我试图在java中的画布上绘制简单的形状（使用Eclipse）。画布显示正确的大小和颜色。然而，形状只出现一秒然后消失。当它们出现时，它们具有正确的尺寸，颜色和位置，但它们会在一秒左右后消失。似乎我写的下面代码中缺少一行，但我无法弄清楚它是什么。
import java.awt.*;
import java.awt.geom.Ellipse2D;
import javax.swing.*;
/**
* Draws simple shapes
*/
public class SimpleDraw2 {
private JFrame frame;
private static Canvas canvas;
private Container contentPane;
private static Graphics graphics;
private Ellipse2D.Double circle;
/**
* Constructor
* Creates canvas and frame
*/
public SimpleDraw2() {
frame = new JFrame("Draw picture");
contentPane = frame.getContentPane();
canvas = new Canvas();
canvas.setSize(250, 250);
canvas.setBackground(Color.YELLOW);
canvas.setForeground(Color.BLUE);
contentPane.add(canvas);
frame.pack();
frame.setVisible(true);
}//end constructor
/**
* Draws two circles
* @param g
*/
public void paint(Graphics g){
circle = new Ellipse2D.Double(125, 125, 50, 50);
g = canvas.getGraphics();
if(g != null){
((Graphics2D) g).fill(circle);
g.setColor(Color.GREEN);
g.fillOval(0, 0, 50, 50);
}
}//end method
/**
* @param args
*/
public static void main(String[] args) {
SimpleDraw2 draw = new SimpleDraw2();
draw.paint(graphics);
}//end main
}//end class
答案 0 :(得分：-1)
啊......在SimpleDraw2.paint（）方法中，你得到画布，然后调用它的paint（）方法。 你不应该这样做。忽略SimpleDraw2.paint（），而是将您的绘图代码发布在canvas.paint（）方法中。这样就可以正确绘制。
SimpleDraw2.paint（）内部的画布Painintg实际上会绘制到画布，但只要重新绘制，所有形状都将被清除。
请勿直接调用paint（Graphics），就像在main（）方法中那样，而是调用repaint（）
答案 1 :(得分：-1)
<强> [增订]
哦差点忘了...通常你应该把你的代码放到paintComponent（）方法中。查看四个测试类。
在v1中，您创建的画布对象将绘制在绘画上方。我不完全知道绘图周期中的内容窗格绘图是什么，以及绘制子组件和/或布局管理器。这可能是原因之一，即使我们不调用super.paint（），画布仍然会被绘制。
V2（和v4）让您可以最大程度地控制绘画周期，使用双/多缓冲可以更加轻松。
V3是最简单的，只是直接绘制到主框架，没有任何变量或特殊魔法。
在v4中，您还可以连接该回调。给你很好的控制，也很简单。
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Ellipse2D.Double;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.WindowConstants;
public class GraphicsPaint {
public static void main(final String[] args) {
new SimpleDraw2v1();
new SimpleDraw2v2();
new SimpleDraw2v3();
new SimpleDraw2v4();
}
}
class SimpleDraw2v1 {
private final JFrame frame;
private static Canvas canvas;
private final Container contentPane;
private static Graphics graphics;
private Ellipse2D.Double circle;
/**
* Constructor
* Creates canvas and frame
*/
public SimpleDraw2v1() {
frame = new JFrame("Draw picture");
frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE); // important
contentPane = frame.getContentPane();
canvas = new Canvas();
canvas.setSize(250, 250);
canvas.setBackground(Color.YELLOW);
canvas.setForeground(Color.BLUE);
contentPane.add(canvas);
frame.pack();
frame.setVisible(true);
}//end constructor
/**
* Draws two circles
* @param g
*/
public void paint(final Graphics g) {
circle = new Ellipse2D.Double(125, 125, 50, 50);
final Graphics2D g2 = ((Graphics2D) g);
g2.fill(circle);
g2.setColor(Color.GREEN);
g2.fillOval(0, 0, 50, 50);
}//end method
}//end class
class SimpleDraw2v2 extends JFrame {
private static final long serialVersionUID = 2057752114790427629L;
public SimpleDraw2v2() {
setTitle("Painting v2");
setDefaultCloseOperation(DISPOSE_ON_CLOSE);
setContentPane(new MyDrawPanel());
setBounds(200, 200, 800, 600);
setVisible(true);
}
}
class MyDrawPanel extends JPanel {
private static final long serialVersionUID = 6483380689207640805L;
@Override public void paint(final Graphics pG) {
// this call (or not overwriting paint() in the first place) results in
super.paint(pG);
// these three calls, in this order. you can test it
// paintComponent(pG);
// paintBorder(pG);
// paintChildren(pG);
final Double circle = new Ellipse2D.Double(125, 125, 50, 50);
final Graphics2D g2 = ((Graphics2D) pG);
g2.fill(circle);
g2.setColor(Color.GREEN);
g2.fillOval(0, 0, 50, 50);
}
@Override protected void paintComponent(final Graphics pG) {
System.out.println("MyDrawPanel.paintComponent()");
super.paintComponent(pG);
}
@Override protected void paintBorder(final Graphics pG) {
System.out.println("MyDrawPanel.paintBorder()");
super.paintBorder(pG);
}
@Override protected void paintChildren(final Graphics pG) {
System.out.println("MyDrawPanel.paintChildren()");
super.paintChildren(pG);
}
}
class SimpleDraw2v3 extends JFrame {
