Java calculator

import java.awt.*;
import java.applet.Applet;
 
public class Applet extends Applet
{
            TextField  tf1 = new TextField(12);
            Label c=new Label("      ");
            Button plus  = new Button("Plus");
            Button ravno  = new Button("=");
            String a1,a2;
            int  n1,n2;
 
        public void init()
       {
          add(tf1);add(plus);add(ravno);add(c);
       }
 
       public boolean action(Event  evt, Object obj)
            {
                if (evt.target == plus)
                {
                    a1= tf1.getText();
                    n1=Integer.parseInt(a1);
                    tf1.setText("");
                    return true;
                }
                if (evt.target == ravno)
                {
                    a2= tf1.getText();
                    n2=Integer.parseInt(a2);n1+=n2;
                    tf1.setText(""+n1);
                    return true;
                }
 
                if (evt.target == tf1)
                {
                    tf1.setText("");
                    return true;
                }
 
                    return false;
            }
    }