given value.

Run-time exceptions are thrown by the predefined operators as follows:

A method invocation expression can also result in an exception being thrown if an exception occurs that causes execution of the method body to complete abruptly. A class instance creation expression can also result in an exception being thrown if an exception occurs that causes execution of the constructor to complete abruptly. Various linkage and virtual machine errors may also occur during the evaluation of an expression. By their nature, such errors are difficult to predict and difficult to handle.

If an exception occurs, then evaluation of one or more expressions may be terminated before all steps of their normal mode of evaluation are complete; such expressions are said to complete abruptly. The terms "complete normally" and "complete abruptly" are also applied to the execution of statements (§14.1). A statement may complete abruptly for a variety of reasons, not just because an exception is thrown.

If evaluation of an expression requires evaluation of a subexpression, abrupt completion of the subexpression always causes the immediate abrupt completion of the expression itself, with the same reason, and all succeeding steps in the normal mode of evaluation are not performed.

15.7 Evaluation Order

The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

It is recommended that code not rely crucially on this specification. Code is usually clearer when each expression contains at most one side effect, as its outermost operation, and when code does not depend on exactly which exception arises as a consequence of the left-to-right evaluation of expressions.

15.7.1 Evaluate Left-Hand Operand First

The left-hand operand of a binary operator appears to be fully evaluated before any part of the right-hand operand is evaluated. For example, if the left-hand operand contains an assignment to a variable and the right-hand operand contains a reference to that same variable, then the value produced by the reference will reflect the fact that the assignment occurred first.

Thus:

class Test {
        public static void main(String[] args) {
                int i = 2;
                int j = (i=3) * i;
                System.out.println(j);
        }
}
prints:

9
It is not permitted for it to print 6 instead of 9.

If the operator is a compound-assignment operator (§15.26.2), then evaluation of the left-hand operand includes both remembering the variable that the left-hand operand denotes and fetching and saving that variable's value for use in the implied combining operation. So, for example, the test program:

class Test {
        public static void main(String[] args) {
                int a = 9;
                a += (a = 3);                                                                   // first example
                System.out.println(a);
                int b = 9;
                b = b + (b = 3);                                                                        // second example
                System.out.println(b);
        }
}
prints:

12
12
because the two assignment statements both fetch and remember the value of the left-hand operand, which is 9, before the right-hand operand of the addition is evaluated, thereby setting the variable to 3. It is not permitted for either example to produce the result 6. Note that both of these examples have unspecified behavior in C, according to the ANSI/ISO standard.

If evaluation of the left-hand operand of a binary operator completes abruptly, no part of the right-hand operand appears to have been evaluated.

Thus, the test program:

class Test {
        public static void main(String[] args) {
                int j = 1;
                try {
                        int i = forgetIt() / (j = 2);
                } catch (Exception e) {
                        System.out.println(e);
                        System.out.println("Now j = " + j);
                }
        }
        static int forgetIt() throws Exception {
                throw new Exception("I'm outta here!");
        }
}
prints:

java.lang.Exception: I'm outta here!
Now j = 1
That is, the left-hand operand forgetIt() of the operator / throws an exception before the right-hand operand is evaluated and its embedded assignment of 2 to j occurs.

15.7.2 Evaluate Operands before Operation

The Java programming language also guarantees that every operand of an operator (except the conditional operators &&, ||, and ? :) appears to be fully evaluated before any part of the operation itself is performed.

If the binary operator is an integer division / (§15.17.2) or integer remainder % (§15.17.3), then its execution may raise an ArithmeticException, but this exception is thrown only after both operands of the binary operator have been evaluated and only if these evaluations completed normally.

So, for example, the program:

class Test {
        public static void main(String[] args) {
                int divisor = 0;
                try {
                        int i = 1 / (divisor * loseBig());
                } catch (Exception e) {
                        System.out.println(e);
                }
        }
        static int loseBig() throws Exception {
                throw new Exception("Shuffle off to Buffalo!");
        }
}
always prints:

java.lang.Exception: Shuffle off to Buffalo!
and not:

java.lang.ArithmeticException: / by zero
since no part of the division operation, including signaling of a divide-by-zero exception, may appear to occur before the invocation of loseBig completes, even though the implementation may be able to detect or infer that the division operation would certainly result in a divide-by-zero exception.

15.7.3 Evaluation Respects Parentheses and Precedence

Java programming language implementations must respect the order of evaluation as indicated explicitly by parentheses and implicitly by operator precedence. An implementation may not take advantage of algebraic identities such as the associative law to rewrite expressions into a more convenient computational order unless it can be proven that the replacement expression is equivalent in value and in its observable side effects, even in the presence of multiple threads of execution (using the thread execution model in §17), for all possible computational values that might be involved.

In the case of floating-point calculations, this rule applies also for infinity and not-a-number (NaN) values. For example, !(x<y) may not be rewritten as x>=y, because these expressions have different values if either x or y is NaN or both are NaN.

Specifically, floating-point calculations that appear to be mathematically associative are unlikely to be computationally associative. Such computations must not be naively reordered.

