问ScrollBars不会添加到面板中

EN

import java.awt.Component;
import java.io.IOException;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JEditorPane;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTree;
import javax.swing.ScrollPaneConstants;
import javax.swing.tree.DefaultMutableTreeNode;

public class SampleSwing extends JFrame{
    public static void main(String[] args) throws IOException{
        new SampleSwing().go();
    }
private void go() throws IOException {

    JPanel a = new JPanel();
    a.setLayout(new BoxLayout(a, BoxLayout.Y_AXIS));

    JScrollPane pane1 = new JScrollPane(a);
    pane1.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
    pane1.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    getContentPane().add(pane1);

    JButton but = new JButton("Hello");
    but.setAlignmentX(Component.CENTER_ALIGNMENT);
    a.add(but);

    JEditorPane pane = new JEditorPane();
    a.add(pane);

    JComboBox list = new JComboBox(new String[]{"One", "Two"});
    list.setAlignmentX(Component.CENTER_ALIGNMENT);
    a.add(list);


    DefaultMutableTreeNode node = new DefaultMutableTreeNode("One");
    DefaultMutableTreeNode node1 = new DefaultMutableTreeNode("Two");
    node.add(node1);
    node1.add(new DefaultMutableTreeNode("Three"));
    node1.add(new DefaultMutableTreeNode("Four"));

    JTree tree = new JTree(node);
    a.add(tree);

    getContentPane().add(a);

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    pack();
    setVisible(true);
    setResizable(false);
    setLocationRelativeTo(null);

}

}