Java Jpanels 和渐变背景

Question

我是一个早期的初学者，正在尝试创建一个 GUI。我有 4 个基本课程（见下文）：

• GuiApp.java - 使用 main 方法，调用 MainFrame 类
• MainFrame.Java - 扩展 JFrame - 添加 MainPpanel
• MainPanel.Java - 扩展 JPanel - 添加面板
• InnerPanel.Java - 扩展 JPanel - 显示 JLabel

我找到了以下代码来添加渐变背景：

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            JFrame frame = new JFrame();
            TestPanel panel = new TestPanel();
            frame.add(panel);
            frame.setSize(200, 200);
            frame.setLocationRelativeTo(null);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setVisible(true);
        }
    });
}

这在添加到 InnerPanel.java 时有效，但在添加到 MainPanel.Java 时无效

没有错误，我只是得到默认的灰色，注意：InnerPanel.Java 设置为透明。

那么如何解决这个问题呢？当然我可以将渐变背景添加到主面板并在其上添加透明面板？

我的 4 节课：

public class GuiApp {
public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new MainFrame();
            }
        });
    }
}
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class MainFrame extends JFrame {
    private MainPanel mainPanel;
    public MainFrame() {
        super("GuiApp");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setSize(300, 500);
        // setBackground(new Color(95, 158, 160));
        setLayout(new BorderLayout());

        mainPanel = new MainPanel();
        add(mainPanel, BorderLayout.CENTER);
    }
}
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public class MainPanel extends JPanel {
    private InnerPanel panel;
    public MainPanel() {
        setLayout(new BorderLayout());
        panel = new InnerPanel();
        add(panel, BorderLayout.CENTER);
    }
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
        int w = getWidth();
        int h = getHeight();
        Color color1 = Color.RED;
        Color color2 = Color.GREEN;
        GradientPaint gp = new GradientPaint(0, 0, color1, 0, h, color2);
        g2d.setPaint(gp);
        g2d.fillRect(0, 0, w, h);
    }
}
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public class InnerPanel extends JPanel {
    public InnerPanel() {
        setBackground(new Color(0, 0, 0, 0));
        JLabel label = new JLabel("testing");
        add(label);
   }

}

Answer 1

所以，你的主要问题在这里：

setBackground(new Color(0, 0, 0, 0));

Swing 只知道如何绘制完全透明或完全不透明的组件，这是通过 opaque 属性控制的。通过提供基于 alpha 的颜色，Swing 将不会知道它应该绘制出现在该组件下方的组件，无论出于何种原因更新时

相反，您应该使用setOpaque 并传递它false

InnerPanel

import java.awt.Color;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class InnerPanel extends JPanel {

    public InnerPanel() {
        setOpaque(false);
        // To prove the point
        setBackground(Color.RED);
        JLabel label = new JLabel("testing");
        add(label);
    }
}

MainPanel

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.JPanel;

public class MainPanel extends JPanel {

    private InnerPanel panel;

    public MainPanel() {
        setLayout(new BorderLayout());
        panel = new InnerPanel();
        add(panel, BorderLayout.CENTER);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
        int w = getWidth();
        int h = getHeight();
        Color color1 = Color.RED;
        Color color2 = Color.GREEN;
        GradientPaint gp = new GradientPaint(0, 0, color1, 0, h, color2);
        g2d.setPaint(gp);
        g2d.fillRect(0, 0, w, h);
    }
}

GuiApp

import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class GuiApp {

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new MainPanel());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

当放在一起会产生...