Lecture Nine, Swing, Garbage collection, JDK utilities

Here you will find information on:

Swing

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class SwingApplication {
    private static String labelPrefix = "Number of button clicks: ";
    private int numClicks = 0;

    public Component createComponents() {
        final JLabel label = new JLabel(labelPrefix + "0    ");

        JButton button = new JButton("I'm a Swing button!");
        button.addActionListener(new ActionListener() {
  public void actionPerformed(ActionEvent e) {
      numClicks++;
      label.setText(labelPrefix + numClicks);
  }
     });

        /*
         * An easy way to put space between a top-level container
         * and its contents is to put the contents in a JPanel
         * that has an "empty" border.
         */
        JPanel pane = new JPanel();
        pane.setBorder(BorderFactory.createEmptyBorder(
             30, //top
             30, //left
             10, //bottom
             30) //right
         );
        pane.setLayout(new GridLayout(0, 1));
        pane.add(button);
        pane.add(label);

        return pane;
    }

    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(
                UIManager.getCrossPlatformLookAndFeelClassName());
        } catch (Exception e) { }

        //Create the top-level container and add contents to it.
        JFrame frame = new JFrame("SwingApplication");
        SwingApplication app = new SwingApplication();
        Component contents = app.createComponents();
        frame.getContentPane().add(contents, BorderLayout.CENTER);

        //Finish setting up the frame, and show it.
        frame.addWindowListener(new WindowAdapter() {
  public void windowClosing(WindowEvent e) {
      System.exit(0);
  }
     });
        frame.pack();
        frame.setVisible(true);
    }
}

Garbage Collection

    Java automatically reclaims memory used by an object when no object variables refer to that object.

   b1 = new Button("Test Button");
   b1 = new Button("Another Button");

   Nothing points to first object("Test button"). It will be garbage collected.

   b1 = new Button("Test Button");
   b2 = b1;
   b2 = new Button("Another Button");

   First object is not garbage collected because b1 still refers to it.

   You can call System.gc() to manually invoke garbage collection.

JDK Utilities

javac

    java compiler. Converts java source code into bytecodes.

    javac -verbose calculator.java

    Tells you which other java classes the compiler needs to create the compile class file and how long it took to do the compilation.

java

    java interpreter.

    java -prof helloworld.java

    Analyzes how much time the program spends in each part of the code. It creates a file called java.prof This file shows how many times each method was called and how many miliseconds were spent executing each one.

javadoc

    Automatically generates HTML documentation for your code.

    /** testdoc - A test file for documentation of JDK */

    public class testdoc {

     /** This methiod is called first by the java interpreter.
         It prints a message to the console */

     // javadoc will ignode this commnet

     public static void main(String[] argv) {
        /* javadoc will also ignore this comment */
        System.out.println("JDK Test");
    }
   }

   javadoc testdoc.java

    Will create a testdoc.html file that uses the comments beginning with multiple asterisks.

javah

    Given a class file, generates the C header files needed by C programs to access the class's data. Performance critical parts of the code can be written in C for better speed.

    javah Filecopy

    will create the file Filecopy.h Note that Filecopy is the name of the class not the file.

jdb

    Java debugger. Use to monitor and control execution of a java program so that bugs can be found.

    alpha>jdb testdoc
    Initializing jdb...
    0xad:class(testdoc)
    > stop in testdoc.main
    Breakpoint set in testdoc.main
    > run
    run testdoc

    Breakpoint hit: running ...
    testdoc.main (testdoc:12)
    main[1] main[1] list
    8            // javadoc will ignode this commnet
    9
    10           public static void main(String[] argv) {
    11              /* javadoc will also ignore this comment */
    12      =>      System.out.println("JDK Test");
    13           }
    14         }
    main[1] clear testdoc.main
    Breakpoint cleared at testdoc.main
    main[1] cont
    main[1] JDK Test

    testdoc exited

    Type help at jdb prompt to get a list of options.

javap

    Examines the bytecodes of a compiled class file and identify its accessible variables and functions.

    javap testdoc

    Compiled from testdoc.java
    public class testdoc extends java.lang.Object {
       public testdoc();
       public static void main(java.lang.String[]);
    }

    javap -c testdoc

    shows each step that will be taken by the VM to execute the class.

Suplemental reading

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