无法在 java 中绘制形状

Question

我很确定这段代码应该在屏幕上的文字旁边绘制一个椭圆形。然而，这个字全部出现了，屏幕的其余部分是黑色的。这似乎发生在任何原始形状上。我想我对 java 相当了解，但图形方面的东西一直让我感到困惑。我对此束手无策，如有任何帮助，我们将不胜感激。

import javax.swing.*;
import java.awt.*;
import java.awt.image.*;
import java.util.ArrayList;

public class Game extends JPanel implements Runnable {
    int W = 4;
    int H = 3;
    int windowSize = 300;
    boolean running;
    static boolean drawHitBoxes = true;
    int FPSLimit = 30;

    private Thread thread;
    private BufferedImage buffer;
    private Graphics2D g;

    public Game() {
        super();
        setPreferredSize(new Dimension(W * windowSize, H * windowSize));
        setFocusable(true);
        requestFocus();
    }

    public void addNotify() {
        super.addNotify();
        if (thread == null) {
            thread = new Thread(this);
            thread.start();
        }
    }

    public void run() {
        running = true;

        buffer = new BufferedImage(W * windowSize, H * windowSize,
                BufferedImage.TYPE_INT_RGB);
        g = (Graphics2D) buffer.getGraphics();

        // citList.add(new Citizen(200, 200, "Joe"));

        long startTime;
        long waitTime;

        long frameTime = 1000 / FPSLimit; // /How long one frame should take
        long currentFrameTime;

        while (running) {
            startTime = System.nanoTime(); // record when loop starts

            gameUpdate();
            gameRender();
            gameDraw();

            // Calculate how long the current frame took
            currentFrameTime = (System.nanoTime() - startTime) / 1000000;
            waitTime = frameTime - currentFrameTime;

            try {
                Thread.sleep(waitTime);
            } catch (Exception e) {
            } // Sleep for the remaining time
        }
    }

    private void gameUpdate() {
        // for(Citizen i:citList){i.update();} //Update citizens
    }

    private void gameRender() {
        g.setColor(Color.WHITE);
        g.drawOval(100, 100, W - 100, H - 100);
        g.setColor(Color.WHITE);
        g.drawString("Text.", 100, 100);
        System.out.println("Drawing white box.");

        // for(Citizen i:citList){i.draw(g);} //Draw citizens
    }

    private void gameDraw() {
        Graphics gMain = this.getGraphics();
        gMain.drawImage(buffer, 0, 0, null);
    }
}

Answer 1

g.drawOval(100, 100, W-100, H-100);

W 是 4，H 是 3，所以因为 W-100 是 -96 而 H-100 是 -97，所以你的第三个和第四个参数是负的，这对Graphics#drawOval(...) 方法，因为椭圆的宽度和高度怎么可能是负数。解决方案：确保在调用此方法时仅使用有意义的正参数。可能你想要的是：

 // but you'll also want to avoid magic numbers such as 100 & 200 as well
 g.drawOval(100, 100, W * windowSize - 200, H * windowSize - 200);

顺便说一句，我自己更喜欢使用被动图形，在 paintComponent 中绘图，每当我看到具有 Graphics 或 Graphics2D 实例字段的 Swing 代码时我都很害怕。此外，您的代码看起来不遵守 Swing 线程规则，因为它似乎是在 Swing 事件线程之外进行 Swing 调用。

Answer 2

我认为最好创建 paintComponent 方法并将您的 gameRender 和 gameDraw 转移到那里，并在您的 while 循环中用 repaint() 替换它们的方法调用。这是有效的代码。

import javax.swing.*;
import java.awt.*;
import java.awt.image.*;
import java.util.ArrayList;

public class Game extends JPanel implements Runnable {
    int W = 4;
    int H = 3;
    int windowSize = 300;
    boolean running;
    static boolean drawHitBoxes = true;
    int FPSLimit = 30;

    private Thread thread;
    private BufferedImage buffer;
    private Graphics2D g;

    public Game() {
        super();
        setPreferredSize(new Dimension(W * windowSize, H * windowSize));
        setFocusable(true);
        requestFocus();
    }

    public void addNotify() {
        super.addNotify();
        if (thread == null) {
            thread = new Thread(this);
            thread.start();
        }
    }

    public void run() {
        running = true;

        buffer = new BufferedImage(W * windowSize, H * windowSize,
                BufferedImage.TYPE_INT_RGB);
        g = (Graphics2D) buffer.getGraphics();

        // citList.add(new Citizen(200, 200, "Joe"));

        long startTime;
        long waitTime;

        long frameTime = 1000 / FPSLimit; // /How long one frame should take
        long currentFrameTime;

        while (running) {
            startTime = System.nanoTime(); // record when loop starts

            gameUpdate();
            //gameRender();
            //gameDraw();
            repaint();

            // Calculate how long the current frame took
            currentFrameTime = (System.nanoTime() - startTime) / 1000000;
            waitTime = frameTime - currentFrameTime;

            try {
                Thread.sleep(waitTime);
            } catch (Exception e) {
            } // Sleep for the remaining time
        }
    }

    private void gameUpdate() {
        // for(Citizen i:citList){i.update();} //Update citizens
    }

    private void gameRender() {
        g.setColor(Color.WHITE);
        //g.drawOval(100, 100, W - 100, H - 100);
        g.drawOval(100, 100, 100, 100);
        g.setColor(Color.WHITE);
        g.drawString("Text.", 100, 100);
        //System.out.println("Drawing white box.");

        // for(Citizen i:citList){i.draw(g);} //Draw citizens
    }

    private void gameDraw(Graphics gMain) {
        //Graphics gMain = this.getGraphics();
        gMain.drawImage(buffer, 0, 0, null);
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        gameRender();
        gameDraw(g);
    }
}

正如@Hovercraft 在他的回答中指出的那样，代码`g.drawOval(100, 100, W - 100, H - 100); 中的 W-100 和 H-100将产生负数。我不确切知道你想要在那里产生什么值，但我只是用 100 替换它们只是为了消除错误。

无法在 java 中绘制形状

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