For example, it is not correct for a Java compiler to rewrite 4.0*x*0.5 as 2.0*x; while roundoff happens not to be an issue here, there are large values of x for which the first expression produces infinity (because of overflow) but the second expression produces a finite result.

So, for example, the test program:

strictfp class Test {
        public static void main(String[] args) {
                double d = 8e+307;
                System.out.println(4.0 * d * 0.5);
                System.out.println(2.0 * d);
        }
}
prints:

Infinity
1.6e+308
because the first expression overflows and the second does not.

In contrast, integer addition and multiplication are provably associative in the Java programming language.

For example a+b+c, where a, b, and c are local variables (this simplifying assumption avoids issues involving multiple threads and volatile variables), will always produce the same answer whether evaluated as (a+b)+c or a+(b+c); if the expression b+c occurs nearby in the code, a smart compiler may be able to use this common subexpression.

15.7.4 Argument Lists are Evaluated Left-to-Right

In a method or constructor invocation or class instance creation expression, argument expressions may appear within the parentheses, separated by commas. Each argument expression appears to be fully evaluated before any part of any argument expression to its right.

Thus:

class Test {
        public static void main(String[] args) {
                String s = "going, ";
                print3(s, s, s = "gone");
        }
        static void print3(String a, String b, String c) {
                System.out.println(a + b + c);
        }
}
always prints:

going, going, gone
because the assignment of the string "gone" to s occurs after the first two arguments to print3 have been evaluated.

If evaluation of an argument expression completes abruptly, no part of any argument expression to its right appears to have been evaluated.

Thus, the example:

class Test {
        static int id;
        public static void main(String[] args) {
                try {
                        test(id = 1, oops(), id = 3);
                } catch (Exception e) {
                        System.out.println(e + ", id=" + id);
                }
        }
        static int oops() throws Exception {
                throw new Exception("oops");
        }
        static int test(int a, int b, int c) {
                return a + b + c;
        }
}
prints:

java.lang.Exception: oops, id=1
because the assignment of 3 to id is not executed.

15.7.5 Evaluation Order for Other Expressions

The order of evaluation for some expressions is not completely covered by these general rules, because these expressions may raise exceptional conditions at times that must be specified. See, specifically, the detailed explanations of evaluation order for the following kinds of expressions:

15.8 Primary Expressions

Primary expressions include most of the simplest kinds of expressions, from which all others are constructed: literals, class literals, field accesses, method invocations, and array accesses. A parenthesized expression is also treated syntactically as a primary expression.


Primary:
        PrimaryNoNewArray
        ArrayCreationExpression

PrimaryNoNewArray:
        Literal
        Type . class
        void . class
        this
        ClassName.this
        ( Expression )
        ClassInstanceCreationExpression
        FieldAccess
        MethodInvocation
        ArrayAccess

15.8.1 Lexical Literals

A literal (§3.10) denotes a fixed, unchanging value.

The following production from §3.10 is repeated here for convenience:


Literal:
        IntegerLiteral
        FloatingPointLiteral
        BooleanLiteral
        CharacterLiteral
        StringLiteral
        NullLiteral
The type of a literal is determined as follows:

Evaluation of a lexical literal always completes normally.

15.8.2 Class Literals

A class literal is an expression consisting of the name of a class, interface, array, or primitive type, or the pseudo-type void, followed by a `.' and the token class. The type of a class literal, C.Class, where C is the name of a class, interface or array type, is Class<C>. If p is the name of a primitive type, let B be the type of an expression of type p after boxing conversion (§5.1.7). Then the type of p.class is Class<B>. The type of void.class is Class<Void>.

A class literal evaluates to the Class object for the named type (or for void) as defined by the defining class loader of the class of the current instance.

It is a compile time error if any of the following occur:

15.8.3 this

The keyword this may be used only in the body of an instance method, instance initializer or constructor, or in the initializer of an instance variable of a class. If it appears anywhere else, a compile-time error occurs.

When used as a primary expression, the keyword this denotes a value that is a reference to the object for which the instance method was invoked (§15.12), or to the object being constructed. The type of this is the class C within which the keyword this occurs. At run time, the class of the actual object referred to may be the class C or any subclass of C.

In the example:

class IntVector {
        int[] v;
        boolean equals(IntVector other) {
                if (this == other)
                        return true;
                if (v.length != other.v.length)
                        return false;
                for (int i = 0; i < v.length; i++)
                        if (v[i] != other.v[i])
                                return false;
                return true;
        }
}
the class IntVector implements a method equals, which compares two vectors. If the other vector is the same vector object as the one for which the equals method was invoked, then the check can skip the length and value comparisons. The equals method implements this check by comparing the reference to the other object to this.

The keyword this is also used in a special explicit constructor invocation statement, which can appear at the beginning of a constructor body (§8.8.7).

15.8.4 Qualified this

Any lexically enclosing instance can be referred to by explicitly qualifying the keyword this.

Let C be the class denoted by ClassName. Let n be an integer such that C is the nth lexically enclosing class of the class in which the qualified this expression appears. The value of an expression of the form ClassName.this is the nth lexically enclosing instance of this (§8.1.3). The type of the expression is C. It is a compile-time error if the current class is not an inner class of class C or C itself.

