Java 回转组件的位置不正确

我希望在

JFrame

的底部有按钮

btn

。我右边有这个。我的代码中的bug在哪里

class MainClass extends JFrame {

    private JSplitPane splitPan=null;

    private void treePanel(){

        DefaultMutableTreeNode nod=new DefaultMutableTreeNode("AAA",true);
        nod.add(new DefaultMutableTreeNode("abcd"));

        JTree tree=new JTree(nod);

        JScrollPane scroll=new JScrollPane(tree);

        splitPan=new JSplitPane(JSplitPane.HORIZONTAL_SPLIT,scroll,new JLabel("aaaaa"));
        splitPan.setSize(this.getMaximumSize());

        add(splitPan);
    }


    public MainClass() {

        setLayout(new BoxLayout(getContentPane(), BoxLayout.X_AXIS));

        treePanel();    
        add(new JButton("btn")); 

        pack();
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(300,200);
        setVisible(true);
    }
}

默认情况下已实现，如果要将其置于

JFrame

的底部，则必须定义

add（new JButton（“btn”，BorderLayout.SOUTH））
您应该使用而不是
X\u轴
：

组件从上到下垂直布置

BoxLayout.Y_轴
。维斯

import javax.swing.*;
import javax.swing.tree.*;

class MainClass extends JFrame {

    private JSplitPane splitPan=null;

    private void treePanel(){
        DefaultMutableTreeNode nod=new DefaultMutableTreeNode("AAA",true);
        nod.add(new DefaultMutableTreeNode("abcd"));
        JTree tree=new JTree(nod);
        JScrollPane scroll=new JScrollPane(tree);
        splitPan=new JSplitPane(JSplitPane.HORIZONTAL_SPLIT,scroll,new JLabel("aaaaa"));
        splitPan.setSize(this.getMaximumSize());
        add(splitPan);
    }

    public MainClass() {
        // this is it!
        setLayout(new BoxLayout(getContentPane(), BoxLayout.Y_AXIS));
        treePanel();
        add(new JButton("btn"));
        pack();
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(300,200);
        setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new MainClass();
            }
        });
    }
}