java - JLabel 未显示

Question

我试图在下一行中给出JLabel，但是当我运行代码时，即使我的输出只显示空行，也没有发生任何事情:

主类:

import javax.swing.JFrame;

public class main {

    public static void main(String[] args){
        test piyu=new test();
        piyu.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        piyu.setSize(300,200);

        piyu.setVisible(true);
    }
}

测试类:

import java.awt.*;
import javax.swing.*;

public class test extends JFrame {

    private JLabel item1,item2;

    public test() {
        super("First Java app");
        JPanel panel=new JPanel();
        panel.setLayout(new GridLayout());
        item1=new JLabel("MY NAME IS XYZ");
        item2=new JLabel("YO");
        item1.setToolTipText("GAME ON");
        panel.add(item1);
        panel.add(item2);
    }
}

Answer 1

你只是少了一行。您永远不会将 JPanel 添加到 JFrame。

class test extends JFrame {

    private JLabel item1,item2;

    public test(){

        super("First Java app");
        JPanel panel=new JPanel();
        panel.setLayout(new GridLayout());
        item1=new JLabel("MY NAME IS XYZ");
        item2=new JLabel("YO");
        item1.setToolTipText("GAME ON");
        panel.add(item1);
        panel.add(item2);
        this.add(panel);

    }
}

另请注意，在这种情况下无需扩展 JFrame。你也可以这样写:

import java.awt.*;
import javax.swing.*;
import javax.swing.JFrame;

public class main {

    public static void main(String[] args){

        JFrame piyu=new JFrame("First Java app");

        JPanel panel=new JPanel();
        JLabel item1,item2;
        panel.setLayout(new GridLayout());
        item1=new JLabel("MY NAME IS XYZ");
        item2=new JLabel("YO");
        item1.setToolTipText("GAME ON");
        panel.add(item1);
        panel.add(item2);
        piyu.add(panel);

        piyu.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        piyu.setSize(300,200);
        piyu.setVisible(true);

    }
}