将JFrame按钮声明为静态变量时，JFrame按钮会很奇怪

Question

So, I am having some really weird stuff going on here.

So, my entire class is this:

public class Test extends JFrame {

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                new Test();

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}


public static void addComponentsToPane(Container pane) {

    JButton button;
pane.setLayout(new GridBagLayout());
GridBagConstraints c = new GridBagConstraints();

button = new JButton("Button 1");
if (shouldWeightX) {
c.weightx = 0.5;
}
c.fill = GridBagConstraints.HORIZONTAL;
c.gridx = 0;
c.gridy = 0;
pane.add(button, c);

button = new JButton("Button 2");
c.fill = GridBagConstraints.HORIZONTAL;
c.weightx = 0.5;
c.gridx = 1;
c.gridy = 0;
pane.add(button, c);

button = new JButton("Button 3");
c.fill = GridBagConstraints.HORIZONTAL;
c.weightx = 0.5;
c.gridx = 2;
c.gridy = 0;
pane.add(button, c);

button = new JButton("Long-Named Button 4");
c.fill = GridBagConstraints.HORIZONTAL;
c.ipady = 50;    
c.weightx = 0.0;
c.gridwidth = 3;
c.gridx = 0;
c.gridy = 1;
pane.add(button, c);

button = new JButton("5");
c.fill = GridBagConstraints.HORIZONTAL;
c.ipady = 20; 
c.weighty = 1.0; 
c.anchor = GridBagConstraints.PAGE_END; 
c.insets = new Insets(10,0,0,0); 
c.gridx = 0;
c.gridwidth = 3;
c.gridy = 3;
pane.add(button, c);

}

private JFrame createAndShowGUI(JFrame frame) {
    frame = new JFrame("GridBagLayoutDemo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    addComponentsToPane(frame.getContentPane());
    frame.pack();
    frame.setVisible(true);
    return frame;
}

JFrame frame = null;
public Test() {

    createAndShowGUI(frame);
    addComponentsToPane(frame.getContentPane());
}

}

So anyway, I am going to focus on this bit:

 JFrame frame = null;
public Test() {

    createAndShowGUI(frame);
    addComponentsToPane(frame.getContentPane());
}

Which, produces this result (which works perfectly fine). However, when I use this code:

 JFrame frame = null;
public Test() {

    frame = createAndShowGUI(frame);
    addComponentsToPane(frame.getContentPane());
}

Which results in buttons 4 and 5 somehow duplicating themselves with a set size at the top of the screen. Also, buttons 4 and 5 appear to have different sizes. Like this: (And when I make the window smaller)

What is causing this? All I am doing is setting a variable from null or a JFrame, which should do nothing.

Answer 1

The first method throws a NullPointerException at the addComponentsToPane method because the frame attribute is never initialized and the second method adds the buttons twice to the frame. I have rewritten the code below and it works fine. You should also have a look at variable shadowing , because your frame attribute gets shadowed by the frame local variable in the addComponentsToPane method whick is rarely a good idea.

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                new Test();

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

public static void addComponentsToPane(Container pane) {

    JButton button;
    pane.setLayout(new GridBagLayout());
    GridBagConstraints c = new GridBagConstraints();

    button = new JButton("Button 1");
    if (true) {
        c.weightx = 0.5;
    }
    c.fill = GridBagConstraints.HORIZONTAL;
    c.gridx = 0;
    c.gridy = 0;
    pane.add(button, c);

    button = new JButton("Button 2");
    c.fill = GridBagConstraints.HORIZONTAL;
    c.weightx = 0.5;
    c.gridx = 1;
    c.gridy = 0;
    pane.add(button, c);

    button = new JButton("Button 3");
    c.fill = GridBagConstraints.HORIZONTAL;
    c.weightx = 0.5;
    c.gridx = 2;
    c.gridy = 0;
    pane.add(button, c);

    button = new JButton("Long-Named Button 4");
    c.fill = GridBagConstraints.HORIZONTAL;
    c.ipady = 50;
    c.weightx = 0.0;
    c.gridwidth = 3;
    c.gridx = 0;
    c.gridy = 1;
    pane.add(button, c);

    button = new JButton("5");
    c.fill = GridBagConstraints.HORIZONTAL;
    c.ipady = 20;
    c.weighty = 1.0;
    c.anchor = GridBagConstraints.PAGE_END;
    c.insets = new Insets(10, 0, 0, 0);
    c.gridx = 0;
    c.gridwidth = 3;
    c.gridy = 3;
    pane.add(button, c);

}

private void createAndShowGUI() {
    frame = new JFrame("GridBagLayoutDemo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

JFrame frame = null;

public Test() {

    createAndShowGUI();
    addComponentsToPane(frame.getContentPane());
    frame.pack();
    frame.setVisible(true);
}