问题使程序运行JAVA

Question

I am having issues trying to get this program to run, I can't seem to find where my error(s) are, can anyone help? When I run the program, I get the following error:

java.lang.NullPointerException
atjava.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at LargestApplet.init(LargestApplet.java:40)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

I can't seem to figure out what exactly I am doing wrong.

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


    public class LargestApplet extends Applet implements ActionListener {

        private static final long serialVersionUID = 1L;
        int number =0;
        double highNumber=-1;
        double lowNumber=-1;



        // Create components for applet
        Label numberLabel = new Label("Enter a number:");
        TextField numberField = new TextField(5);
        Button okButton;
        Button cancelButton;
        Label highNumberOutputLabel = new Label("The Highest number is: 0         ");

        public void init() {
            add(numberLabel);
            add(numberField);
            numberField.requestFocus();
            add(okButton);
            add(cancelButton);
            add(highNumberOutputLabel);
            setSize(400, 500);  // Sets the size of the applet window
        }
        public void actionPerformed(ActionEvent e) {
            int number = 0, highNumber = -1;

            if (numberField.getText().length() == 0) {
                numberField.requestFocus();
                JOptionPane.showMessageDialog(null,
                        "Number Cannot blank", "D A T A   E R R O R",
                        JOptionPane.ERROR_MESSAGE);
                return;
            }
            try {
                number = Integer.parseInt(numberField.getText());
            } catch (NumberFormatException ex) {
                numberField.requestFocus();
                JOptionPane.showMessageDialog(null, "Number is invalid",
                        "D A T A   E R R O R", JOptionPane.ERROR_MESSAGE);
                return;
            }
            if (number < 0 || number > 10) {
                numberField.requestFocus();
                JOptionPane.showMessageDialog(null,
                        "Number must be between 0 and 10",
                        "D A T A   E R R O R", JOptionPane.ERROR_MESSAGE);
                return;
            }
            // Determine highest number
            Integer [] numberAr = {number};
            for(int i = 0; i < numberAr.length; i++)
            {
                number += numberAr[i];
                if (numberAr[i] < lowNumber)
                    lowNumber = numberAr[i];
                else if (numberAr[i] > highNumber)
                    highNumber = numberAr[i];
            }

            // Display the results
            highNumberOutputLabel.setText("The Highest Number is:   "
                    + (highNumber));




}
        }

Answer 1

Looks like you are trying to add buttons which are not initialized

 add(okButton);
 add(cancelButton);

You should initialize both of them before adding them

Button okButton = new Button(..);
Button cancelButton = new Button(..);

Answer 2

You have to previously initialize both Button objects before adding to the Applet's content pane.

Also, your Applet isn't "listening" to the events generated by the buttons, you also need to check what type of event is being processed in the actionPerformed method.

I know I shouldn't give you the fish straight away, but here's a possible solution:

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


public class LargestApplet extends Applet implements ActionListener {

    private static final long serialVersionUID = 1L;
    int number =0;
    double highNumber=-1;
    double lowNumber=-1;



    // Create components for applet
    Label numberLabel = new Label("Enter a number:");
    TextField numberField = new TextField(5);
    Button okButton = new Button("OK");
    Button cancelButton = new Button("Cancel");
    Label highNumberOutputLabel = new Label("The Highest number is: 0         ");

    public void init() {
        add(numberLabel);
        add(numberField);
        numberField.requestFocus();

        add(okButton);
        add(cancelButton);
        add(highNumberOutputLabel);

        okButton.addActionListener(this);
        cancelButton.addActionListener(this);

        okButton.setActionCommand("ok");
        cancelButton.setActionCommand("cancel");

        setSize(400, 500);  // Sets the size of the applet window
    }
    public void actionPerformed(ActionEvent e) {
        int number = 0, highNumber = -1;

        String command = e.getActionCommand();

        if(command.equals("cancel"))
            System.exit(0);

        if (numberField.getText().length() == 0) {
            numberField.requestFocus();
            JOptionPane.showMessageDialog(null,
                    "Number Cannot blank", "D A T A   E R R O R",
                    JOptionPane.ERROR_MESSAGE);
            return;
        }
        try {
            number = Integer.parseInt(numberField.getText());
        } catch (NumberFormatException ex) {
            numberField.requestFocus();
            JOptionPane.showMessageDialog(null, "Number is invalid",
                    "D A T A   E R R O R", JOptionPane.ERROR_MESSAGE);
            return;
        }
        if (number < 0 || number > 10) {
            numberField.requestFocus();
            JOptionPane.showMessageDialog(null,
                    "Number must be between 0 and 10",
                    "D A T A   E R R O R", JOptionPane.ERROR_MESSAGE);
            return;
        }
        // Determine highest number
        Integer [] numberAr = {number};
        for(int i = 0; i < numberAr.length; i++)
        {
            number += numberAr[i];
            if (numberAr[i] < lowNumber)
                lowNumber = numberAr[i];
            else if (numberAr[i] > highNumber)
                highNumber = numberAr[i];
        }

        // Display the results
        highNumberOutputLabel.setText("The Highest Number is:   "
                + (highNumber));
    }
}

You should study the AWT and Swing libraries in order to understand some key concepts and design patterns. Good luck.