15.8.5 Parenthesized Expressions

A parenthesized expression is a primary expression whose type is the type of the contained expression and whose value at run time is the value of the contained expression. If the contained expression denotes a variable then the parenthesized expression also denotes that variable.

The use of parentheses only effects the order of evaluation, with one fascinating exception.

Discussion

Consider the case if the smallest possible negative value of type long. This value, 9223372036854775808L, is allowed only as an operand of the unary minus operator (§3.10.1). Therefore, enclosing it in parentheses, as in -(9223372036854775808L) causes a compile time error.

In particular, the presence or absence of parentheses around an expression does not (except for the case noted above) affect in any way:

15.9 Class Instance Creation Expressions

A class instance creation expression is used to create new objects that are instances of classes.


ClassInstanceCreationExpression:
   new TypeArgumentsopt ClassOrInterfaceType ( ArgumentListopt )
ClassBodyopt
        Primary. new TypeArgumentsopt Identifier TypeArgumentsopt (
ArgumentListopt ) ClassBodyopt

ArgumentList:
        Expression
        ArgumentList , Expression
A class instance creation expression specifies a class to be instantiated, possibly followed by type arguments (if the class being instantiated is generic (§8.1.2)), followed by (a possibly empty) list of actual value arguments to the constructor. It is also possible to pass explicit type arguments to the constructor itself (if it is a generic constructor (§8.8.4)). The type arguments to the constructor immediately follow the keyword new. It is a compile-time error if any of the type arguments used in a class instance creation expression are wildcard type arguments (§4.5.1). Class instance creation expressions have two forms:

A class instance creation expression can throw an exception type E iff either:

Both unqualified and qualified class instance creation expressions may optionally end with a class body. Such a class instance creation expression declares an anonymous class (§15.9.5) and creates an instance of it.

We say that a class is instantiated when an instance of the class is created by a class instance creation expression. Class instantiation involves determining what class is to be instantiated, what the enclosing instances (if any) of the newly created instance are, what constructor should be invoked to create the new instance and what arguments should be passed to that constructor.

15.9.1 Determining the Class being Instantiated

If the class instance creation expression ends in a class body, then the class being instantiated is an anonymous class. Then:

If a class instance creation expression does not declare an anonymous class, then:

The type of the class instance creation expression is the class type being instantiated.

15.9.2 Determining Enclosing Instances

Let C be the class being instantiated, and let i the instance being created. If C is an inner class then i may have an immediately enclosing instance. The immediately enclosing instance of i (§8.1.3) is determined as follows:

In addition, if C is an anonymous class, and the direct superclass of C, S, is an inner class then i may have an immediately enclosing instance with respect to S which is determined as follows:

15.9.3 Choosing the Constructor and its Arguments

Let C be the class type being instantiated. To create an instance of C, i, a constructor of C is chosen at compile-time by the following rules:

Note that the type of the class instance creation expression may be an anonymous class type, in which case the constructor being invoked is an anonymous constructor.

15.9.4 Run-time Evaluation of Class Instance Creation Expressions

At run time, evaluation of a class instance creation expression is as follows.

First, if the class instance creation expression is a qualified class instance creation expression, the qualifying primary expression is evaluated. If the qualifying expression evaluates to null, a NullPointerException is raised, and the class instance creation expression completes abruptly. If the qualifying expression completes abruptly, the class instance creation expression completes abruptly for the same reason.

Next, space is allocated for the new class instance. If there is insufficient space to allocate the object, evaluation of the class instance creation expression completes abruptly by throwing an OutOfMemoryError (§15.9.6).

The new object contains new instances of all the fields declared in the specified class type and all its superclasses. As each new field instance is created, it is initialized to its default value (§4.12.5).

Next, the actual arguments to the constructor are evaluated, left-to-right. If any of the argument evaluations completes abruptly, any argument expressions to its right are not evaluated, and the class instance creation expression completes abruptly for the same reason.

Next, the selected constructor of the specified class type is invoked. This results in invoking at least one constructor for each superclass of the class type. This process can be directed by explicit constructor invocation statements (§8.8) and is described in detail in §12.5.

The value of a class instance creation expression is a reference to the newly created object of the specified class. Every time the expression is evaluated, a fresh object is created.

15.9.5 Anonymous Class Declarations

An anonymous class declaration is automatically derived from a class instance creation expression by the compiler.

An anonymous class is never abstract (§8.1.1.1). An anonymous class is always an inner class (§8.1.3); it is never static (§8.1.1, §8.5.2). An anonymous class is always implicitly final (§8.1.1.2).

15.9.5.1 Anonymous Constructors

An anonymous class cannot have an explicitly declared constructor. Instead, the compiler must automatically provide an anonymous constructor for the anonymous class. The form of the anonymous constructor of an anonymous class C with direct superclass S is as follows:

In all cases, the throws clause of an anonymous constructor must list all the checked exceptions thrown by the explicit superclass constructor invocation statement contained within the anonymous constructor, and all checked exceptions thrown by any instance initializers or instance variable initializers of the anonymous class.

Note that it is possible for the signature of the anonymous constructor to refer to an inaccessible type (for example, if such a type occurred in the signature of the superclass constructor cs). This does not, in itself, cause any errors at either compile time or run time.

