What is a Java NullPointerException? [duplicate]

Answer 1

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.

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

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