java 图形显示有关问题（setVisible和repaint有关问题）

package frameDemo;

import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.GridLayout;

import javax.swing.*;

public class calculator  {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		calculator c=new calculator();

	}
	
	private JFrame _calculateJFrame;
	private JButton _mutilJButten,_DivJButton,_AddJButton,_subJButton;
	private JTextField jTextFileFirst,jTextFileSecond,jTextFileLast;
	private JPanel jPanelFirst,jPanelSecond,jPanelThird,jPanelFour;
	
	public calculator(){
		
		//_calculateJFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
		_calculateJFrame=new JFrame("Calculator");
		_calculateJFrame.setLayout(new GridLayout(4,1));
		_calculateJFrame.setBounds(40, 30, 400, 380);
		_calculateJFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
		

                //注意这行代码，如果放到最后不会显示在画布上添加的东西
		_calculateJFrame.setVisible(true);
		
		//添加四个画布
		jPanelFirst = new JPanel();
		jPanelSecond=new JPanel();
		jPanelThird=new JPanel();
		jPanelFour=new JPanel();
		
		_calculateJFrame.add(jPanelFirst);
		_calculateJFrame.add(jPanelSecond);
		_calculateJFrame.add(jPanelThird);
		_calculateJFrame.add(jPanelFour);
		
		//_calculateJFrame.setVisible(true);
		
		
		jPanelFirst.setLayout(null);
		jPanelFirst.setBackground(Color.yellow);
		jTextFileFirst=new JTextField();
		
		jPanelFirst.add(jTextFileFirst);
		
		jTextFileFirst.setSize( jPanelFirst.getWidth(),jPanelFirst.getHeight());
		//jTextFileFirst.setSize(100, 100);
		//jTextFileFirst.setBackground(Color.black);
		
		jPanelSecond.setLayout(null);
		jPanelSecond.setBackground(Color.red);
		jTextFileSecond=new JTextField();
		jTextFileSecond.setSize( jPanelSecond.getWidth(),jPanelSecond.getHeight());
		jPanelSecond.add(jTextFileSecond);
		
		
		jPanelThird.setLayout(null);
		jPanelThird.setBackground(Color.blue);
		
		//添加+ - * /
		_AddJButton=new JButton("+");
		jPanelThird.add(_AddJButton);
		//_AddJButton.setBounds(jPanelThird.getLocation().x,jPanelThird.getLocation().y,jPanelThird.getWidth()/4,jPanelThird.getHeight());