Graphics2d制作盒子

Question

我是个java新手，我想做一个在屏幕上画一个方框的程序，除了paintComponent()没有用之外，其他的都是正确的。

import java.awt.*; 
import javax.swing.*;
import java.awt.Graphics2D.*;
public class Frame extends JPanel
{
    @Override //this section creates the box
    public void paintComponent(Graphics g){
    Graphics2D g2 = (Graphics2D) g;
     super.paintComponent(g);
     
     Graphics2D g2d = (Graphics2D) g;
     
     g2d.drawRect(150,150,20,20);
    
    }
    public static void createWindow() //this section creates the frame
    {
        JFrame frame = new JFrame("Simple GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    //set closing bebavior
        frame.setSize(400, 400);                                 //set the size of jframe
        frame.setLocationRelativeTo(null);                       //center the jframe 
        frame.setVisible(true);                                  //show the frame
    }
    //main method  
    public static void main(String[] args) 
    {
        createWindow();//launch your creaWindow method  
        paintComponent();
    }
}       

Answer 1

您必须复习Java的基础知识。您的错误在于，您必须创建此类框架的对象，并将其添加到已创建的JFrame中。下面是正确的代码@Hh000。

import java.awt.*; 
import javax.swing.*;
import java.awt.Graphics2D.*;
public class Frame extends JPanel{

    @Override //this section creates the box
    public void paintComponent(Graphics g){
     super.paintComponent(g);
     g.setColor(Color.BLACK);
     g.drawRect(150,150,20,20);
    }
    
    //main method  
    public static void main(String[] args){ 
        JFrame frame = new JFrame("Simple GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  //set closing bebavior
        Frame frameObject = new Frame();                       //Making a class object
        frame.add(frameObject);                                //Adding the object into the JFrame
        frame.setSize(400, 400);                               //set the size of jframe
        frame.setLocationRelativeTo(null);                     //center the jframe 
        frame.setVisible(true);
    }
}