java – 使用getGraphics()绘制对象而不扩展JFrame

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.

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);
    }
}

解决方法:

如果要更改绘制组件的方式(添加矩形),则需要在该组件中重新定义paintComponent().在您的代码中,您使用的是getGraphics().

您不应该在组件上调用getGraphics().你做的任何绘画(返回的图形)将是临时的,并且在下次Swing确定需要重新绘制组件时将丢失.

相反,您应该覆盖paintComponent(Graphics)方法(JComponent或JPanel),并使用作为参数接收的Graphics对象在此方法中进行绘制.

检查this link以进一步阅读.

以下是您的代码,已更正.

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：

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);
    }
}

标签：graphics2d,paintcomponent,java,swing
来源： https://codeday.me/bug/20190915/1806479.html