/*
 * File: SemanticError.java
 * Author: Java, Java, Java
 * Description: This applet illustrates a common semantic
 *  error, that of redeclaring a global variabe within a method.
 *  Note that a Button b1 is declared at the class
 *  level, but never instantiated. Another Button b1 is
 *  declared within the init() method, and this one is
 *  instantiated and is given an ActionListener. However, because this
 *  one has local scope it cannot be referred to outside of the init()
 *  method. So in actionPerfored(), b1 refers to the global button,
 *  which was not instantiated. This causes a NullPointerException to
 *  be thrown.

 *  To fix this problem, don't redeclare b1 in the init() method.
 *  Just instantiate it there:
 *       b1 = new Button("Local");
 */

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class SemanticError extends Applet implements ActionListener
{
    private Button b1;           // This b1 has class scope but is not instantiated

    public void init()
    {                            // Declare and instantiate a local b1
        Button b1 = new Button ("Local"); // This b1 has local scope
        add(b1);                   // The local b1 is added to the applet
        b1.addActionListener(this);
    } // init()

    public void actionPerformed(ActionEvent e)
    {                  
        b1.setLabel("Global");     // This refers to the uninstantiated b1.
    } // actionPerformed()
} // SemanticError