15.9.6 Example: Evaluation Order and Out-of-Memory Detection

If evaluation of a class instance creation expression finds there is insufficient memory to perform the creation operation, then an OutOfMemoryError is thrown. This check occurs before any argument expressions are evaluated.

So, for example, the test program:

class List {
        int value;
        List next;
        static List head = new List(0);
        List(int n) { value = n; next = head; head = this; }
}
class Test {
        public static void main(String[] args) {
                int id = 0, oldid = 0;
                try {
                        for (;;) {
                                ++id;
                                new List(oldid = id);
                        }
                } catch (Error e) {
                        System.out.println(e + ", " + (oldid==id));
                }
        }
}
prints:

java.lang.OutOfMemoryError: List, false
because the out-of-memory condition is detected before the argument expression oldid = id is evaluated.

Compare this to the treatment of array creation expressions (§15.10), for which the out-of-memory condition is detected after evaluation of the dimension expressions (§15.10.3).

15.10 Array Creation Expressions

An array instance creation expression is used to create new arrays (§10).


ArrayCreationExpression:
   new PrimitiveType DimExprs Dimsopt
        new ClassOrInterfaceType DimExprs Dimsopt
      new PrimitiveType Dims ArrayInitializer
        new ClassOrInterfaceType Dims ArrayInitializer
        



DimExprs:
        DimExpr
        DimExprs DimExpr

DimExpr:
        [ Expression ]

Dims:
        [ ]
        Dims [ ]
An array creation expression creates an object that is a new array whose elements are of the type specified by the PrimitiveType or ClassOrInterfaceType. It is a compile-time error if the ClassOrInterfaceType does not denote a reifiable type (§4.7). Otherwise, the ClassOrInterfaceType may name any named reference type, even an abstract class type (§8.1.1.1) or an interface type (§9).

Discussion

The rules above imply that the element type in an array creation expression cannot be a parameterized type, other than an unbounded wildcard.

The type of the creation expression is an array type that can denoted by a copy of the creation expression from which the new keyword and every DimExpr expression and array initializer have been deleted.

For example, the type of the creation expression:

new double[3][3][]
is:

double[][][]
The type of each dimension expression within a DimExpr must be a type that is convertible (§5.1.8) to an integral type, or a compile-time error occurs. Each expression undergoes unary numeric promotion (§). The promoted type must be int, or a compile-time error occurs; this means, specifically, that the type of a dimension expression must not be long.

If an array initializer is provided, the newly allocated array will be initialized with the values provided by the array initializer as described in §10.6.

15.10.1 Run-time Evaluation of Array Creation Expressions

At run time, evaluation of an array creation expression behaves as follows. If there are no dimension expressions, then there must be an array initializer. The value of the array initializer is the value of the array creation expression. Otherwise:

First, the dimension expressions are evaluated, left-to-right. If any of the expression evaluations completes abruptly, the expressions to the right of it are not evaluated.

Next, the values of the dimension expressions are checked. If the value of any DimExpr expression is less than zero, then an NegativeArraySizeException is thrown.

Next, space is allocated for the new array. If there is insufficient space to allocate the array, evaluation of the array creation expression completes abruptly by throwing an OutOfMemoryError.

Then, if a single DimExpr appears, a single-dimensional array is created of the specified length, and each component of the array is initialized to its default value (§4.12.5).

If an array creation expression contains N DimExpr expressions, then it effectively executes a set of nested loops of depth N-1 to create the implied arrays of arrays.

For example, the declaration:

float[][] matrix = new float[3][3];
is equivalent in behavior to:

float[][] matrix = new float[3][];
for (int d = 0; d < matrix.length; d++)
        matrix[d] = new float[3];
and:

Age[][][][][] Aquarius = new Age[6][10][8][12][];
is equivalent to:

Age[][][][][] Aquarius = new Age[6][][][][];
for (int d1 = 0; d1 < Aquarius.length; d1++) {
        Aquarius[d1] = new Age[10][][][];
        for (int d2 = 0; d2 < Aquarius[d1].length; d2++) {
                Aquarius[d1][d2] = new Age[8][][];
                for (int d3 = 0; d3 < Aquarius[d1][d2].length; d3++) {
                        Aquarius[d1][d2][d3] = new Age[12][];
                }
        }
}
with d, d1, d2 and d3 replaced by names that are not already locally declared. Thus, a single new expression actually creates one array of length 6, 6 arrays of length 10, 6 x 10 = 60 arrays of length 8, and 6 x 10 x 8 = 480 arrays of length 12. This example leaves the fifth dimension, which would be arrays containing the actual array elements (references to Age objects), initialized only to null references. These arrays can be filled in later by other code, such as:

Age[] Hair = { new Age("quartz"), new Age("topaz") };
Aquarius[1][9][6][9] = Hair;

A multidimensional array need not have arrays of the same length at each level.

Thus, a triangular matrix may be created by:

float triang[][] = new float[100][];
for (int i = 0; i < triang.length; i++)
        triang[i] = new float[i+1];

15.10.2 Example: Array Creation Evaluation Order

In an array creation expression (§15.10), there may be one or more dimension expressions, each within brackets. Each dimension expression is fully evaluated before any part of any dimension expression to its right.

