Mywindow() {
        setSize(300, 300);
        setVisible(true);
        Container con = getContentPane();
        con.setLayout(new GridLayout(1, 2));
        button1 = new JButton("button1");
        button2 = new JButton("button2");
        text = new JTextArea(6, 12);
        scroll = new JScrollPane(text);
        splitOne = new JSplitPane(JSplitPane.VERTICAL_SPLIT, true, button1,
                button2);
        splitTwo = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, true, splitOne,
                scroll);
        interframe = new JInternalFrame("Remember to init");   《======= 这里啊
        interframe.setSize(200, 200);
        interframe.setVisible(true);
        Container interCon = interframe.getContentPane();
        interCon.setLayout(new FlowLayout());
        interCon.add(splitTwo);
        JDesktopPane desk = new JDesktopPane();
        desk.add(interframe);
        getContentPane().add(desk);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

------解决方案--------------------
interframe没有new - - 2L已经正解了 就不贴代码了

------解决方案--------------------
Java code

import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JInternalFrame;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import javax.swing.JTextArea;


public class swingFrame {
    public static void main(String args[]) {
        Mywindow win = new Mywindow();
        win.validate();
    }
}

class Mywindow extends JFrame {
    JButton button1, button2;

    JTextArea text;

    JScrollPane scroll;

    JInternalFrame interframe = new JInternalFrame();//没有初始化 对三楼无话可说了

    JSplitPane splitOne, splitTwo;

    Mywindow() {
        setSize(300, 300);
        setVisible(true);
        Container con = getContentPane();
        con.setLayout(new GridLayout(1, 2));
        button1 = new JButton("button1");
        button2 = new JButton("button2");
        text = new JTextArea(6, 12);
        scroll = new JScrollPane(text);
        splitOne = new JSplitPane(JSplitPane.VERTICAL_SPLIT, true, button1,
                button2);
        splitTwo = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, true, splitOne,
                scroll);
        
        interframe.setSize(new Dimension(200, 200));
        interframe.setVisible(true);
        Container interCon = interframe.getContentPane();
        interCon.setLayout(new FlowLayout());
        interCon.add(splitTwo);
        JDesktopPane desk = new JDesktopPane();
        desk.add(interframe);
        getContentPane().add(desk);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}