使用null LayoutManager时,为什么我的JScrollPane与JTextArea不可见？

Question

我正在尝试在JScrollPane中显示一个JTextArea,但是当我运行我的(简化)程序时,我只得到一个空框架:

import java.awt.Container;
import java.awt.Dimension;    
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;

public class ScrollPaneTest extends JFrame {
    private Container myCP; 
    private JTextArea resultsTA;
    private JScrollPane scrollPane;

    public ScrollPaneTest() {
        setSize(500, 500);
        setLocation(100, 100);
        myCP = this.getContentPane();
        myCP.setLayout(null);

        resultsTA = new JTextArea("Blah blah");
        scrollPane = new JScrollPane(resultsTA,
                JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
                JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scrollPane.setPreferredSize(new Dimension(200, 100));
        scrollPane.setLocation(100, 300);
        myCP.add(scrollPane);

        setVisible(true);
        addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
    }

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

我使用null LayoutManager与我教的教科书一致.

Answer 1

这将有效:

public class ScrollPaneTest extends JFrame {
    private Container myCP; 
    private JTextArea resultsTA;
    private JScrollPane scrollPane;

    public ScrollPaneTest() {
        setSize(500, 500);
        setLocation(100, 100);
        myCP = this.getContentPane();
        myCP.setLayout(null);

        resultsTA = new JTextArea("Blah blah");
        resultsTA.setBounds(10, 10, 150, 30);

        scrollPane = new JScrollPane(resultsTA,JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scrollPane.setPreferredSize(new Dimension(200, 100));
        scrollPane.setBounds(0, 0, 500, 500);

        myCP.add(scrollPane);
        setVisible(true);
        addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
    }

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

如果使用null布局,则必须指定边界.

编辑

setBounds()方法考虑了方法的任务setLocation().

例如, setBounds(x,y,w,h);

第一个将设置该组件相对于其容器的x/y位置.第二个2(w/h)将设置该组件的大小.

换一种说法:-

1. setBounds(int x,int y,int witdh,int height) - 设置组件的大小和位置
2. setLocation(int x,int y) - 设置组件的位置