Thus:

class Test {
        public static void main(String[] args) {
                int i = 4;
                int ia[][] = new int[i][i=3];
                System.out.println(
                        "[" + ia.length + "," + ia[0].length + "]");
        }
}
prints:

[4,3]
because the first dimension is calculated as 4 before the second dimension expression sets i to 3.

If evaluation of a dimension expression completes abruptly, no part of any dimension expression to its right will appear to have been evaluated. Thus, the example: 

class Test {
        public static void main(String[] args) {
                int[][] a = { { 00, 01 }, { 10, 11 } };
                int i = 99;
                try {
                        a[val()][i = 1]++;
                } catch (Exception e) {
                        System.out.println(e + ", i=" + i);
                }
        }
        static int val() throws Exception {
                throw new Exception("unimplemented");
        }
}
prints:

java.lang.Exception: unimplemented, i=99
because the embedded assignment that sets i to 1 is never executed.

15.10.3 Example: Array Creation and Out-of-Memory Detection

If evaluation of an array creation expression finds there is insufficient memory to perform the creation operation, then an OutOfMemoryError is thrown. This check occurs only after evaluation of all dimension expressions has completed normally.

So, for example, the test program:

class Test {
        public static void main(String[] args) {
                int len = 0, oldlen = 0;
                Object[] a = new Object[0];
                try {
                        for (;;) {
                                ++len;
                                Object[] temp = new Object[oldlen = len];
                                temp[0] = a;
                                a = temp;
                        }
                } catch (Error e) {
                        System.out.println(e + ", " + (oldlen==len));
                }
        }
}
prints:

java.lang.OutOfMemoryError, true
because the out-of-memory condition is detected after the dimension expression oldlen = len is evaluated.

Compare this to class instance creation expressions (§15.9), which detect the out-of-memory condition before evaluating argument expressions (§15.9.6).

15.11 Field Access Expressions

A field access expression may access a field of an object or array, a reference to which is the value of either an expression or the special keyword super. (It is also possible to refer to a field of the current instance or current class by using a simple name; see §6.5.6.)


FieldAccess:
        Primary . Identifier
   super . Identifier
        ClassName .super . Identifier
The meaning of a field access expression is determined using the same rules as for qualified names (§6.6), but limited by the fact that an expression cannot denote a package, class type, or interface type.

15.11.1 Field Access Using a Primary

The type of the Primary must be a reference type T, or a compile-time error occurs. The meaning of the field access expression is determined as follows:

Note, specifically, that only the type of the Primary expression, not the class of the actual object referred to at run time, is used in determining which field to use.

Thus, the example:

class S { int x = 0; }
class T extends S { int x = 1; }
class Test {
        public static void main(String[] args) {
                T t = new T();
                System.out.println("t.x=" + t.x + when("t", t));
                S s = new S();
                System.out.println("s.x=" + s.x + when("s", s));
                s = t;
                System.out.println("s.x=" + s.x + when("s", s));
        }


       static String when(String name, Object t) {
                return " when " + name + " holds a "
                        + t.getClass() + " at run time.";
        }
}
produces the output:

t.x=1 when t holds a class T at run time.
s.x=0 when s holds a class S at run time.
s.x=0 when s holds a class T at run time.
The last line shows that, indeed, the field that is accessed does not depend on the run-time class of the referenced object; even if s holds a reference to an object of class T, the expression s.x refers to the x field of class S, because the type of the expression s is S. Objects of class T contain two fields named x, one for class T and one for its superclass S.

This lack of dynamic lookup for field accesses allows programs to be run efficiently with straightforward implementations. The power of late binding and overriding is available, but only when instance methods are used. Consider the same example using instance methods to access the fields:

class S { int x = 0; int z() { return x; } }
class T extends S { int x = 1; int z() { return x; } }
class Test {
        public static void main(String[] args) {
                T t = new T();
                System.out.println("t.z()=" + t.z() + when("t", t));
                S s = new S();
                System.out.println("s.z()=" + s.z() + when("s", s));
                s = t;
                System.out.println("s.z()=" + s.z() + when("s", s));
        }
        static String when(String name, Object t) {
                return " when " + name + " holds a "
                        + t.getClass() + " at run time.";
        }
}
Now the output is:

t.z()=1 when t holds a class T at run time.
s.z()=0 when s holds a class S at run time.
s.z()=1 when s holds a class T at run time.
The last line shows that, indeed, the method that is accessed does depend on the run-time class of referenced object; when s holds a reference to an object of class T, the expression s.z() refers to the z method of class T, despite the fact that the type of the expression s is S. Method z of class T overrides method z of class S.

The following example demonstrates that a null reference may be used to access a class (static) variable without causing an exception:

class Test {
        static String mountain = "Chocorua";
        static Test favorite(){
                System.out.print("Mount ");
                return null;
        }
        public static void main(String[] args) {
                System.out.println(favorite().mountain);
        }
}
It compiles, executes, and prints:

Mount Chocorua

Even though the result of favorite() is null, a NullPointerException is not thrown. That "Mount " is printed demonstrates that the Primary expression is indeed fully evaluated at run time, despite the fact that only its type, not its value, is used to determine which field to access (because the field mountain is static).

