public abstract class AbstractClass extends JPanel {

    private JLabel jLabel = null;

    public AbstractClass() {
        super();
        initialize();
    }

    private void initialize() {
        GridBagConstraints gridBagConstraints = new GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 0;
        jLabel = new JLabel();
        jLabel.setText("JLabel");
        this.setSize(300, 200);
        this.setLayout(new GridBagLayout());
        this.add(jLabel, gridBagConstraints);
    }

}

public class A extends AbstractClass {

}