java - 在默认 JFrame 面板上使用 paintComponent()

Question

我想向 JFrame 添加渐变背景。在 JFrame 中，我没有创建 JPanel，而是使用了在创建 JFrame 时自动添加的 JPanel。我在 JPanel 实例上找到的所有在线调用 paintComponent() 的示例，但如前所述，在我的例子中我没有专门实例化一个。我知道 paintComponent() 需要在组件上调用，因此不能在 JFrame 上调用，那么我是否需要创建并添加 JPanel 才能实现此目的？我过去曾成功地这样做过，但我正在努力使我的代码尽可能精简，所以如果不需要，我不想创建第二个 JPanel。这是我的代码:

public class Main extends JFrame {

private static final long serialVersionUID = 1L;

JPanel pane;

public Main() {

    int WIDTH = 700, HEIGHT = 400;
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBackground(Color.GRAY);
    setSize(WIDTH, HEIGHT);
    setLocationRelativeTo(null);

    pane = (JPanel) getContentPane();
    pane.setOpaque(false);
    pane.setLayout(new BorderLayout());
    pane.setBorder(new EmptyBorder(20, 20, 20, 20));

    // SOME OTHER STUFF
    }
    protected void paintComponent(Graphics grphcs) {
    Graphics2D g2d = (Graphics2D) grphcs;
    g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    GradientPaint gp = new GradientPaint(0, 0, new Color(255, 195, 225), 0, getHeight(), new Color(139, 207, 236));
    g2d.setPaint(gp);
    paintComponent(grphcs);
    }
}

我将不胜感激。

Answer 1

so do I need to create and add a JPanel in order to achieve this?

是的。

but I'm trying to keep my code as streamlines as possible, so I don't want to create a second JPanel if I don't need to.

这里不会浪费，因为简化上述代码的方法是去掉 extends JFrame。没有必要这样做，并且有很多理由不这样做。让类扩展 JFrame 是在逼迫自己创建和显示 JFrame，而这通常需要更大的灵 active 。事实上，我敢打赌，我创建的和看到的大多数 Swing GUI 代码都不扩展 JFrame，而且实际上您很少想这样做这。更常见的是，您的 GUI 类将用于创建 JPanel，然后可以将其放入 JFrames 或 JDialogs，或 JTabbedPanes，或通过 CardLayouts 在任何需要的地方进行交换。这将大大增加您的 GUI 编码的灵 active 。

例如，

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.*;
import javax.swing.border.EmptyBorder;

public class MainPanel extends JPanel {
    private static final long serialVersionUID = 1L;
    private static final int PREF_W = 700;
    private static final int PREF_H = 400;
    private static final Color GRAD_COLOR_1 = new Color(255, 195, 225);
    private static final Color GRAD_COLOR_2 = new Color(139, 207, 236);

    public MainPanel() {
        setBackground(Color.GRAY);
        setSize(WIDTH, HEIGHT);
        setLayout(new BorderLayout());
        setBorder(new EmptyBorder(20, 20, 20, 20));

        // SOME OTHER STUFF
    }

    // to set the preferred size of the JPanel/GUI
    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }

    @Override
    protected void paintComponent(Graphics grphcs) {
        super.paintComponent(grphcs); // ** don't forget this.
        Graphics2D g2d = (Graphics2D) grphcs;
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        GradientPaint gp = new GradientPaint(0, 0, GRAD_COLOR_1, 0, getHeight(), GRAD_COLOR_2);
        g2d.setPaint(gp);
        g2d.fillRect(0, 0, getWidth(), getHeight());
    }

    private static void createAndShowGui() {
        MainPanel mainPanel = new MainPanel();

        // create the JFrame when/if needed
        JFrame frame = new JFrame("Main");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            createAndShowGui();
        });
    }
}

---

But doesn't it still create unneeded JPanels?

那是在担心错误的事情。创建了一个 JPanel，即 contentPane，这不会对您的程序或您的资源使用产生真正的不利影响。

My actual class extends JPanel, so when I instantiate that there is one created, plus the default panel which was created when we created the JFrame. Isn't that kind of the same as what I had before, I mean I could have just created a second JPanel and added it to my main class (JFrame) and called paintComponent() on it.

如上所述——不要小题大做，也不要过早优化。如果您担心程序中的资源滥用，请分析它，然后解决具有实际影响的问题。

What are the advantages of extending JPanel instead of JFrame

我已经提到了很多 - 我上面的回答特别令人困惑吗？