Re: doubt

I have made some change in the program.I put the layout before adding
components,still it doesnot work.The program is given below.

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

class test extends JFrame implements ActionListener
{
        public JTextField textBox;
        public JButton one;
        public JButton two;
        public JButton three;
        public JButton four;
        public JButton five;
        public JButton six;
        public JButton seven;
        public JButton eight;
        public JButton nine;
        public JButton zero;
        public JButton plus;
        public JButton minus;
        public JButton star;
        public JButton div;
        public JButton equal;
    test()
    {
        setSize(450,350);
        setLayout(new FlowLayout());
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        textBox = new JTextField(15);
        one=new JButton("1");
        two=new JButton("2");
        three=new JButton("3");
        four=new JButton("4");
        five=new JButton("5");
        six=new JButton("6");
        seven=new JButton("7");
        eight=new JButton("8");
        nine=new JButton("9");
        zero=new JButton("0");
        plus = new JButton("+");
        minus = new JButton("-");
        star = new JButton("*");
        div = new JButton("/");
        equal=new JButton("=");

        add(textBox);
        add(one);
        add(two);
        add(three);
        add(four);
        add(five);
        add(six);
        add(seven);
        add(eight);
        add(nine);
        add(zero);
        add(plus);
        add(minus);
        add(star);
        add(div);
        add(equal);

        one.addActionListener(this);
        two.addActionListener(this);
        three.addActionListener(this);
        four.addActionListener(this);
        five.addActionListener(this);
        six.addActionListener(this);
        seven.addActionListener(this);
        eight.addActionListener(this);
        nine.addActionListener(this);
        zero.addActionListener(this);
        plus.addActionListener(this);
        minus.addActionListener(this);
        star.addActionListener(this);
        div.addActionListener(this);
        equal.addActionListener(this);

    }
    public void actionPerformed(ActionEvent e)
            {
                String str1=null,str2=null,str3=null,str4=null;
                int x,y,z=0;
                while(e.getSource()!=equal)
                {

                        if(e.getSource()==one)
                            str1=str1+"1";
                        else if(e.getSource()==two)
                            str1=str1+"2";
                        else if(e.getSource()==three)
                            str1=str1+"3";
                        else if (e.getSource()==four)
                            str1=str1+"4";
                        else if(e.getSource()==five)
                            str1=str1+"5";
                        else if(e.getSource()==six)
                            str1=str1+"6";
                        else if(e.getSource()==seven)
                            str1=str1+"7";
                        else if(e.getSource()==eight)
                            str1=str1+"8";
                        else if(e.getSource()==nine)
                            str1=str1+"9";
                        else if(e.getSource()==zero)
                            str1=str1+"0";
                        else if(e.getSource()==plus)
                        {
                            str3="+";
                            str2=str1;
                                    str1=null;
                        }
                        else if(e.getSource()==minus)
                        {
                                str3="-";
                                str2=str1;
                                str1=null;
                        }
                        else if(e.getSource()==star)
                        {
                                str3="*";
                                str2=str1;
                                str1=null;
                        }
                        else if(e.getSource()==div)
                        {
                                str3="/";
                                str2=str1;
                                str1=null;
                        }
                        textBox.setText(str1);

              }
              x=Integer.parseInt(str2);
              y=Integer.parseInt(str1);
              if(str3=="+")
               z=x+y;
              else if(str3=="-")
               z=x-y;
              else if(str3=="*")
               z=x*y;
              else if(str3=="/")
               z=x/y;
             textBox.setText(Integer.toString(z));
            }

}
public class Calculator
{
    public static void main(String args[])
    {
        test obj = new test();

    }
}

        Please help me to correct this program.