What is a Java NullPointerException? [duplicate]

Question

This question already has an answer here:

• What is a NullPointerException, and how do I fix it? 12 answers

I created a Java Application and get this Exception:

Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at javax.swing.JFrame.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at Executer.<init>(Executer.java:21)
    at Executer.main(Executer.java:14 

Here is the code:

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

public class Executer {

private JLabel lblCommand;
private JTextField txtEnter;
private JButton btNext, btPrevious;
private JPanel panel;

public static void main(String[] args) {
    new Executer();
}
public Executer() {
    JFrame frame = new JFrame("Execute Script");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(900,400);
    frame.setVisible(true);
    frame.add(panel); 
    frame.setVisible(true);
    MyPanel();
    Text();
    Buttons();
    Fields();
}
public void MyPanel() {
    panel = new JPanel();
    panel.setLayout(null);
}
public void Text(){
    lblCommand = new JLabel("Enter Here");
    lblCommand.setBounds(135, 50, 150, 20);
    Font styleOne = new Font("Arial", Font.BOLD, 13);
    lblCommand.setFont(styleOne);
    panel.add(lblCommand);
}

public void Fields () {
    txtEnter = new JTextField();
    txtEnter.setBounds(210, 50, 150, 20);
    panel.add(txtEnter);
}
public void Buttons() {
    btNext = new JButton ("Next");
    btNext.setBounds(380,325,100,20);
    panel.add(btNext);

    btPrevious = new JButton ("Previous");
    btPrevious.setBounds(260,325,100,20);
    panel.add(btPrevious);
}}

What is a NullPointerException? How would I find out?

Answer 1

Problem in this line frame.add(panel); Panel is not initialized at that point, move this line MyPanel(); before adding to initialize it.

Answer 2

Yes you are adding panel to the frame before creating object of JPanel. Anyway change your constructor with this:

public Executer() {
        JFrame frame = new JFrame("Execute Script");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(900, 400);

        MyPanel();
        Text();
        Buttons();
        Fields();

        frame.add(panel);
        frame.setVisible(true);

    }

thanks.

Answer 3

You need to instantiate panel before adding it. If you use panel before calling MyPanel(), panel is still null, hence the NullPointerException.

While you're here, give this a glance. http://geosoft.no/development/javastyle.html

Method names in Java should be mixed case starting with a lower case letter, e.g. myPanel() instead of MyPanel(). To most of us, MyPanel() looks like a constructor at first glance because you improperly styled it.

Additionally, MyPanel, Text, Fields, and Buttons should all be private methods, as it would be improper for an external class to call them.

Answer 4

As others have said, you need to create the JPanel before you try to add it to the JFrame. In fact, you should typically create all components inside the JPanel as well. I suggest that you move the calls to

Text();
Buttons();
Fields();

from the Executer constructor to the MyPanel() method and call MyPanel() before calling frame.add(panel);.

In addition, you do not need to call frame.setVisible(true); twice. Also, you should use a LayoutManager rather than calling panel.setLayout(null);. See the Oracle tutorial for Using Layout Managers.