图形 - 如何使用方法create(int x, int y, int width, int height)和translate(int x, int y)？

我试图做我的计算机科学作业，但当我尝试使用以下方法时我陷入困境。

1. public Graphics create(int x,int y,int width,int height)

   基于此 Graphics 对象创建一个新的 Graphics 对象，但具有新的平移和剪辑区域。

   参数：

   • x - x 坐标。
   • y - y 坐标。
   • width - 剪切矩形的宽度。
   • height - 剪切矩形的高度。
      

2. public abstract void translate(int x,int y)

   将图形上下文的原点平移到当前坐标系中的点 (x, y)。

谁能解释并举例说明如何使用它们？

我正在尝试这样做..

public Graphics drawPlayer1()
{
    myPencil.up();
    myPencil.move(-620,300);
    myPencil.down();
    myPencil.fillCircle(20);
    myPencil.up();
    myPencil.move(-590,300);
    myPencil.drawString("Player1: " + player1);
    p1.create(-620,300,40,40);
    return p1;
}//end drawPlayer1

当涉及到 p1.create(-620,300,40,40); 时，它向我抛出了 nullPointerException;

我和安德鲁一起讨论这个，我从未使用过Graphics#create(int, int, int, int)。我确实用Graphics#create though.

基本上，创建方法将创建一个新的图形上下文，它是原始图形上下文的副本。这使您可以在不影响原始文件的情况下操作副本。如果您正在对无法（轻松）撤消的图形执行操作，这一点很重要。

将图形上下文简单“清零”到新位置。 Swing 绘制过程对其绘制的每个组件执行此操作。基本上之前paint被调用时，图形上下文被转换到组件位置，这意味着组件内的所有绘制都是从 0x0 开始完成的

public class TestGraphics01 {

    public static void main(String[] args) {
        new TestGraphics01();
    }

    public TestGraphics01() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestGraphicsPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestGraphicsPane extends JPanel {

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            FontMetrics fm = g.getFontMetrics();

            // This creates a "copy" the graphics context, it's translated
            // to the x, y position within the current graphics context
            // and has a width and height.  If the width or height is outside
            // the current graphics context, then it is truncated...
            // It's kind of like clip, except what ever you do to this copy
            // does not effect the graphics context it came from...
            // This would be simular to setting the clipping region, just it 
            // won't effect the parent Graphics context it was copied from...
            Graphics create = g.create(100, 100, 200, 200);
            create.setColor(Color.GREEN);
            create.fillRect(0, 0, 200, 200);
            create.setColor(Color.YELLOW);
            create.drawString("I'm inside...", 0, fm.getAscent());
            create.dispose();

            // But I remain uneffected...
            g.drawString("I'm outside...", 0, fm.getAscent());

            // I will effect every thing draw afterwards...
            g.setColor(Color.RED);
            int y = 50 - (fm.getHeight() / 2) + fm.getAscent();
            g.translate(50, y);
            g.drawString("I'm half way", 0, 0);
            // You must reset the translation if you want to reuse the graphics OR
            // you didn't create a copy...
            g.translate(-50, -y);

            y = 350 - (fm.getHeight() / 2) + fm.getAscent();
            g.translate(300, y);
            g.drawString("I'm half way", 0, 0);
            // You must reset the translation if you want to reuse the graphics OR
            // you didn't create a copy...
            g.translate(-300, -y);

        }

    }

}