15.11.2 Accessing Superclass Members using super

The special forms using the keyword super are valid only in an instance method, instance initializer or constructor, or in the initializer of an instance variable of a class; these are exactly the same situations in which the keyword this may be used (§15.8.3). The forms involving super may not be used anywhere in the class Object, since Object has no superclass; if super appears in class Object, then a compile-time error results.

Suppose that a field access expression super.name appears within class C, and the immediate superclass of C is class S. Then super.name is treated exactly as if it had been the expression ((S)this).name; thus, it refers to the field named name of the current object, but with the current object viewed as an instance of the superclass. Thus it can access the field named name that is visible in class S, even if that field is hidden by a declaration of a field named name in class C.

The use of super is demonstrated by the following example:

interface I { int x = 0; }
class T1 implements I { int x = 1; }
class T2 extends T1 { int x = 2; }
class T3 extends T2 {
        int x = 3;
        void test() {
                System.out.println("x=\t\t"+x);
                System.out.println("super.x=\t\t"+super.x);
                System.out.println("((T2)this).x=\t"+((T2)this).x);
                System.out.println("((T1)this).x=\t"+((T1)this).x);
                System.out.println("((I)this).x=\t"+((I)this).x);
        }
}

class Test {
        public static void main(String[] args) {
                new T3().test();
        }
}

which produces the output:

x=              3
super.x=        2
((T2)this).x=   2
((T1)this).x=   1
((I)this).x=    0

Within class T3, the expression super.x is treated exactly as if it were:

((T2)this).x
Suppose that a field access expression T.super.name appears within class C, and the immediate superclass of the class denoted by T is a class whose fully qualified name is S. Then T.super.name is treated exactly as if it had been the expression ((S)T.this).name.

Thus the expression T.super.name can access the field named name that is visible in the class named by S, even if that field is hidden by a declaration of a field named name in the class named by T.

It is a compile-time error if the current class is not an inner class of class T or T itself.

15.12 Method Invocation Expressions

A method invocation expression is used to invoke a class or instance method.


MethodInvocation:
        MethodName ( ArgumentListopt )
        Primary . NonWildTypeArgumentsopt Identifier ( ArgumentListopt )
        super . NonWildTypeArgumentsopt Identifier ( ArgumentListopt )
        ClassName . super . NonWildTypeArgumentsopt Identifier ( ArgumentListopt )
        TypeName . NonWildTypeArguments Identifier ( ArgumentListopt )
The definition of ArgumentList from §15.9 is repeated here for convenience:


ArgumentList:
        Expression
        ArgumentList , Expression

Resolving a method name at compile time is more complicated than resolving a field name because of the possibility of method overloading. Invoking a method at run time is also more complicated than accessing a field because of the possibility of instance method overriding.

Determining the method that will be invoked by a method invocation expression involves several steps. The following three sections describe the compile-time processing of a method invocation; the determination of the type of the method invocation expression is described in §15.12.3.

15.12.1 Compile-Time Step 1: Determine Class or Interface to Search

The first step in processing a method invocation at compile time is to figure out the name of the method to be invoked and which class or interface to check for definitions of methods of that name. There are several cases to consider, depending on the form that precedes the left parenthesis, as follows:

15.12.2 Compile-Time Step 2: Determine Method Signature

The second step searches the type determined in the previous step for member methods. This step uses the name of the method and the types of the argument expressions to locate methods that are both accessible and applicable, that is, declarations that can be correctly invoked on the given arguments. There may be more than one such method, in which case the most specific one is chosen. The descriptor (signature plus return type) of the most specific method is one used at run time to perform the method dispatch.

A method is applicable if it is either applicable by subtyping (§15.12.2.2), applicable by method invocation conversion (§15.12.2.3), or it is an applicable variable arity method (§15.12.2.4).

The process of determining applicability begins by determining the potentially applicable methods (§15.12.2.1). The remainder of the process is split into three phases.

Discussion

The purpose of the division into phases is to ensure compatibility with older versions of the Java programming language.

The first phase (§15.12.2.2) performs overload resolution without permitting boxing or unboxing conversion, or the use of variable arity method invocation. If no applicable method is found during this phase then processing continues to the second phase.

Discussion

This guarantees that any calls that were valid in older versions of the language are not considered ambiguous as a result of the introduction of variable arity methods, implicit boxing and/or unboxing.

The second phase (§15.12.2.3) performs overload resolution while allowing boxing and unboxing, but still precludes the use of variable arity method invocation. If no applicable method is found during this phase then processing continues to the third phase.

Discussion

This ensures that a variable arity method is never invoked if an applicable fixed arity method exists.

The third phase (§15.12.2.4) allows overloading to be combined with variable arity methods, boxing and unboxing.

Deciding whether a method is applicable will, in the case of generic methods (§8.4.4), require that actual type arguments be determined. Actual type arguments may be passed explicitly or implicitly. If they are passed implicitly, they must be inferred (§15.12.2.7) from the types of the argument expressions.

