NullPointerException while getting font from my JButton

Issue

I wanted to create button which would auto resize while the string inside of it cannot be displayed (it is too big).

I got something like that:

public class ResizingButton extends JButton {
    public ResizingButton(String txt) {
        super(txt);
        addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                setText(JOptionPane.showInputDialog("Text"));
            }
        });
    }

    @Override
    public void setText(String arg0) {
        super.setText(arg0);
        FontMetrics metrics = getFontMetrics(getFont());//nullPointerException !!!
        int width = metrics.stringWidth(getText());
        int height = metrics.getHeight();
        Dimension newDimension = new Dimension(width + 40, height + 10);
        setPreferredSize(newDimension);
        setBounds(new Rectangle(getLocation(), getPreferredSize()));
    }
}

I wanted to use that class:

public class Zadanie2 extends JFrame {
    public Zadanie2() {
        createGUI();
    }

    private void createGUI() {
        setSize(200, 80);
        //setLayout(new GridLayout());
        add(new ResizingButton("tekst"));
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
    new Zadanie2();
    }
}

But I am getting null pointer exception in setText() method? Why is that happening, and how can I repair it? getFont() returns null and then getFontMetrics(null) throws exception.

Stack trace:

Exception in thread "main" java.lang.NullPointerException
    at java.util.concurrent.ConcurrentHashMap.hash(Unknown Source)
    at java.util.concurrent.ConcurrentHashMap.get(Unknown Source)
    at sun.font.FontDesignMetrics.getMetrics(Unknown Source)
    at sun.swing.SwingUtilities2.getFontMetrics(Unknown Source)
    at javax.swing.JComponent.getFontMetrics(Unknown Source)
    at ResizingButton.setText(ResizingButton.java:26)
    at javax.swing.AbstractButton.init(Unknown Source)
    at javax.swing.JButton.<init>(Unknown Source)
    at javax.swing.JButton.<init>(Unknown Source)
    at ResizingButton.<init>(ResizingButton.java:13)
    at Zadanie2.createGUI(Zadanie2.java:14)
    at Zadanie2.<init>(Zadanie2.java:8)
    at Zadanie2.main(Zadanie2.java:20)

Solution

Long story short:
Change super(txt) to

super();
setText(txt);

Short explanation
As the stack-trace indicates, calling super(txt) (with a String argument) calls at some point AbstractButton's init method:

protected void init(String text, Icon icon) {
    if (text != null) {
        setText(text);
    }
    ...

As you can see, IF the text argument is not null, method setText is called. But your overriden setText method requires the Font to be set (which at this point isn't).

So, calling the no-argument constructor first (super()), causes the text argument passed to init to be null, thus avoiding the problem. Only after super() returns (at which point the Font has been properly set), is it safe to call setText(txt) explicitely and everything works fine.

(Long story short plus happy ending !)

Answered By - gkalpak