How do i draw a triangle in java?

问题:

Ok so first of all i know how to draw a rectangle and circles and ect with g.drawRect or g.drawOval but there is no g.drawtriangle so can you guys tell me if there is a way to draw a triangle with out me having to draw it out each side of the triangle.

回答1:

You may use Graphics.drawPolygon(int[], int[], int) where the first int[] is the set of x values, the second int[] is the set of y values, and the int is the length of the array. (In a triangle's case, the int is going to be 3)

Example:

graphics.drawPolygon(new int[] {10, 20, 30}, new int[] {100, 20, 100}, 3);

Output:

回答2:

I would use a Path2D object, and would place my first point with its moveTo(...) method, and then add additional points with its lineTo(...) method. Then I could draw it or fill it via Graphics2D#draw(...) and Graphics2D#fill(...). Also calling closePath() on it will make sure that your triangle closes appropriately.

For example, the following code produces:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.RenderingHints;
import java.awt.geom.Path2D;    
import javax.swing.*;

public class Path2DExample extends JPanel {
   private static final int PREF_W = 600;
   private static final int PREF_H = PREF_W;
   private static final Color COLOR_1 = Color.blue;
   private static final Color COLOR_2 = Color.red;
   private static final Paint GRADIENT_PAINT = new GradientPaint(0, 0, COLOR_1, 20, 20, COLOR_2, true);
   private Path2D myPath = new Path2D.Double();

   public Path2DExample() {
      double firstX = (PREF_W / 2.0) * (1 - 1 / Math.sqrt(3));
      double firstY = 3.0 * PREF_H / 4.0;

      myPath.moveTo(firstX, firstY);
      myPath.lineTo(PREF_W - firstX, firstY);
      myPath.lineTo(PREF_W / 2.0, PREF_H / 4.0);
      myPath.closePath();
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;

      // to smooth out the jaggies
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(GRADIENT_PAINT);  // just for fun!
      g2.fill(myPath);  // fill my triangle
   }

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

   private static void createAndShowGui() {
      Path2DExample mainPanel = new Path2DExample();

      JFrame frame = new JFrame("Path2DExample");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

---

An additional benefit to use of a Path2D object is that if you want to drag the Shape, it's not hard to do with a MouseListener and MouseMotionListener, say something like:

private class MyMouseAdapter extends MouseAdapter {
  private Point pPressed = null;

  @Override
  public void mousePressed(MouseEvent e) {
     if (e.getButton() != MouseEvent.BUTTON1) {
        return;
     }
     if (myPath.contains(e.getPoint())) {
        pPressed = e.getPoint();
     }
  }

  public void mouseDragged(MouseEvent e) {
     drag(e);
  }

  @Override
  public void mouseReleased(MouseEvent e) {
     drag(e);
     pPressed = null;
  }

  private void drag(MouseEvent e) {
     if (pPressed == null) {
        return;
     }
     Point p = e.getPoint();
     int tx = p.x - pPressed.x;
     int ty = p.y - pPressed.y;
     AffineTransform at = AffineTransform.getTranslateInstance(tx, ty);
     myPath.transform(at);
     pPressed = p;
     repaint();
  };

}

The whole thing could look like:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.AffineTransform;
import java.awt.geom.Path2D;
import javax.swing.*;

@SuppressWarnings("serial")
public class Path2DExample extends JPanel {
   private static final int PREF_W = 600;
   private static final int PREF_H = PREF_W;
   private static final Color COLOR_1 = Color.blue;
   private static final Color COLOR_2 = Color.red;
   private static final Paint GRADIENT_PAINT = new GradientPaint(0, 0, COLOR_1,
         20, 20, COLOR_2, true);
   private Path2D myPath = new Path2D.Double();

   public Path2DExample() {
      double firstX = (PREF_W / 2.0) * (1 - 1 / Math.sqrt(3));
      double firstY = 3.0 * PREF_H / 4.0;

      myPath.moveTo(firstX, firstY);
      myPath.lineTo(PREF_W - firstX, firstY);
      myPath.lineTo(PREF_W / 2.0, PREF_H / 4.0);
      myPath.closePath();

      MyMouseAdapter myMouseAdapter = new MyMouseAdapter();
      addMouseListener(myMouseAdapter);
      addMouseMotionListener(myMouseAdapter);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(GRADIENT_PAINT);
      g2.fill(myPath);
   }

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

   private class MyMouseAdapter extends MouseAdapter {
      private Point pPressed = null;

      @Override
      public void mousePressed(MouseEvent e) {
         if (e.getButton() != MouseEvent.BUTTON1) {
            return;
         }
         if (myPath.contains(e.getPoint())) {
            pPressed = e.getPoint();
         }
      }

      public void mouseDragged(MouseEvent e) {
         drag(e);
      }

      @Override
      public void mouseReleased(MouseEvent e) {
         drag(e);
         pPressed = null;
      }

      private void drag(MouseEvent e) {
         if (pPressed == null) {
            return;
         }
         Point p = e.getPoint();
         int tx = p.x - pPressed.x;
         int ty = p.y - pPressed.y;
         AffineTransform at = AffineTransform.getTranslateInstance(tx, ty);
         myPath.transform(at);
         pPressed = p;
         repaint();
      };

   }

   private static void createAndShowGui() {
      Path2DExample mainPanel = new Path2DExample();

      JFrame frame = new JFrame("Path2DExample");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}