private static final long serialVersionUID = 2057752114790427629L;
public SimpleDraw2v3() {
setTitle("Painting v3");
setDefaultCloseOperation(DISPOSE_ON_CLOSE);
setBounds(200, 200, 800, 600);
setVisible(true);
}
@Override public void paint(final Graphics pG) {
super.paint(pG);
final Double circle = new Ellipse2D.Double(125, 125, 50, 50);
final Graphics2D g2 = ((Graphics2D) pG);
g2.fill(circle);
g2.setColor(Color.GREEN);
g2.fillOval(0, 0, 50, 50);
}
}
class SimpleDraw2v4 extends JFrame {
private static final long serialVersionUID = 2057752114790427629L;
public SimpleDraw2v4() {
setTitle("Painting v4");
setDefaultCloseOperation(DISPOSE_ON_CLOSE);
setContentPane(new JPanel() {
private static final long serialVersionUID = -8011251549484904282L;
@Override public void paint(final Graphics pG) {
super.paint(pG);
paintShapes((Graphics2D) pG); // use this call
}
@Override protected void paintComponent(Graphics pG) {
super.paintComponent(pG);
paintShapes((Graphics2D) pG); // or this call
}
});
setBounds(200, 200, 800, 600);
setVisible(true);
}
protected void paintShapes(final Graphics2D pG) {
final Double circle = new Ellipse2D.Double(125, 125, 50, 50);
pG.fill(circle);
pG.setColor(Color.GREEN);
pG.fillOval(0, 0, 50, 50);
}
@Override public void paint(final Graphics pG) {
super.paint(pG);
}
}
答案 2 :(得分：-1)
import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.geom.Ellipse2D;
import java.util.Scanner;
/**
* Draws simple shapes
*/
public class SimpleDraw5 {
private Frame frame;
private static MyCanvas canvas;
private static Graphics graphics;
private Panel panel;
Ellipse2D.Double circle;
/**
* Draws blue ellipses on yellow canvas
* @param args
*/
public static void main(String[] args) {
SimpleDraw5 draw = new SimpleDraw5();
Scanner scan = new Scanner(System.in);
draw.addShapes();
int test;
boolean exit = false;
while (exit == false){
System.out.println("Draw another?");
System.out.println("Enter 1 to keep drawing, 2 to close window "
+ "3 to stop but keep window open>");
test = scan.nextInt();
if(test == 1){
draw.addShapes();
}
else if (test == 2)
System.exit(0);
else
exit = true;
}
}//end main
/**
* Constructor
* Creates canvas and frame
*/
public SimpleDraw5() {
frame = new Frame("Draw picture");
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent windowEvent){
System.exit(0);
} } );
panel = new Panel();
frame.add(panel);
canvas = new MyCanvas();
panel.add(canvas);
frame.pack();
frame.setVisible(true);
}
public void addShapes(){
canvas.update(canvas.getGraphics());
frame.pack();
frame.setVisible(true);
}
/**
* Returns circle set with paramenters
* @param x - X coordinate
* @param y - Y coordinate
* @param height
* @param width
* @return - object of class Ellipse2D.Double
*/
public static Ellipse2D.Double getCircle(int x, int y, int height, int width){
Ellipse2D.Double circle = new Ellipse2D.Double(x,y,height, width);
return circle;
}//end method
/**
* Returns "default" circle
* @return - object of class Ellipse2D.Double
*/
public static Ellipse2D.Double getCircle(){
Ellipse2D.Double circle = new Ellipse2D.Double(0,0,50, 50);
return circle;
}//end method
/**
* Inner class- defines canvas
*
*/
class MyCanvas extends Canvas{
private static final long serialVersionUID = 1L;
private Ellipse2D.Double circle;
/**
* Constructor. Sets canvas size and colors
*/
public MyCanvas(){
setSize(250, 250);
setBackground(Color.YELLOW);
setForeground(Color.BLUE);
}//end
/* (non-Javadoc)
* @see java.awt.Canvas#paint(java.awt.Graphics)
*/
public void paint(Graphics g){
super.paint(g);
frame.setVisible(true);
}//end method
/* (non-Javadoc)
* @see java.awt.Canvas#update(java.awt.Graphics)
*/
public void update(Graphics g){
g = this.getGraphics();
g.setColor(Color.BLUE);
Scanner scan = new Scanner(System.in);
System.out.println("Enter x");
int x = scan.nextInt();
System.out.println("Enter y");
int y = scan.nextInt();
System.out.println("Enter w");
int w = scan.nextInt();
System.out.println("Enter h");
int h = scan.nextInt();
Ellipse2D.Double circle = getCircle(x,y,w,h);
((Graphics2D) g).fill(circle);
((Graphics2D) g).draw(circle);
graphics = g;
}//end method
}//end inner class
}//end class