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 个基本类(class)(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？我当然可以将渐变背景添加到主面板并在其上添加透明面板吗？

我的 4 门类(class):

public class GuiApp {
public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new MainFrame();
            }
        });
    }
}
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
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);
    }
}
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
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);
    }
}
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
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 不会知道当任何一个由于某种原因更新时它应该绘制出现在该组件下方的组件

相反，您应该使用 setOpaque 并传递它 false

内面板

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);
    }
}

主面板

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);
            }
        });
    }
}

当组合在一起时会生成......