java - How to change the dimension of a component in a JFrame

Question

Suppose I have a JPanel in a JFrame. When I invoke a method that changes the preferred size of that JPanel, it does not change.

The code looks something like this:

public class SomePanel extends JPanel{

    public SomePanel(){
        setPreferredSize( new Dimension( 390, 40 ) );
        setBackground( Color.BLACK );
    }

    public void expand(){
        setPreferredSize( new Dimension( 390, 200 ) );
    }

    public static void main( String args[] ){
        JFrame frame = new JFrame();
        frame.setSize( 450, 500 );
        frame.setLayout( new FlowLayout() );

        SomePanel somePanel = new SomePanel();

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

        somePanel.expand();
    }
}

Is there something that I have to do first? I have tried so check the size of the JPanel when expand() is invoked. The height of the JPanel before and after setting the preferred size remains at 40.

I have also tried to use a Dimension variable, and that did not work either.

    Dimension dimension;

    public SomePanel(){
        dimension = new Dimension( 390, 40 );
        ...
    }

    public expand(){
        dimension.setSize( 390, 200 );
        setPreferredSize( dimension );
    } 

Answer 1

Add frame.pack(); after somePanel.expand(); in your main() method. It will be done.