Java Calc Problem

package Calculator;

import java.awt.Color;

import javax.swing.*;

import java.awt.event.*;


public class CalcPanel extends JPanel implements ActionListener {
	String num1="";
	String num2="";
	boolean usingFirst=true;

	JTextField display;
	JButton b1;
	JButton b2;
	JButton b3;
	JButton b4;
	JButton b5;
	JButton b6;
	JButton b7;
	JButton b8;
	JButton b9;
	JButton bequals;
	JButton bsum;
	JButton bdifference;
	JButton bproduct;
	JButton bquotient;
	JButton bdecimal;
	JButton b0;
	JButton bclear;

	public CalcPanel()
	{
		this.setBackground(Color.gray);
		setLayout(null);
		display=new JTextField();
		b0=new JButton("0");
		b1=new JButton("1");
		b2=new JButton("2");
		b3=new JButton("3");
		b4=new JButton("4");
		b5=new JButton("5");
		b6=new JButton("6");
		b7=new JButton("7");
		b8=new JButton("8");
		b9=new JButton("9");
		bclear=new JButton("c");
		bequals=new JButton("=");
		bsum=new JButton("+");
		bdifference=new JButton("-");
		bproduct=new JButton("*");
		bquotient=new JButton("/");
		bdecimal=new JButton(".");
		display.setBounds(0,0,200,350);
		b0.setBounds(0,275,100,50);
		bdecimal.setBounds(100,275,50,50);
		bequals.setBounds(150,225,50,100);
		b1.setBounds(0,225,50,50);
		b2.setBounds(50,225,50,50);
		b3.setBounds(100,225,50,50);
		b4.setBounds(0,175,50,50);
		b5.setBounds(50,175,50,50);
		b6.setBounds(100,175,50,50);
		bsum.setBounds(150,125,50,100);
		b7.setBounds(0,125,50,50);
		b8.setBounds(50,125,50,50);
		b9.setBounds(100,125,50,50);
		bproduct.setBounds(50,75,50,50);
		bdifference.setBounds(150,75,50,50);
		bquotient.setBounds(100,75,50,50);
		bclear.setBounds(0,75,50,50);
		display.setBounds(0,0,200,75);


		add(b0);
		add(bdecimal);
		add(bequals);
		add(b1);
		add(b2);
		add(b3);
		add(b4);
		add(b5);
		add(b6);
		add(b7);
		add(b8);
		add(b9);
		add(bsum);
		add(bdifference);
		add(bproduct);
		add(bquotient);
		add(bclear);
		add(display);

		b1.addActionListener(this);
		b2.addActionListener(this);
		b3.addActionListener(this);
		b4.addActionListener(this);
		b5.addActionListener(this);
		b6.addActionListener(this);
		b7.addActionListener(this);
		b8.addActionListener(this);
		b9.addActionListener(this);
		b0.addActionListener(this);
		bequals.addActionListener(this);
		bsum.addActionListener(this);
		bclear.addActionListener(this);
		bdecimal.addActionListener(this);
		bdifference.addActionListener(this);
		bproduct.addActionListener(this);
		bquotient.addActionListener(this);


	}

	public void actionPerformed(ActionEvent e){
		String s=e.getActionCommand();
		if(s.equals("1")||s.equals("2")||s.equals("3")||s.equals("4")||s.equals("5")||s.equals("6")||s.equals("7")||s.equals("8")||s.equals("9")||s.equals("0")||s.equals("."))
		{
			if(usingFirst)
			{num1=num1+s;
			display.setText(num1);
			}
			else
			{ num2=num2+s;
			display.setText(num2);
			}

		}
		if(s.equals("+"))
		{
			usingFirst=false;
		}
		if(s.equals("-"))
		{
			usingFirst=false;
		}
		if(s.equals("*"))
		{
			usingFirst=false;
		}
		if(s.equals("/"))
		{
			usingFirst=false;
		}
		if(s.equals("="))
		{
			Double total=Double.parseDouble(num1)+Double.parseDouble(num2);
			display.setText(""+total);
			usingFirst=false;
			num1=Double.toString(total);
			num2="";

		}
		if(s.equals("c"))
		{
			display.setText("");
			usingFirst=true;
			num1="";
			num2="";
		}
	}

	}