向JScrollPane添加一个按钮 - Java

Question

我有这个Frame课程;如您所见，我向splitPane课程添加了Frame：

public class Frame extends JFrame {

    private static final long serialVersionUID = 1L;
    private static final int X_AXIS = 175;
    private static final int Y_AXIS = 25;
    private static final int WIDTH= 930;
    private static final int HEIGHT = 600;

    private SplitPane splitPane;

    public Frame() {

        this.setTitle("My Frame");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setBounds(X_AXIS,Y_AXIS,WIDTH,HEIGHT);

        splitPane = new SplitPane();
        this.getContentPane().add(splitPane);   
    }

}

这是SplitPane课程。我在左边添加了JScrollPane：

import javax.swing.JScrollPane;
import javax.swing.JSplitPane;

public class SplitPane extends JSplitPane{

    private static final long serialVersionUID = 1L;
    private static final int SPLIT_WEIGHT  = 250;
    private JScrollPane leftPanel;

    public SplitPane() {

        leftPanel = new LeftPanel();
        this.setOrientation(JSplitPane.HORIZONTAL_SPLIT);
        this.setDividerLocation(SPLIT_WEIGHT);
        this.setLeftComponent(leftPanel);
    }
}

最后这是LeftPanel class：

import javax.swing.JButton;
import javax.swing.JScrollPane;
import javax.swing.ScrollPaneLayout;

public class LeftPanel extends JScrollPane{

    private static final long serialVersionUID = 1L;

    public LeftPanel() {    

        this.setLayout(new ScrollPaneLayout());
        JButton button = new JButton("Hello World!");
        this.add(button);   
    }

}

该按钮不会出现。 如何才能显示我的按钮"Hello World!"？