If several applicable methods have been identified during one of the three phases of applicability testing, then the most specific one is chosen, as specified in section §15.12.2.5. See the following subsections for details.

15.12.2.1 Identify Potentially Applicable Methods

A member method is potentially applicable to a method invocation if and only if all of the following are true:

Discussion

The clause above implies that a non-generic method may be potentially applicable to an invocation that supplies explicit type parameters. Indeed, it may turn out to be applicable. In such a case, the type parameters will simply be ignored.

This rule stems from issues of compatibility and principles of substitutability. Since interfaces or superclasses may be generified independently of their subtypes, we may override a generic method with a non-generic one. However, the overriding (non-generic) method must be applicable to calls to the generic method, including calls that explicitly pass type parameters. Otherwise the subtype would not be substitutable for its generified supertype.

Whether a member method is accessible at a method invocation depends on the access modifier (public, none, protected, or private) in the member's declaration and on where the method invocation appears.

The class or interface determined by compile-time step 1 (§15.12.1) is searched for all member methods that are potentially applicable to this method invocation; members inherited from superclasses and superinterfaces are included in this search.

In addition, if the method invocation has, before the left parenthesis, a MethodName of the form Identifier, then the search process also examines all methods that are (a) imported by single-static-import declarations (§7.5.3) and static-import-on-demand declarations (§7.5.4) within the compilation unit (§7.3) within which the method invocation occurs, and (b) not shadowed (§6.3.1) at the place where the method invocation appears.

If the search does not yield at least one method that is potentially applicable, then a compile-time error occurs.

15.12.2.2 Phase 1: Identify Matching Arity Methods Applicable by Subtyping

Let m be a potentially applicable method (§15.12.2.1), let e1, ..., en be the actual argument expressions of the method invocation and let Ai be the type of ei, 1in. Then:

The method m is applicable by subtyping if and only if both of the following conditions hold:

If no method applicable by subtyping is found, the search for applicable methods continues with phase 2 (§15.12.2.3). Otherwise, the most specific method (§15.12.2.5) is chosen among the methods that are applicable by subtyping.

15.12.2.3 Phase 2: Identify Matching Arity Methods Applicable by Method Invocation Conversion

Let m be a potentially applicable method (§15.12.2.1), let e1, ..., en be the actual argument expressions of the method invocation and let Ai be the type of ei, 1in. Then:

The method m is applicable by method invocation conversion if and only if both of the following conditions hold:

If no method applicable by method invocation conversion is found, the search for applicable methods continues with phase 3 (§15.12.2.4). Otherwise, the most specific method (§15.12.2.5) is chosen among the methods that are applicable by method invocation conversion.

15.12.2.4 Phase 3: Identify Applicable Variable Arity Methods

Let m be a potentially applicable method (§15.12.2.1) with variable arity, let e1, ..., ek be the actual argument expressions of the method invocation and let Ai be the type of ei, 1ik. Then:

The method m is an applicable variable-arity method if and only if all three of the following conditions hold:

If no applicable variable arity method is found, a compile-time error occurs. Otherwise, the most specific method (§15.12.2.5) is chosen among the applicable variable-arity methods.

15.12.2.5 Choosing the Most Specific Method

If more than one member method is both accessible and applicable to a method invocation, it is necessary to choose one to provide the descriptor for the run-time method dispatch. The Java programming language uses the rule that the most specific method is chosen.

The informal intuition is that one method is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time type error.

One fixed-arity member method named m is more specific than another member method of the same name and arity if all of the following conditions hold:

In addition, one variable arity member method named m is more specific than another variable arity member method of the same name if either:

The above conditions are the only circumstances under which one method may be more specific than another.

A method m1 is strictly more specific than another method m2 if and only if m1 is more specific than m2 and m2 is not more specific than m1.

A method is said to be maximally specific for a method invocation if it is accessible and applicable and there is no other method that is applicable and accessible that is strictly more specific.

If there is exactly one maximally specific method, then that method is in fact the most specific method; it is necessarily more specific than any other accessible method that is applicable. It is then subjected to some further compile-time checks as described in §15.12.3.

It is possible that no method is the most specific, because there are two or more methods that are maximally specific. In this case:

15.12.2.6 Method Result and Throws Types

The exception types of the throws clause of the chosen method are determined as follows:

A method invocation expression can throw an exception type E iff either:

15.12.2.7 Inferring Type Arguments Based on Actual Arguments

In this section, we describe the process of inferring type arguments for method and constructor invocations. This process is invoked as a subroutine when testing for method (or constructor) applicability (§15.12.2.2 - §15.12.2.4).

Discussion

The process of type inference is inherently complex. Therefore, it is useful to give an informal overview of the process before delving into the detailed specification.

Inference begins with an initial set of constraints. Generally, the constraints require that the statically known types of the actual arguments are acceptable given the declared formal argument types. We discuss the meaning of "acceptable" below.

Given these initial constraints, one may derive a set of supertype and/or equality constraints on the formal type parameters of the method or constructor.

Next, one must try and find a solution that satisfies the constraints on the type parameters. As a first approximation, if a type parameter is constrained by an equality constraint, then that constraint gives its solution. Bear in mind that the constraint may equate one type parameter with another, and only when the entire set of constraints on all type variables is resolved will we have a solution.

