为什么JFrame的大小超过了ContentPane的大小，即使它具有定义的大小？

Question

我注意到秋千上的容器之间有奇怪的行为。

为了举例说明该测试，我创建了一个JFrame和一个JPanel，并将面板设置为
contentPane。我将首选和最大的JPanel大小定义为400,300。所有
这些都可以在下面的示例中看到：

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class ExcessiveSpacePanelTest {
    JFrame frame;
    JPanel panel;

    public void initGUI(){

        frame =  new JFrame();

        panel = new JPanel();
        panel.setBorder(BorderFactory.createLineBorder(Color.RED, 1));
        panel.setPreferredSize(new Dimension(400, 300));
        panel.setMaximumSize(new Dimension(400, 300));


        frame.setContentPane(panel);
        frame.pack();

        System.out.println("panel size: [" + panel.getSize().width + "," + panel.getSize().height +"]");
        System.out.println("frame size: [" + frame.getSize().width + "," + frame.getSize().height+"]");

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() ->{
            new ExcessiveSpacePanelTest().initGUI();
        });
    }
}

The result is:

令我惊讶的是，终端的输出为：

面板尺寸：[400,300]
框架尺寸：[416,338]

我不明白为什么框架会增加这个额外的空间，甚至
在组件中什么也没有迫使框架调整大小。

我尝试通过在panel.setBorder (BorderFactory.createEmptyBorder(0, 0, 0, 0));前面添加行来在面板上设置零边缘，pack();但
结果仍然相同。

这样做的问题是java给我一个错误的信息，因为面板
正在缩放框架的大小，从上面的结果可以看出
，两者的大小不一样。

两种措施的报告是否正确？为什么会发生这种情况？

Answer 1

您可能会发现比较
不同平台上类似程序的结果会有所帮助。尤其是，

• The red Border is inside the panel; your usage conforms to the API recommendation, “we recommend that you put the component in a JPanel and set the border on the JPanel.”

• The frame width matches the panel width on Mac OS X because the heavyweight peer used for the top-level container has no vertical border on Mac OS X; the Windows peer has its own border.

• The frame height includes the drag bar and frame border: 38 pixels on Widows, 22 on Mac OS X.

• Don’t use setPreferredSize() when you really mean to override getPreferredSize().

• As an aside, the frame’s add() method forwards to the content pane.

  panel size: [400,300]
  frame size: [400,322]

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class ExcessiveSpacePanelTest {
    JFrame frame;
    JPanel panel;

    public void initGUI(){
        frame = new JFrame();
        panel = new JPanel() {
            @Override
            public Dimenshtml" target="_blank">ion getPreferredSize() {
                return new Dimension(400, 300);
            }
        };
        panel.setBorder(BorderFactory.createLineBorder(Color.RED, 1));
        frame.add(panel);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        frame.setVisible(true);

        System.out.println("panel size: [" + panel.getSize().width + "," + panel.getSize().height +"]");
        System.out.println("frame size: [" + frame.getSize().width + "," + frame.getSize().height+"]");
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() ->{
            new ExcessiveSpacePanelTest().initGUI();
        });
    }
}