java - 当我使用不同的变量时，为什么 java 复制而不是创建一个新的 Instant？

Question

我写了一个相当简单的代码，但我遇到了一个让我困惑的错误。

所以我有一个类创建两个完全不同的变量并使用 new 关键字创建它们

Player playerLeft = new Player(5,150);
Player playerRight = new Player( 150,150);

玩家类:

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

public class Player extends JComponent {

    private int posY;
    private int posX;

    public Player(int x, int y) {
        posX = x;
        posY = y;
        //repaint();
    }

    public float getMovementY() {
        return movementY;
    }

    public void setMovementY(int movementY) {
        this.movementY = movementY;
    }

    int movementY = 0;

    public void paintComponent(Graphics g) {
        Graphics2D _g2 = (Graphics2D) g;
        Rectangle rect = new Rectangle(posX, posY, 20, 150);
        _g2.fill(rect);
    }

    public void setLocation(int x, int y) {
        posY = y;
        posX = x;
        repaint();
    }

    public void move() {
        setLocation(posX, posY + movementY);
    }
}

可能是我对 Java 不了解，但对我来说，当我尝试实例化 playerRight 时，它只会覆盖 player left 并仅 drawsOut playerRight。

完整代码如下:

import javax.swing.*;
import java.awt.*;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.Timer;
import java.util.TimerTask;

public class mainJFrame extends JFrame implements KeyListener {

    int relativeTimeMillsec = 0;

    Player playerLeft = new Player(5, 150);
    Player playerRight = new Player(150, 150);

    Timer timer = new Timer();
    TimerTask task = new TimerTask() {
        @Override
        public void run() {
            relativeTimeMillsec++;
            refreshTimeText(relativeTimeMillsec);
            calcMovements();
        }
    };

    //components
    JLabel timeCounterLabel = new JLabel("Time: " + 0, SwingConstants.CENTER);

    public mainJFrame() {
        createComponents();

        addKeyListener(this);
    }

    public void createComponents() {
        this.setTitle("The title");
        this.setSize(800, 600);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);

        timer.scheduleAtFixedRate(task, 0, 10);

        JButton testButton = new JButton("Label");
        testButton.setSize(100, 25);
        testButton.setLocation(this.getWidth() / 2 - testButton.getWidth() / 2, this.getHeight() / 2 - testButton.getHeight() / 2);

        timeCounterLabel.setSize(200, 25);
        timeCounterLabel.setLocation(this.getWidth() / 2 - timeCounterLabel.getWidth() / 2, 10);

        //playerRight = new Player(this.getWidth()-45,this.getHeight()/2);

        // this.add(testButton);
        this.add(timeCounterLabel);
        this.add(playerLeft);
        this.add(playerRight);
    }

    public void paintComponent(Graphics g) {
        {
            super.repaint();
        }

    }

    @Override
    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_S) {
            playerLeft.movementY = +2;
        } else if (e.getKeyCode() == KeyEvent.VK_W) {
            playerLeft.movementY = -2;
        }
        if (e.getKeyCode() == KeyEvent.VK_UP) {
            playerRight.movementY = +2;
        } else if (e.getKeyCode() == KeyEvent.VK_DOWN) {
            playerRight.movementY = -2;
        }
    }

    @Override
    public void keyReleased(KeyEvent e) {

    }

    @Override
    public void keyTyped(KeyEvent e) {

    }

    private double calcRealRelativeTime(int _relTime) {
        return relativeTimeMillsec / (double) 100;

    }

    private void refreshTimeText(int _relTime) {
        timeCounterLabel.setText("Time: " + Math.round(calcRealRelativeTime(_relTime)));
    }

    private void calcMovements() {
        playerLeft.move();
        playerRight.move();

    }
}

Answer 1

了解 JFrame 的 contentPane(包含其组件的容器)默认使用 BorderLayout，此代码:

    this.add(timeCounterLabel);
    this.add(playerLeft);
    this.add(playerRight);

正在将所有组件添加到相同的默认 BorderLayout.CENTER 位置，这意味着添加的任何组件都将替换之前添加的组件。

但更重要的是，您的问题是一个常见问题，源于您让 Player 类从 GUI 组件扩展而来。不要这样做，因为那样的话您将很难绘制多个 Player 对象并让它们轻松交互(正如您所发现的)。相反，让 Player 成为一个逻辑(非组件)类，并且只有 一个 类扩展 JPanel 并完成所有绘图。这个类可以保存 Player 对象，可能保存在一个集合中，例如 ArrayList<Player>。 ，然后在其 paintComponent 方法中遍历集合。

其他问题:

• 不要使用 java.util.Timer和 java.util.TimerTask用于 Swing 动画，因为这些类不遵循 Swing 线程规则。改用 javax.swing.Timer .
• 学习和使用Java naming conventions .变量名应全部以小写字母开头，而类名应以大写字母开头。了解这一点并遵循这一点将使我们能够更好地理解您的代码，并使您能够更好地理解其他人的代码
• 如果/当您重写绘画方法时，例如 paintComponent ，一定要在你的override中调用super的方法，通常在第一行，以免打断绘画链。另外，使用 @Override此方法和您认为您可能会覆盖的任何其他方法之前的注释，以便编译器捕获可能的错误。

例如(但不是完整的例子)

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

@SuppressWarnings("serial")
public class SimpleAnimation extends JPanel {
    private static final int PREF_W = 800;
    private static final int PREF_H = 600;
    private static final int TIMER_DELAY = 20;
    private Player2 playerLeft = new Player2(5, 150, Color.RED);
    private Player2 playerRight = new Player2(150, 150, Color.BLUE);

    public SimpleAnimation() {
        playerLeft.setySpeed(1);
        playerRight.setySpeed(-1);
        new Timer(TIMER_DELAY, new TimerListener()).start();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        playerLeft.draw(g);
        playerRight.draw(g);
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }

    private class TimerListener implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            playerRight.move();
            playerLeft.move();
            repaint();
        }
    }

    private static void createAndShowGui() {
        SimpleAnimation mainPanel = new SimpleAnimation();

        JFrame frame = new JFrame("SimpleAnimation");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

class Player2 {
    private static final int RECT_WIDTH = 20;
    private static final int RECT_HEIGHT = 50;
    private int x;
    private int y;
    private int ySpeed;
    private Color color;

    public Player2(int x, int y, Color color) {
        this.x = x;
        this.y = y;
        this.color = color;
    }

    public void setX(int x) {
        this.x = x;
    }

    public void setY(int y) {
        this.y = y;
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public void setySpeed(int ySpeed) {
        this.ySpeed = ySpeed;
    }

    public int getySpeed() {
        return ySpeed;
    }

    public void setLocation(int x, int y) {
        setX(x);
        setY(y);
    }

    public void move() {
        setLocation(x, y + ySpeed);
    }

    public void draw(Graphics g) {
        g.setColor(color);
        g.fillRect(x, y, RECT_WIDTH, RECT_HEIGHT);
    }
}