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

public class complete_Flowlayout {
    /*****************main method************************/
    public static void main(String[] args) {
        new mypanel("frame");
    }
    /**********************类***********************/
    public static class mypanel extends JFrame implements panel{
    mypanel(String s){
        super(s);
        setBounds(300,100,500,500);
        //准备写接口的方法

        
        setVisible(true);
        
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        }
    }
   
    /*******************interface****************/
    public interface panel{
        void build1();
        void build2();
        void build3();
        void build4();
    }
   
   
   
}



   

修改后的代码，建议下载一个IntelliJ IDE，会给出编译错误以及修改建议的

package buildPanelInFrame;

import javax.swing.*;

public class complete_Flowlayout {
    /**
     * **************main method***********************
     */
    public static void main(String[] args) {
        new mypanel("frame");
    }

    /**
     * ****************interface***************
     */
    public interface panel {
        void build1();
        void build2();
        void build3();
        void build4();
    }

    /**
     * *******************类**********************
     */
    public static class mypanel extends JFrame implements panel {
        mypanel(String s) {
            super(s);
            setBounds(300, 100, 500, 500);
            //准备写接口的方法
            setVisible(true);
            setDefaultCloseOperation(EXIT_ON_CLOSE);
        }
        public void build1(){}
        public void build2(){}
        public void build3(){}
        public void build4(){}
    }
}

你的myPanel类在类complete_Flowlayout里面，也就是说你使用了内部类