java - setBackground 语句不起作用？

Question

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

public class Snowman extends JApplet {
//---------------------------------------------
// Draws a snowman.
//---------------------------------------------
public void paint (Graphics page)
{
    final int MID = 150;
    final int TOP = 50;

    setBackground (Color.cyan);

    page.setColor(Color.blue);
    page.fillRect(0, 175, 300, 50); // ground

    page.setColor (Color.yellow);
    page.fillOval (-40, -40, 80, 80); // sun

    page.setColor (Color.white);
    page.fillOval (MID-20, TOP, 40, 40); // head
    page.fillOval (MID-35, TOP+35, 70, 50); // upper torso
    page.fillOval (MID-50, TOP+80, 100, 60); // lower torso

    page.setColor (Color.black);
    page.fillOval(MID-10, TOP+10, 5, 5);
    page.fillOval(MID+5, TOP+10, 5, 5);

    page.drawArc(MID-10, TOP+20, 20, 10, 190, 160); // smile

    page.drawLine (MID-25, TOP+60, MID-50, TOP+40); // left arm
    page.drawLine (MID+25, TOP+60, MID+55, TOP+60); // right arm

    page.drawLine (MID-20, TOP+5, MID+20, TOP+5); // brim of hat
    page.fillRect(MID-15, TOP-20, 30, 25); // top of hat
}
}

这是全部代码。 setBackground 是在我声明两个最终变量之后声明的，在此先感谢，我从一本书“Java 软件解决方案”中获得了这段代码，我一遍又一遍地查看它，但没有运气:/提前感谢:)

Answer 1

//<applet code='Snowman' width=300 height=200></applet>
import javax.swing.*;
import java.awt.*;

public class Snowman extends JApplet {
//---------------------------------------------
// Draws a snowman.
//---------------------------------------------
    public void init() {
        add(new SnowmanPanel());
        validate();
    }
}

class SnowmanPanel extends JPanel {

    final int MID = 150;
    final int TOP = 50;

    SnowmanPanel() {
        setBackground (Color.cyan);
    }

    public void paintComponent(Graphics page)
    {
        super.paintComponent(page);

        page.setColor(Color.blue);
        page.fillRect(0, 175, 300, 50); // ground

        page.setColor (Color.yellow);
        page.fillOval (-40, -40, 80, 80); // sun

        page.setColor (Color.white);
        page.fillOval (MID-20, TOP, 40, 40); // head
        page.fillOval (MID-35, TOP+35, 70, 50); // upper torso
        page.fillOval (MID-50, TOP+80, 100, 60); // lower torso

        page.setColor (Color.black);
        page.fillOval(MID-10, TOP+10, 5, 5);
        page.fillOval(MID+5, TOP+10, 5, 5);

        page.drawArc(MID-10, TOP+20, 20, 10, 190, 160); // smile

        page.drawLine (MID-25, TOP+60, MID-50, TOP+40); // left arm
        page.drawLine (MID+25, TOP+60, MID+55, TOP+60); // right arm

        page.drawLine (MID-20, TOP+5, MID+20, TOP+5); // brim of hat
        page.fillRect(MID-15, TOP-20, 30, 25); // top of hat
    }
}

一般建议。

• 不要在顶级 Swing 组件中绘制。而是将自定义绘画移动到 JPanel 或 JComponent 中并在那里绘画。对于后者的自定义绘画，覆盖 paintComponent(Graphics)
• 在做自定义绘画的时候，记得调用super.paintComponent(Graphics)
• 在构造函数(或 init())中而不是在 paint 方法中设置颜色。

其他建议

• 对于静态图像，您还可以将其绘制到 BufferedImage 并将图像放在 JLabel 中的 ImageIcon 中。哪个更简单。
• 如果这本书急于创建小程序，请将其扔掉。小程序比标准应用程序难得多，新手不应尝试。