calculator runtimedesign using 1 textfield (s5)

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Arrays;
import javax.swing.*;
public class singletextfieldcalculator extends JFrame implements ActionListener{

    JButton zero, one, two, three, four, five, six, seven, eight, nine, backspace;

    JTextField field;

    JButton plus, minus, multiply, divide, equal;

    String operation=null;

    int a, b, result;

    public singletextfieldcalculator(){
    super("Calculator");

    Container c = getContentPane();
    JPanel p = new JPanel();

    field = new JTextField(15);
    p.add(field);

    backspace = new JButton("backspace");
    backspace.addActionListener(this);
    p.add(backspace);

    one = new JButton("1");
    one.addActionListener(this);
    p.add(one);

    two = new JButton("2");
    two.addActionListener(this);
    p.add(two);

    three = new JButton("3");
    three.addActionListener(this);
    p.add(three);

    four = new JButton("4");
    four.addActionListener(this);
    p.add(four);

    five = new JButton("5");
    five.addActionListener(this);
    p.add(five);

    six = new JButton("6");
    six.addActionListener(this);
    p.add(six);

    seven = new JButton("7");
    seven.addActionListener(this);
    p.add(seven);

    eight = new JButton("8");
    eight.addActionListener(this);
    p.add(eight);

    nine = new JButton("9");
    nine.addActionListener(this);
    p.add(nine);

    zero = new JButton("0");
    zero.addActionListener(this);
    p.add(zero);

    plus = new JButton("+");
    plus.addActionListener(this);
    p.add(plus);

    minus = new JButton("-");
    minus.addActionListener(this);
    p.add(minus);

    multiply = new JButton("x");
    multiply.addActionListener(this);
    p.add(multiply);

    divide = new JButton("/");
   divide.addActionListener(this);
    p.add(divide);

    equal = new JButton("=");
    equal.addActionListener(this);
    p.add(equal);

    c.add(p);
    setSize(200,250);
    setVisible(true);

    }

    public static void main(String args[]){
        new singletextfieldcalculator();
    }
    public void actionPerformed(ActionEvent e) {

        if(e.getSource()==backspace){
            String text = field.getText();
             String text2 = text.substring(0, text.length()-1);
             field.setText(""+text2);
            }

        else if(e.getSource() == zero)
          field.setText(field.getText()+0);

        else if(e.getSource() == one)
          field.setText(field.getText()+1);

        else if (e.getSource() == two)
          field.setText(field.getText()+2);

        else if (e.getSource() == three)
          field.setText(field.getText()+3);

        else if (e.getSource() == four)
          field.setText(field.getText()+4);

    else if (e.getSource() == five)
          field.setText(field.getText()+5);

     else if (e.getSource() == six)
          field.setText(field.getText()+6);

     else if (e.getSource() == seven)
          field.setText(field.getText()+7);

     else if (e.getSource() == eight)
          field.setText(field.getText()+8);

    else if (e.getSource() == nine)
          field.setText(field.getText()+9);

      if(e.getSource() == plus){
       a = Integer.parseInt(field.getText());
      field.setText("");
      operation = "plus";
      }

 else if (e.getSource() == minus) {
      a = Integer.parseInt(field.getText());
      field.setText("");
      operation = "minus";
      }

    else if (e.getSource() == multiply) {
       a = Integer.parseInt(field.getText());
      field.setText("");
      operation = "multiply";
      }

   else if (e.getSource() == divide) {
       a = Integer.parseInt(field.getText());
      field.setText("");
      operation = "divide";
      }

   else if (e.getSource() == equal) {
      b = Integer.parseInt(field.getText());
      field.setText("");
     if(operation.equals("plus")) {
      result = a+b;
      field.setText(""+result);
      }

     else if(operation.equals("minus")) {
         result = a-b;
          field.setText(""+result);
      }

     else if(operation.equals("multiply")) {
      result = a*b;
      field.setText(""+result);
      }

     else if(operation.equals("divide")){
      result = a/b;
      field.setText(""+result);
      }
      operation=null;

      }
    }
  }