A supertype constraint T :> X implies that the solution is one of supertypes of X. Given several such constraints on T, we can intersect the sets of supertypes implied by each of the constraints, since the type parameter must be a member of all of them. We can then choose the most specific type that is in the intersection.

Computing the intersection is more complicated than one might first realize. Given that a type parameter is constrained to be a supertype of two distinct invocations of a generic type, say List<Object> and List<String>, the naive intersection operation might yield Object. However, a more sophisticated analysis yields a set containing List<?>. Similarly, if a type parameter, T, is constrained to be a supertype of two unrelated interfaces I and J, we might infer T must be Object, or we might obtain a tighter bound of I & J. These issues are discussed in more detail later in this section.

We will use the following notational conventions in this section:

Inference begins with a set of initial constraints of the form A << F, A = F, or A >> F, where U << V indicates that type U is convertible to type V by method invocation conversion (§5.3), and U >> V indicates that type V is convertible to type U by method invocation conversion.

Discussion

In a simpler world, the constraints could be of the form A <: F - simply requiring that the actual argument types be subtypes of the formal ones. However, reality is more involved. As discussed earlier, method applicability testing consists of up to three phases; this is required for compatibility reasons. Each phase imposes slightly different constraints. If a method is applicable by subtyping (§15.12.2.2), the constraints are indeed subtyping constraints. If a method is applicable by method invocation conversion (§15.12.2.3), the constraints imply that the actual type is convertible to the formal type by method invocation conversion. The situation is similar for the third phase (§15.12.2.4), but the exact form of the constraints differ due to the variable arity.

These constraints are then reduced to a set of simpler constraints of the forms T :> X, T = X or T <: X, where T is a type parameter of the method. This reduction is achieved by the procedure given below:

Discussion

It may be that the initial constraints are unsatisfiable; we say that inference is overconstrained. In that case, we do not necessarily derive unsatisfiable constraints on the type parameters. Instead, we may infer actual type arguments for the invocation, but once we substitute the actual type arguments for the formal type parameters, the applicability test may fail because the actual argument types are not acceptable given the substituted formals.

An alternative strategy would be to have type inference itself fail in such cases. Compilers may choose to do so, provided the effect is equivalent to that specified here.

Given a constraint of the form A << F, A = F, or A >> F:

Discussion

This follows from the covariant subtype relation among array types. The constraint A << F, in this case means that A << U[]. A is therefore necessarily an array type V[], or a type variable whose upper bound is an array type V[] - otherwise the relation A << U[] could never hold true. It follows that V[] << U[]. Since array subtyping is covariant, it must be the case that V << U.

Discussion

For simplicity, assume that G takes a single type argument. If the method invocation being examined is to be applicable, it must be the case that A is a subtype of some invocation of G. Otherwise, A << F would never be true.

In other words, A << F, where F = G<U>, implies that A << G<V> for some V. Now, since U is a type expression (and therefore, U is not a wildcard type argument), it must be the case that U = V, by the non-variance of ordinary parameterized type invocations.

The formulation above merely generalizes this reasoning to generics with an arbitrary number of type arguments.

Discussion

Again, let's keep things as simple as possible, and consider only the case where G has a single type argument.

A <<F in this case means A << G<? extends U>. As above, it must be the case that A is a subtype of some invocation of G. However, A may now be a subtype of either G<V>, or G<? extends V>, or G<? super V>. We examine these cases in turn. The first variation is described (generalized to multiple arguments) by the sub-bullet directly above. We therefore have A = G<V> << G<? extends U>. The rules of subtyping for wildcards imply that V << U.

Discussion

Extending the analysis above, we have A = G<? extends V> << G<? extends U>. The rules of subtyping for wildcards again imply that V << U.

Discussion

Here, we have A = G<? super V> << G<? extends U>. In general, we cannot conclude anything in this case. However, it is not necessarily an error. It may be that U will eventually be inferred to be Object, in which case the call may indeed be valid. Therefore, we simply refrain from placing any constraint on U.

Discussion

As usual, we consider only the case where G has a single type argument.

A <<F in this case means A << G<? super U>. As above, it must be the case that A is a subtype of some invocation of G. A may now be a subtype of either G<V>, or G<? extends V>, or G<? super V>. We examine these cases in turn. The first variation is described (generalized to multiple arguments) by the sub-bullet directly above. We therefore have A = G<V> << G<? super U>. The rules of subtyping for wildcards imply that V >> U.

Discussion

We have A = G<? super V> << G<? super U>. The rules of subtyping for lower-bounded wildcards again imply that V >> U.

Discussion

Here, we have A = G<? extends V> << G<? super U>. In general, we cannot conclude anything in this case. However, it is not necessarily an error. It may be that U will eventually be inferred to the null type, in which case the call may indeed be valid. Therefore, we simply refrain from placing any constraint on U.

Discussion

Such a constraint is never part of the initial constraints. However, it can arise as the algorithm recurses. We have seen this occur above, when the constraint A << F relates two parameterized types, as in G<V> << G<U>.

Discussion

Clearly, if the array types U[] and V[] are the same, their component types must be the same.