当我们为添加到JScrollPane的JPanel使用setPreferredSize时，为什么它向JScrollPane显示滚动条 [英] Why it shows scrollbar to JScrollPane when we use setPreferredSize for the JPanel which is added to JScrollPane

问题描述

请参见以下代码块

import java.awt.Color;
import java.awt.Dimension;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
public class test extends JFrame {
    public test(){

    this.setBounds(0,0,300,700);

    JPanel pnltemp= new JPanel();

    //pnltemp.setBounds(0,0,400,1000);
    pnltemp.setPreferredSize(new Dimension(400,1000));

    JScrollPane scrtemp= new JScrollPane();


    scrtemp.getViewport().add(pnltemp);
    this.getContentPane().add(scrtemp);

    this.getContentPane().add(scrtemp);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setVisible(true);

}

public static void main(String args[]){
    new test();

}
}

1. 当我们通过注释"pnltemp.setBounds(0,0,400,1000);"运行上述代码时并将其替换为"pnltemp.setPreferredSize(new Dimension(400,1000));"该窗口显示水平和垂直滚动条. 但是当我们通过注释"pnltemp.setPreferredSize(new Dimension(400,1000));"运行同一程序时并替换为"pnltemp.setBounds(0,0,400,1000)"时，该窗口将不显示水平和垂直滚动条.

1. When we run the above code by commenting "pnltemp.setBounds(0,0,400,1000);" and replacing it with "pnltemp.setPreferredSize(new Dimension(400,1000));" the window shows horizontal and vertical scrollbars. but when we run the same programm by commenting "pnltemp.setPreferredSize(new Dimension(400,1000));" and replacing it wiht "pnltemp.setBounds(0,0,400,1000)" the window does not shows horizontal and vertical scrollbars.

为什么同一程序通过更改setBounds和setPreferredSize方法的行为有所不同；因为这两种方法在行为上看起来都是一样的.

why same program behaves differently by changing the setBounds and setPreferredSize method; as these two methods are looks same in behaviour.

还是这样的事情，当我们使用JScrollPane获取滚动条时，必须使用setPreferredSize();.我们要在JScrollPane中添加的组件的方法.

Or is it something like this that when we are using the JScrollPane to get the Scrollbar we must use the setPreferredSize(); method for the componant that we are going to add in JScrollPane.

1. 我的第二个问题是，当我们将pnltemp添加到scrtemp时，即，将JPanel直接添加到JScrollPane时，当我们说时，它不会给出错误的意思.

scrtemp.add(pnltemp);

scrtemp.add(pnltemp);

它没有给出任何错误，但是也没有在scrtemp中显示pnltemp和滚动条. 但是当我们键入 scrtemp.getViewPort.add(pnltemp);

it does not give any error but it also does not show the pnltemp and scrollbar in scrtemp. but when we type scrtemp.getViewPort.add(pnltemp);

它没有给出任何错误，但它还在scrtemp中显示了pnltemp和滚动条.

it does not give any error but it also shows the pnltemp and scrollbar in scrtemp.

我通过将背景颜色分配给JPanel和JScrollPane进行了检查.

I checked this by assigning the background color to JPanel and JScrollPane.

有人可以解释这个吗?

谢谢！

推荐答案

来自 JScrollPane JavaDocs.

From the JScrollPane JavaDocs.

默认情况下，JScrollPane使用ScrollPaneLayout处理其子组件的布局. ScrollPaneLayout通过以下两种方法之一确定制作视口视图的大小:

By default JScrollPane uses ScrollPaneLayout to handle the layout of its child Components. ScrollPaneLayout determines the size to make the viewport view in one of two ways:

1. 如果视图实现Scrollable，则使用getPreferredScrollableViewportSize，getScrollableTracksViewportWidth和getScrollableTracksViewportHeight的组合，否则
使用
2. getPreferredSize.

1. If the view implements Scrollable a combination of getPreferredScrollableViewportSize, getScrollableTracksViewportWidth and getScrollableTracksViewportHeight is used, otherwise
2. getPreferredSize is used.

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