Java SWING 绘画不正确

Question

我遇到了 JFrame 无法正确绘制的问题。有人可以看看解决方案吗？我尝试过使用 BoxLayouts，但它也不起作用。在我将鼠标悬停在按钮上之前，这些按钮不会显示。

所有内容都在一个文件中并可执行。

此致

package Executable;

import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.util.ArrayList;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Whiteboard implements ActionListener 
{
JFrame frame;
public static Color drawColor = Color.BLACK;
public static String drawShape = "line";
JButton redButton, blueButton, blackButton, greenButton, circleButton,         lineButton, rectangleButton;

public static void main(String[] args)
{
    EventQueue.invokeLater(new Runnable()
    {
        public void run()
        {
            try
            {
                Whiteboard window = new Whiteboard();
                window.frame.setVisible(true);
            }
            catch (Exception e)
            {
                e.printStackTrace();
            }
        }
    });
}

public Hw11()
{
    initialize();
}

private void initialize()
{
    frame = new JFrame();
    frame.setBounds(100, 100, 686, 464);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().setLayout(null);

    JPanel toolpanel = new JPanel();
    toolpanel.setBounds(10, 11, 652, 48);
    frame.getContentPane().add(toolpanel);
    toolpanel.setLayout(null);

    blackButton = new JButton();
    blackButton.setBackground(Color.BLACK);
    blackButton.setBounds(10, 11, 89, 26);
    blackButton.addActionListener(this);
    toolpanel.add(blackButton);

    redButton = new JButton();
    redButton.setBackground(Color.RED);
    redButton.setBounds(101, 11, 89, 26);
    redButton.addActionListener(this);
    toolpanel.add(redButton);

    greenButton = new JButton();
    greenButton.setBackground(Color.GREEN);
    greenButton.setBounds(192, 11, 89, 26);
    greenButton.addActionListener(this);
    toolpanel.add(greenButton);

    blueButton = new JButton();
    blueButton.setBackground(Color.BLUE);
    blueButton.setBounds(283, 11, 89, 26);
    blueButton.addActionListener(this);
    toolpanel.add(blueButton);

    circleButton = new JButton("Circle");
    circleButton.setBounds(465, 11, 89, 26);
    circleButton.addActionListener(this);
    toolpanel.add(circleButton);

    lineButton = new JButton("Line");
    lineButton.setBounds(374, 11, 89, 26);
    lineButton.addActionListener(this);
    toolpanel.add(lineButton);

    rectangleButton = new JButton("Rect");
    rectangleButton.setBounds(555, 11, 89, 26);
    rectangleButton.addActionListener(this);
    toolpanel.add(rectangleButton);

    DrawPanel whiteboard = new DrawPanel();
    whiteboard.setBounds(10, 63, 652, 351);
    frame.getContentPane().add(whiteboard);
}

public void actionPerformed(ActionEvent e) 
{
    if (e.getSource() == redButton)
        drawColor = Color.RED;
    else if (e.getSource() == blueButton)
        drawColor = Color.BLUE;
    else if (e.getSource() == blackButton)
        drawColor = Color.BLACK;
    else if (e.getSource() == greenButton)
        drawColor = Color.GREEN;
    else if (e.getSource() == lineButton)
        drawShape = "line";
    else if (e.getSource() == circleButton)
        drawShape = "circle";
    else if (e.getSource() == rectangleButton)
        drawShape = "rect";
}
}

class DrawPanel extends JPanel implements MouseListener, MouseMotionListener
{
private Point p1, p2, p3;
ArrayList<Shape> shapes = new ArrayList<>();
Shape temp;

public DrawPanel()
{
    setBackground(Color.WHITE);
    addMouseListener(this);
    addMouseMotionListener(this);
}

public void mousePressed(MouseEvent e)
{
    p1 = e.getPoint();
}

public void mouseReleased(MouseEvent e)
{
    p2 = e.getPoint();
    Shape s = null;
    if (Hw11.drawShape.equals("line"))
    {
        s = new Line(p1, p2, Hw11.drawColor);
    }
    else if (Hw11.drawShape.equals("circle"))
    {
        s = new Circle(p1, p2, Hw11.drawColor);
    }
    else if (Hw11.drawShape.equals("rect"))
    {
        s = new Rectangle(p1, p2, Hw11.drawColor);
    }
    shapes.add(s);
    Graphics g = getGraphics();
    paint(g);
}

