Arrays of Objects

It takes several steps to create an array of objects in Java.  In the example shown below

	JButton [] btn;
	btn = new JButton[3];
	
	btn[0] = new JButton("Button 1");
	btn[1] = new JButton("Button 2");
	btn[2] = new JButton("Button 3");

three distinct actions occured.  First, a reference to the array was created:

	JButton [] btn;

Then the array object was created, and a reference to that array object was saved in btn:

	btn = new JButton[3];

The array object contains three references to JButton objects.  No JButton objects have yet been created -- there are only three JButton references at this point.

Next, the three JButton objects are created and their references are saved in the btn array:

	btn[0] = new JButton("Button 0");
	btn[1] = new JButton("Button 1");
	btn[2] = new JButton("Button 2");

Note that loop would be more appropriate here, but I wanted to spell out the creation of the three JButton objects.  Click here  for a discussion of the differences between arrays of built-in types and arrays of objects.

Here is a Java application that creates ten JButton objects and places them in a window:

   import java.awt.*;
   import java.awt.event.*;
   import javax.swing.*;
   
   public class ArrayOfButtons
   {
      public static void main(String [] args)
      {
         MyFrame frame = new MyFrame("Alan Window");
         frame.setSize(300, 200);
         frame.setLocation(100, 75);
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.setVisible(true);
      }
   }
   
   class MyFrame extends JFrame
   {
      JButton [] buttons = new JButton[10];
   
      public MyFrame(String s)
      {
         super(s);
         setLayout(new FlowLayout());
   
         int i = 0;
         while (i < 10)
         {
            buttons[i] = new JButton("Button " + i);  // Create each JButton object
            add(buttons[i]);
            i++;
         }
      }
   }

This code creates the following frame window:

In this next example, a text field has been added to the window, and event handling for the buttons that reports which button has been clicked:

   import java.awt.*;
   import java.awt.event.*;
   import javax.swing.*;
   
   public class ArrayOfButtons
   {
      public static void main(String [] args)
      {
         MyFrame frame = new MyFrame("Alan Window");
         frame.setSize(300, 200);
         frame.setLocation(100, 75);
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.setVisible(true);
      }
   }
   
   class MyFrame extends JFrame
   {
      JButton [] buttons = new JButton[10];
      JTextField status = new JTextField(12);
   
      public MyFrame(String s)
      {
         super(s);
         setLayout(new FlowLayout());
   
         int i = 0;
         while (i < 10)
         {
            buttons[i] = new JButton("Button " + i);  // Create each JButton object
            add(buttons[i]);
            i++;
         }
         
         add(status);
         status.setEnabled(false);
         
         ReportClick handler = new ReportClick();
         
         i = 0;
         while (i < 10)
         {
            buttons[i].addActionListener(handler);
            i++;
         }
      }
      
      class ReportClick implements ActionListener
      {
         public void actionPerformed(ActionEvent event)
         {
            JButton btn = (JButton) event.getSource();
            String s = btn.getText();
            status.setText(s);
         }
      }
   }

When run, the following window is created:

.

Exercises

1. Create a Java application that puts 35 text fields in a window.  Each text field should be five characters in size.
2. Create a Java application that puts 15 buttons in a window with the names "Button 0", "Button 1", etc.  Also show a text field with the initial contents of "Times clicked: 0".  Each time a button is clicked the "Times clicked" count goes up by one (i.e., after six buttons have been clicked the text field shows "Times clicked: 6").
3. Write a program that will act as a simple calculator.  Present ten buttons where each button displays a single digit (0 through 9), a non-editable JTextField for showing the current total, and a clear button. Pressing a button will add its value into the running total kept in the JTextField. Clear will set the total back to zero.
4. Create a Java application that puts 15 buttons in a window with the names "Button 0", "Button 1", etc.  Whenever a button is clicked the text on the face of the button changes to "Clicked!".