Java Beans Tutorial, Pt. 4: Invoking Reflected Methods

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[Tutorial Contents]

Now you'll see how to invoke a method whose name has been typed into a TextField. This is an artificial example of reflection. In fact few programs really require reflection, but application builder tools rely heavily on reflection and introspection (which is built on top of reflection to support Beans).

You're now going to add two methods which will be invoked by using reflection. Given a string naming a method, a Method object will be retrieved and subsequently invoked with the proper arguments. In order to support the reflection classes, add the following package import statement at the end of the current list of imports:

import java.lang.reflect.*;

Now have a look at the methods that will be called based on TextField input events or Choice ItemSelected events:

First add a paint method that draws a rectangle of variable size:

public void paint(Graphics g) {
  g.drawRect(x,y,w,h);
  }

public class ChoiceApplet04 extends Applet
{
  ...
  int x,y,w,h;
  ...
}

  public void big() {
    System.out.println("big()");
    x=10; y=50; w=100; h=100;
    repaint();
  }

  public void small() {
    System.out.println("small()");
    x=10; y=50; w=20; h=20;
    repaint();
  }

The first draws a big rectangle the second draws a small one. You will not be calling these methods directly, but indirectly throught the Reflect lookup mechansism. This is the essence of last four sections of exercises. The method which does the lookup and invocation is called invoke:

  public void invoke(String mName) {
    try {
      Method m = 
        ChoiceApplet04.class.getDeclaredMethod(
        mName, null);
      m.invoke(this, null);
    }
    catch (Exception ex) {
      System.err.println("no such method: "+mName);
    }
  }

   ChoiceApplet04.class.getDeclaredMethod(mName, null);

Now that you have the desired Method object in the variable m, you can invoke it as follows:

    m.invoke(this, null);

Just for tidiness, you'll want to keep the text displayed in the TextField consistent with the selection in the Choice component and vice versa. Add the following code to the actionPerformed method of the inner class handling events for theTextField:

    if (mName.equals("big") || 
      mName.equals("small"))
      theChoice.select(mName);
    else
      theChoice.select("unknown");

    invoke(mName);

  theChoice.addItemListener(new ItemListener() {
    public void itemStateChanged(ItemEvent e) {
      System.out.println(
        "ENTER----->theChoice ActionListener");
      String mName = (String) e.getItem();
      System.out.println("mName: " + mName);
      String pString =  
        e.paramString();  // useful for Debugging
      System.out.println("pString: " + pString);

      theTextField.setText(mName);

      invoke(mName);
      System.out.println(
        "EXIT------>theChoice ActionListener");
    }
  });

  theTextField.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
      System.out.println(
        "ENTER----->theTextField ActionListener");
      String mName = e.getActionCommand();
      System.out.println("mName: " + mName);

      if (mName.equals("big") || 
        mName.equals("small"))
        theChoice.select(mName);
      else
        theChoice.select("unknown");

      invoke(mName);
      System.out.println(
        "EXIT------>theTextField ActionListener");
    }
  });
}

Hopefully that wasn't too bad. Once you get the hang of inner classes, events and event listeners, you'll be on solid ground for programming JDK 1.1 applications and applets. Reflection is worth knowing about in certain situations. As mentioned if your are building programming environments and tools that need to find out the capabilities of classes at run time, you'll find Reflection is indespensible. For typical programs, however, you should focus on the new event model.

You'll find more detailed examples of event handlers, inner classes, reflection, and introspection in the next four sections starting with

#usemacro BEANSLINK (
  LINKFILE $NEXTFILE
  LINKTEXT Automatically Resized Beans
)

You can use TestChoiceApplet04.html to test the applet.

Program Source Code

A makefile for this lesson automates source code compilation, JAR file construction, and copying of JAR files to the appropriate BeanBox directory. You'll have to edit several of the variables in the makefile to indicate the location of your JDK 1.1 and BDK installation directories.

You may want to look at the final source file for ChoiceApplet04.java to verify changes.