使用getGraphics（）绘制对象而不扩展JFrame [英] Drawing an object using getGraphics() without extending JFrame

问题描述

如何在没有类（扩展 JFrame ）的情况下绘制对象？我找到了 getGraphics 方法，但它没有绘制对象。

How can I draw an object without a class (which extends JFrame)? I found getGraphics method but it doesnt draw the object.

import javax.swing.*;
import java.awt.*;
public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(600, 400);

        JPanel panel = new JPanel();
        frame.add(panel);

        Graphics g = panel.getGraphics();
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);
    }
}

推荐答案

如果如果要更改组件的绘制方式（添加矩形），则需要在该组件中重新定义 paintComponent（）。在您的代码中，您使用 getGraphics（）。

If you want to change the way your component is being drawn (you are adding rectangles), you need to redefine paintComponent() in that component. In your code, you are using getGraphics().

您不应该调用组件上的getGraphics（）。你做的任何绘画（返回 Graphics ）都是临时的，并且在下次Swing确定需要重新绘制一个组件时会丢失。

You shouldn't call getGraphics() on a component. Any painting you do (to the Graphics returned) will be temporary and will be lost the next time Swing determines a component needs to be repainted.

相反，你应该覆盖 paintComponent（Graphics）方法（ JComponent 或 JPanel ），并使用收到的 Graphics 对象作为参数在此方法中进行绘制。

Instead, you should override the paintComponent(Graphics) method (of the JComponent or JPanel), and do the painting in this method, using the Graphics object received as argument.

检查此链接以进一步阅读。

以下是您的代码，已更正。

Below is your code, corrected.

import javax.swing.*;
import java.awt.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(600, 400);

        JPanel panel = new JPanel() {
            @Override
            public void paintComponent(Graphics g) {
                super.paintComponent(g);
                g.setColor(Color.BLUE);
                g.fillRect(0, 0, 100, 100);
            }
        };
        frame.add(panel);

        // Graphics g = panel.getGraphics();
        // g.setColor(Color.BLUE);
        // g.fillRect(0, 0, 100, 100);

        frame.validate(); // because you added panel after setVisible was called
        frame.repaint(); // because you added panel after setVisible was called
    }
}

另一个版本，完全相同，但可能更清楚：

Another version, does the exact same thing, but may be clearer to you:

import javax.swing.*;
import java.awt.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(600, 400);

        JPanel panel = new MyRectangleJPanel(); // changed this line
        frame.add(panel);

        // Graphics g = panel.getGraphics();
        // g.setColor(Color.BLUE);
        // g.fillRect(0, 0, 100, 100);

        frame.validate(); // because you added panel after setVisible was called
        frame.repaint(); // because you added panel after setVisible was called
    }
}

/* A JPanel that overrides the paintComponent() method and draws a rectangle */
class MyRectangleJPanel extends JPanel {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);
    }
}

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