图形无法正确清除

Question

I am having problems, I want a square (fly) to be drawn and redrawn to show movement. Fine in this code when the button is pressed the fly does "move", but the old squares do not delete. I have tried enviromentPanel.repaint() updateui() and removeall() and I cannot get it to work, if I use any of them then none of the shapes appear and I get a blank screen.

import java.util.Random;


public class Fly implements Runnable{
private int xPosition;
private int yPosition;
private boolean eaten;

public Fly(){
    Random randomGenerator = new Random();
    xPosition = randomGenerator.nextInt(690) + 10;
    yPosition = randomGenerator.nextInt(690) + 10;
    eaten = false;
}

public int getxPosition() {
    return xPosition;
}

public void setxPosition(int xPosition) {
    this.xPosition = xPosition;
}

public int getyPosition() {
    return yPosition;
}

public void setyPosition(int yPosition) {
    this.yPosition = yPosition;
}

public boolean isEaten() {
    return eaten;
}

public void setEaten(boolean eaten) {
    this.eaten = eaten;
}

public void move(){
      Random randomGenerator = new Random();

        int xChange = -10 + randomGenerator.nextInt(20);
        int yChange = -10 + randomGenerator.nextInt(20);
        xPosition = xPosition + xChange;
        yPosition = yPosition + yChange;
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        move();

}

@Override
public String toString() {
    return "Fly [xPosition=" + xPosition + ", yPosition=" + yPosition
            + ", eaten=" + eaten + "]";
}

@Override
public void run() {
    move();
}


}

import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Panel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import javax.swing.*;
import java.awt.*;
import javax.imageio.ImageIO;

public class Enviroment2 implements Runnable,ActionListener{ 
private JFrame frame;
private JPanel enviromentPanel,totalGUI,enviromentButtonPanel;
private JButton newFrogButton, resetButton, hungryButton;
private JTextField enterName;
private JLabel hungryLabel;
private ArrayList<Frog> frogs;
private ArrayList<Fly> flys;



public Enviroment2(){
totalGUI = new JPanel();
flys = new ArrayList<Fly>();
frogs = new ArrayList<Frog>();
enviromentPanel = new JPanel();
enviromentButtonPanel = new JPanel();
newFrogButton = new JButton("New Frog");
enterName = new JTextField("Enter name");

hungryButton = new JButton("Hungry!");

resetButton = new JButton("Reset");
frame = new JFrame("[=] Hungry Cyber Pet [=]");
JFrame.setDefaultLookAndFeelDecorated(true);

frame.setContentPane(runEnviroment());

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(740, 800);
frame.setVisible(true);



}



public JPanel runEnviroment(){

totalGUI.setLayout(null);

enviromentPanel.setLayout(null);
enviromentPanel.setLocation(10, 10);
enviromentPanel.setSize(700, 700);
enviromentPanel.setBackground(Color.WHITE);
totalGUI.add(enviromentPanel);


FlowLayout experimentLayout = new FlowLayout();
enviromentButtonPanel.setLayout(experimentLayout);
enviromentButtonPanel.setLocation(10, 710);
enviromentButtonPanel.setSize(700, 50);
totalGUI.add(enviromentButtonPanel);

newFrogButton.setLocation(0, 0);
newFrogButton.setSize(120, 30);
newFrogButton.addActionListener(this);
enviromentButtonPanel.add(newFrogButton);

enterName.setLocation(140,0);
enterName.setSize(120,30);
enviromentButtonPanel.add(enterName);

hungryButton.setLocation(280, 0);
hungryButton.setSize(120, 30);
hungryButton.addActionListener(this);
enviromentButtonPanel.add(hungryButton);

resetButton.setLocation(420, 0);
resetButton.setSize(120, 30);
resetButton.addActionListener(this);
enviromentButtonPanel.add(resetButton);


totalGUI.setOpaque(true);

return totalGUI;
}


public void draw(){     

Graphics paper = enviromentPanel.getGraphics();

for (int i = 0; i <= flys.size()-1; i++){
    System.out.println("hi");
    paper.setColor(Color.BLACK);

    paper.fillRect(flys.get(i).getxPosition(), flys.get(i).getyPosition(), 10,      10);




}
try {
    Thread.sleep(1000);
} catch (InterruptedException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

draw();

}

public void actionPerformed(ActionEvent e) {
 if(e.getSource() == newFrogButton){
     Frog frog = new Frog(enterName.getText());
     frogs.add(frog);
     Fly fly = new Fly();
     Thread t = new Thread(fly);
     t.start();
     flys.add(fly);
     showFlys();

  }
  else if(e.getSource() == hungryButton){
  }
  else if(e.getSource() == resetButton){
      frogs.clear();
      flys.clear();
      System.out.println(frogs);
      System.out.println(flys);


  }
}


public void showFlys(){
for (int i = 0; i <= flys.size()-1; i++){
    System.out.println(flys.get(i));
}

}



@Override
public void run() {
draw(); 
}



}

Answer 1

This Graphics paper = enviromentPanel.getGraphics() is the start of your problems, this is not how custom painting is done.

getGraphics returns the graphics context that was used in the last paint cycle, at best it's a snap shot, at worst it can be null .

You should never maintain a reference to any Graphics context you didn't create. They can change and painting on it out of turn can produce unexpected results.

Instead, you should override the paintComponent method (probably in environmentPanel ) and do all your custom painting in it.

Your second problem is your violating the thread rules of Swing - you should never create or modify any UI component in any thread other then the EDT

You might like to take a read through

• Performing Custom Painting
• Painting in AWT and Swing
• Concurrency in Swing