JButton button

[david john]

This code compiles and runs fine. But if inside the go method while creating the instance of JButton if we mention it like JButton button = new JButton("click this") then it throws nullPointerException when we click on the button. I am not able to understand what is going wrong if reference type is also mentioned during the instance creation.
Thank you in advance.
This code will work

import javax.swing.*;
import java.awt.event.*;
 
public class guiAct implements ActionListener {
JButton button;
 
public static void main (String[] args){
guiAct gui = new guiAct();
gui.go();
}
 
public void go(){
JFrame frame = new JFrame();
button = new JButton("Click This");

button.addActionListener(this);

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(button);
frame.setSize(300,300);
frame.setVisible(true);
}
public void actionPerformed(ActionEvent event){
button.setText("I've been clicked");
}
}

This code will throw nullPointerException.

import javax.swing.*;
import java.awt.event.*;
 
public class guiAct implements ActionListener {
JButton button;
 
public static void main (String[] args){
guiAct gui = new guiAct();
gui.go();
}
 
public void go(){
JFrame frame = new JFrame();
JButton button = new JButton("Click This");

button.addActionListener(this);

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(button);
frame.setSize(300,300);
frame.setVisible(true);
}
public void actionPerformed(ActionEvent event){
button.setText("I've been clicked");
}
}

[ November 23, 2008: Message edited by: david john ]

[Andre Uhres]

The reference to "button" in the "actionPerformed" method
does not know the button that you declare locally in the "go" method.
So, an Exception is thrown, because the instance variable "button"
is not initialised.

[marc weber]

Originally posted by david john:
... I am not able to understand what is going wrong if reference type is also mentioned during the instance creation...

To expand on what Andre said, when you specify the type before the variable name, you are declaring a new variable instead of using an existing variable. In this example, you have 2 different variables called "button." One is declared as an instance variable in the class guiAct, and the other is declared as a local variable within the scope of the method go. The "button" inside go is assigned to an instance, but the "button" in guiAct has a default value of null.

[david john]

Marc, Andre... Thank you very much for the reply! Got the point.