为什么此代码的JTextArea占据整个JFrame？

Question

I expect part of my frame contains the JTextArea but it occupies entirely. I cannot trace the error here.

import java.awt.*;    
import javax.swing.*;

public class EchoServer 
{
   public static void main(String args[])
   {
       CalcFrame c = new CalcFrame();
       CalcTextArea a = new CalcTextArea();
   } 
}

class CalcTextArea 
{
    JTextArea historyDisplayer  = new JTextArea("",50,20);
    CalcTextArea()
    {  
          //historyDisplayer.setVisible(true);
          historyDisplayer.insert("Hello World", 0);              
          Color bg = new Color(23,34,56);              
          historyDisplayer.setBackground(bg);               
          historyDisplayer.setBackground(bg);
    }       
}

class CalcFrame extends CalcTextArea
{
    JFrame frame = new JFrame(); 
    CalcFrame()
    {
        frame.setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
        frame.setTitle("CALCULATOR");
        frame.setVisible(true);
        frame.add(historyDisplayer);

    }
    private static int  DEFAULT_WIDTH = 299,DEFAULT_HEIGHT = 190; 
}

Answer 1

JFrame by default uses BorderLayout . When you just add something onto a BorderLayout component like JFrame , it would add to the very center of the BorderLayout (if you did not specify where to add the component), and it takes up the entire JFrame .

You should use the correct layout to adjust them.

Answer 2

You can try using Absolute layout. It's on the Layouts Pallet.

Or enable it with :

frame = new JFrame();
... //your code here

// to set absolute layout.
frame.getContentPane().setLayout(null);

This way, you can freely place the control anywhere you like.