本文介绍了用按钮在屏幕上移动球。无法编程初始位置。的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
所以我正在做这个练习，我需要创建一个程序，按下四个按钮之一，在屏幕上移动一个小球。我已经完成了，但后来我想让初始位置位于屏幕的中心，所以我将值getWidth（）/ 2分配给xCoord，将getHeight（）/ 2分配给yCoord（首先我没有构造函数，然后当它不起作用时我添加了构造函数并添加了repaint（），因此将调用paintComponent（））但是当我启动程序时，球仍然在左上角。我怎样才能解决这个问题？
P.S.我将非常感谢对代码的任何评论。谢谢。
So I'm doing this exercise, where I need to create a program that will be moving a little ball on the screen by pressing one of four buttons. I've completed it, but then I wanted to make the initial position to be in the center of the screen, so I assigned the values getWidth()/2 to xCoord and getHeight()/2 to yCoord (first I did it without the constructor, then when it didn't work I added the constructor and added repaint(), so the paintComponent() would be called) but the ball is still in the top left corner when I start the program. How can I fix this?P.S. I will appreciate any comments on the code in general, as well. Thank you.
package movingaball;
import java.awt.BorderLayout;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MovingABall extends JFrame {
private JButton jbtLeft = new JButton("Left");
private JButton jbtRight = new JButton("Right");
private JButton jbtUp = new JButton("Up");
private JButton jbtDown = new JButton("Down");
private BallPanel ballPanel = new BallPanel();
public MovingABall () {
JPanel buttonPanel = new JPanel();
buttonPanel.add(jbtLeft);
buttonPanel.add(jbtRight);
buttonPanel.add(jbtUp);
buttonPanel.add(jbtDown);
this.add(ballPanel);
this.add(buttonPanel, BorderLayout.SOUTH);
jbtLeft.addActionListener(new ButtonListener());
jbtRight.addActionListener(new ButtonListener());
jbtUp.addActionListener(new ButtonListener());
jbtDown.addActionListener(new ButtonListener());
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
MovingABall mainWondow = new MovingABall();
mainWondow.setTitle("Moving a ball");
mainWondow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mainWondow.setSize(300, 200);
mainWondow.setVisible(true);
}
class ButtonListener implements ActionListener {
public void actionPerformed(ActionEvent buttonPressed) {
if (buttonPressed.getSource() == jbtLeft)
ballPanel.left();
else if (buttonPressed.getSource() == jbtRight)
ballPanel.right();
else if (buttonPressed.getSource() == jbtUp)
ballPanel.up();
else if (buttonPressed.getSource() == jbtDown)
ballPanel.down();
}
}
class BallPanel extends JPanel {
private int xCoord = 10;
private int yCoord = 10;
public BallPanel() {
xCoord = getWidth()/2;
yCoord = getHeight()/2;
repaint();
}
@Override
public void setBackground(Color bg) {
super.setBackground(bg); //To change body of generated methods, choose Tools | Templates.
}
public void left() {
xCoord-=5;
repaint();
}
public void right() {
xCoord+=5;
repaint();
}
public void up() {
yCoord-=5;
repaint();
}
public void down() {
yCoord+=5;
repaint();
}
protected void paintComponent(Graphics aBall) {
super.paintComponent(aBall);
System.out.println("X" + getWidth());
aBall.drawOval(xCoord, yCoord, 10, 10);
}
}
}
推荐答案
在代码中查看评论。
import java.awt.BorderLayout;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MovingABall extends JFrame {
private JButton jbtLeft = new JButton("Left");
private JButton jbtRight = new JButton("Right");
private JButton jbtUp = new JButton("Up");
private JButton jbtDown = new JButton("Down");
private BallPanel ballPanel = new BallPanel();
public MovingABall () {
JPanel buttonPanel = new JPanel();
buttonPanel.add(jbtLeft);
buttonPanel.add(jbtRight);
buttonPanel.add(jbtUp);
buttonPanel.add(jbtDown);
ballPanel.setBackground(Color.RED);
this.add(ballPanel);
this.add(buttonPanel, BorderLayout.SOUTH);
jbtLeft.addActionListener(new ButtonListener());
jbtRight.addActionListener(new ButtonListener());
jbtUp.addActionListener(new ButtonListener());
jbtDown.addActionListener(new ButtonListener());
}
public static void main(String[] args) {
// Should be called on the EDT!
MovingABall mainWondow = new MovingABall();
mainWondow.setTitle("Moving a ball");
mainWondow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// Don't pack here. Instead return a preferred size for the
// custom comonent end..
//mainWondow.setSize(300, 200);
// ..pack() the window.
mainWondow.pack();
mainWondow.setVisible(true);
}
class ButtonListener implements ActionListener {
public void actionPerformed(ActionEvent buttonPressed) {
if (buttonPressed.getSource() == jbtLeft)
ballPanel.left();
else if (buttonPressed.getSource() == jbtRight)
ballPanel.right();
else if (buttonPressed.getSource() == jbtUp)
ballPanel.up();
else if (buttonPressed.getSource() == jbtDown)
ballPanel.down();
}
}
class BallPanel extends JPanel {
private int xCoord = -1;
private int yCoord = -1;
private Dimension preferredSize = new Dimension(300,200);
/* Harmful to our logic..
public BallPanel() {
xCoord = getWidth()/2;
yCoord = getHeight()/2;
repaint();
}
*/
/* A good compiler would remove this..
@Override
public void setBackground(Color bg) {
super.setBackground(bg);
} */
public void left() {
xCoord-=5;
repaint();
}
public void right() {
xCoord+=5;
repaint();
}
public void up() {
yCoord-=5;
repaint();
}
public void down() {
yCoord+=5;
repaint();
}
/** Suggest a size to the layout manager. */
@Override
public Dimension getPreferredSize() {
return preferredSize;
}
protected void paintComponent(Graphics aBall) {
super.paintComponent(aBall);
// This will center the ball if it is the first time painted
// OR if the x or y co-ord goes off the left/top edge.
// Further logic left to user..
if (xCoord<0 || yCoord<0) {
xCoord = getWidth()/2;
yCoord = getHeight()/2;
}
System.out.println("X" + getWidth());
aBall.drawOval(xCoord, yCoord, 10, 10);
}
}
}
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