CODEWITHSUNDEEP
javaswingjframejpaneljcomponent
hjaltist
hjaltist

Reputation: 59

Unexpected runtime error with custom control (Java Swing GUI)

So I'm currently trying to build up a GUI. But I can't seem to be able to find the error I get. This is my error:

Exception in thread "main" java.lang.NullPointerException
     at java.awt.Container.addImpl(Unknown Source)
     at java.awt.Container.add(Unknown Source)
     at GlobalTest.<init>(GlobalTest.java:41)
     at GlobalTest.main(GlobalTest.java:7)

And here is my code:

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

public class GlobalTest extends JPanel {     
     public static void main(String[] args) {
          JFrame window = new JFrame("Global Test");
          window.setContentPane(new GlobalTest());
          window.pack();
          window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          window.setResizable(false);
          Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
          window.setLocation((screensize.width - window.getWidth()) / 2,
                            (screensize.height - window.getHeight()) / 2);
          window.setVisible(true);
     }

 //---------------------------------------------------------------------------------------------------


 private JTextField textField1, textField2, textField3;
 private JButton calculateButton;
 private JTextArea textArea1;



 public GlobalTest() {
     setLayout(new GridLayout(1,3,3,3));
     setBackground(Color.BLUE);
     setBorder(BorderFactory.createLineBorder(Color.BLACK, 2));

     JPanel panel1 = new JPanel();
     JPanel panel2 = new JPanel();
     JPanel panel3 = new JPanel();

     add(panel1);
     add(panel2);
     add(panel3);

     panel1.setLayout(new GridLayout(3,1,2,2));
     panel1.setBorder(BorderFactory.createEmptyBorder(0,5,0,5));
     panel1.add(textField1);
     panel1.add(textField2);
     panel1.add(textField3);

     panel2.setBorder(BorderFactory.createEmptyBorder(0,5,0,5));
     panel2.add(textArea1);
     panel2.setPreferredSize(new Dimension(200,200));

     panel3.setBorder(BorderFactory.createEmptyBorder(0,5,0,5));
     panel3.add(calculateButton);
 }

}

So sorry for the indenting but can anyone tell me why I can't run this code?

Upvotes: 1

Views: 105

Answers (3)

jonD
jonD

Reputation: 36

You are using your variables textFieldX and textArea without instanciating them.

In your globaltest method, add your instanciations

textField1 = new JTextField() ...etc ! and you should get rid of that error

Upvotes: 2

Frakcool
Frakcool

Reputation: 11163

You missed instanciating these variables.

private JTextField textField1 = new JTextField(""), textField2 = new JTextField(""), textField3 = new JTextField("");
private JButton calculateButton = new JButton ("Text of Button");
private JTextArea textArea1 = new JTextArea("");

So they were null when you were calling them. That's why you got that error.

Upvotes: 3

Ivar Reukers
Ivar Reukers

Reputation: 7719

You should call:

textfield1 = new JTextField();

if the amount of characters is specified,

textfield1 = new JTextField(amount);

Upvotes: 3

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