Java Odd pack()行为 [英] Java Odd pack() Behavior

问题描述

我的主要问题是设置JFrame时的以下代码:

My main issue is with the following piece of code when setting up a JFrame:

public Frame(){
  JPanel panel = new JPanel();
  add(panel);
  panel.setPreferredSize(new Dimension(200, 200));

  pack(); // This is the relevant code
  setResizable(false); // This is the relevant code

  setVisible(true);
}

使用以下打印语句，我们会收到错误的面板尺寸:

With the following print statements we receive faulty dimensions for panel:

System.out.println("Frame: " + this.getInsets());
System.out.println("Frame: " + this.getSize());
System.out.println("Panel: " + panel.getInsets());
System.out.println("Panel: " + panel.getSize());

Output:
Frame: java.awt.Insets[top=25,left=3,bottom=3,right=3]
Frame: java.awt.Dimension[width=216,height=238]
Panel: java.awt.Insets[top=0,left=0,bottom=0,right=0]
Panel: java.awt.Dimension[width=210,height=210]

我发现将相关代码修改为以下内容可以解决此问题:

I have discovered that modifying the relevant code to the following fixes the issue:

public Frame(){
  JPanel panel = new JPanel();
  add(panel);
  panel.setPreferredSize(new Dimension(200, 200));

  setResizable(false); // Relevant code rearranged
  pack(); // Relevant code rearranged

  setVisible(true);
}

这将为我们的面板生成正确的尺寸(使用与之前相同的打印语句)

This produces the correct dimensions for our panel (using same print statements as earlier):

Frame: java.awt.Insets[top=25,left=3,bottom=3,right=3]
Frame: java.awt.Dimension[width=206,height=228]
Panel: java.awt.Insets[top=0,left=0,bottom=0,right=0]
Panel: java.awt.Dimension[width=200,height=200]

我浏览了一些文档，但找不到这10个像素来自何处. 有人知道为什么会这样吗?

I have looked through some documentation but could not find out where these 10 pixels come from. Does anybody know why exactly this is the case?

推荐答案

JFrame源自Frame，在setResizable(...)的Frame源代码中，您将看到以下注释:

JFrame derives from Frame, and in the Frame source code for the setResizable(...) you'll see this comment:

    // On some platforms, changing the resizable state affects
    // the insets of the Frame. If we could, we'd call invalidate()
    // from the peer, but we need to guarantee that we're not holding
    // the Frame lock when we call invalidate().

因此，在调用setResizable(false)之后调用pack() 是有意义的.

Because of this, it makes sense to call pack() after calling setResizable(false).

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