public void paintBackground()
{
    setBackground(Color.WHITE);
}

@Override
public void paintComponent(Graphics g)
{
    Graphics2D g2 = (Graphics2D) g;
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.7f));
    paintBackground();
    super.paintComponent(g2);
    temp.draw(g);
    for (int i = 0; i < shapes.size(); i++)
    {
        Shape s = shapes.get(i);
        s.draw(g);
    }
}

public void mouseDragged(MouseEvent e)
{
    p3 = e.getPoint();

    if (Hw11.drawShape.equals("line"))
    {
        temp = new Line(p1, p3, Hw11.drawColor);
    }
    else if (Hw11.drawShape.equals("circle"))
    {
        temp = new Circle(p1, p3, Hw11.drawColor);
    }
    else if (Hw11.drawShape.equals("rect"))
    {
        temp = new Rectangle(p1, p3, Hw11.drawColor);
    }
    Graphics g = getGraphics();
    paint(g);
}

public void mouseMoved(MouseEvent e)
{

}

public void mouseClicked(MouseEvent e)
{

}

public void mouseEntered(MouseEvent e)
{

}

public void mouseExited(MouseEvent e)
{

}
}

abstract class Shape
{
Point p1;
Point p2;
Color c;

public Shape(Point p1, Point p2, Color c)
{
    this.p1 = p1;
    this.p2 = p2;
    this.c = c;
}

public abstract void draw(Graphics g);
}

class Line extends Shape
{
public Line(Point p1, Point p2, Color c)
{
    super(p1, p2, c);
}

@Override
public void draw(Graphics g)
{
    g.setColor(c);
    g.drawLine(p1.x, p1.y, p2.x, p2.y);//draw line p1 - p2
}

}


class Circle extends Shape
{
public Circle(Point p1, Point p2, Color c)
{
    super(p1, p2, c);
}

@Override
public void draw(Graphics g)
{
    g.setColor(c);
    int r = (int)Math.sqrt((p2.x-p1.x)*(p2.x-p1.x) + (p2.y-p1.y)*(p2.y-p1.y));
    g.drawOval(p1.x, p1.y - (r/2), r, r);//draw circle p1 - p2
}
}

class Rectangle extends Shape
{
public Rectangle(Point p1, Point p2, Color c)
{
    super(p1, p2, c);
}

@Override
public void draw(Graphics g)
{
    g.setColor(c);
    int width = p2.x - p1.x;
    int height = p2.y - p1.y;
g.drawRect(p1.x, p1.y, width, height);//draw rect p1 - p2
}
}

Answer 1

您的 Shape temp 声明为 class 字段，但它在 public void mouseDragged(MouseEvent e) 中初始化。所以在第一次拖动操作之前它是空的，并且在 temp.draw(g); 中它给出了一个异常。

Answer 2

这里除了 NullPointerException 之外还有多个问题。

你不应该直接调用 paint:

Graphics g = getGraphics();
paint(g);

使用 repaint(); 而不是 paint(getGraphics());。

你不应该修改传递给 paintComponent 的 Graphics:

Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(...);
g2.setComposite(...);

这可能会意外地转移到您尝试绘制的组件之外的其他组件。

相反，创建一个副本：

Graphics2D g2 = (Graphics2D) g.create();

g2.setRenderingHint(...);
g2.setComposite(...);
...

// and dispose it at the end of paintComponent
g2.dispose();

您不应该在 paintComponent:

中修改组件状态

public void paintBackground()
{
    setBackground(Color.WHITE);
}

@Override
public void paintComponent(Graphics g)
{
    ...
    paintBackground();

这样做会导致无休止的重绘循环，因为 setBackground 如果颜色不同就会重绘。

而是使用 g.fillRect(...) 或 g.clearRect(...)。

Java SWING 绘画不正确

Java SWING Not Painting Properly

java

swing

nullpointerexception

mouse-listeners

paintcomponent