java - JButton.actionPerformed: null pointer exception

Question

I'm working through a book, and the following code throws a NPE at runtime when the JButton is clicked, at the line button.actionPerformed. I've done my best to be sure my code is exactly what is in the book, can someone point out my problem? (the book was written for java 5, I'm using the latest java 7, this shouldn't make a difference in the following code as far as I know though)

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

public class SimpleGui implements ActionListener {
JButton button;
public static void main(String[] args) {
    SimpleGui gui = new SimpleGui();
    gui.go();
}

public void go() {
    JFrame frame = new JFrame();
    JButton button = new JButton("click here");

    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, argh!");
}

}

Answer 1

The reason is this Line:

JButton button = new JButton("click here");

Here you are creating new local JButton object which is shadowing the member variable button . Hence button is still null. You should instead use:

button = new JButton("click here");