Java AWT：设置画布背景改变整个框架的背景

Question

为什么改变java.awt.canvas的背景颜色就改变了整个框架的背景？

public class Gui extends Frame {

    public Gui() {

        setSize(800, 600);
        setLocation(0, 0);

        Canvas cd=new Canvas();
        cd.setSize(500, 300);
        cd.setBackground(Color.BLUE);
        add(cd);


        addWindowListener(new WindowAdapter() {

            @Override
            public void windowClosing(WindowEvent arg0) {
                System.exit(0);
            }

        });
    }

    @Override
    public void paint(Graphics g) {
        paintComponents(g);
    }

    public static void main(String[] args) {

        Gui g=new Gui();
        g.setVisible(true);

    }

}

这可能是问题所在吗（我在Ubuntu上运行这个）？
还是我在这里遗漏了什么？

Answer 1

Frame的默认布局是BorderLayout（尽管许多使用基于AWT的Frame类的示例代码是在默认布局为FlowLayout时编写的）。如果没有布局约束，添加到BorderLayout的组件最终将处于Center约束中，该约束将扩展组件，使其符合可用的高度和宽度。

相反，我们可以使用GridBagLayout。当我们将单个组件添加到GridBagLayout而不受任何约束时，它将居中并尊重大小。例如。（请参阅代码中的进一步注释。）

import java.awt.*;
import java.awt.event.*;

public class Gui extends Frame {

    public Gui() {
        setSize(400, 200);
        setLocationByPlatform(true);
        setLayout(new GridBagLayout());

        Canvas cd=new Canvas();
        // TODO: Override rather than set & make it preferred rather than actual
        cd.setSize(300, 100); 
        cd.setBackground(Color.BLUE);
        add(cd);

        addWindowListener(new WindowAdapter() {

            @Override
            public void windowClosing(WindowEvent arg0) {
                System.exit(0);
            }
        });
    }

    @Override
    public void paint(Graphics g) {
        //paintComponents(g); // wrong method!
        super.paint(g); // right method, but does nothing different to original!
    }

    public static void main(String[] args) {
        // TODO: AWT/Swing based GUIs should be started on the EDT
        Gui g=new Gui();
        g.setVisible(true);
    }
}