Learning Java Program

import java.awt.*;
import java.awt.event.*;
import java.applet.*;
 /*
<applet code="Cal" width=300 height=300>
</applet>
*/
public class Cal extends Applet
implements ActionListener
{
        String msg=" ";
        int v1,v2,result;
        TextField t1;
        Button b[]=new Button[10];
        Button add,sub,mul,div,clear,mod,EQ;
        char OP;
        public void init()
        {
                Color k=new Color(120,89,90);
                setBackground(k);
                t1=new TextField(10);
                GridLayout gl=new GridLayout(4,5);
                setLayout(gl);
                for(int i=0;i<10;i++)
                {
                        b[i]=new Button(""+i);
                }
                add=new Button("add");
                sub=new Button("sub");
                mul=new Button("mul");
                div=new Button("div");
                mod=new Button("mod");
                clear=new Button("clear");
                EQ=new Button("EQ");
                t1.addActionListener(this);
                add(t1);
                for(int i=0;i<10;i++)
                {
                        add(b[i]);
                }
                add(add);
                add(sub);
                add(mul);
                add(div);
                add(mod);
                add(clear);
                add(EQ);
                for(int i=0;i<10;i++)
                {
                        b[i].addActionListener(this);
                }
                add.addActionListener(this);
                sub.addActionListener(this);
                mul.addActionListener(this);
                div.addActionListener(this);
                mod.addActionListener(this);
                clear.addActionListener(this);
                EQ.addActionListener(this);
        }
        public void actionPerformed(ActionEvent ae)
        {
                String str=ae.getActionCommand();
                char ch=str.charAt(0);
                if ( Character.isDigit(ch))
                t1.setText(t1.getText()+str);
                else
                if(str.equals("add"))
                {
                        v1=Integer.parseInt(t1.getText());
                        OP='+';
                        t1.setText("");
                }
                else if(str.equals("sub"))
                {
                        v1=Integer.parseInt(t1.getText());
                        OP='-';
                        t1.setText("");
                }
                else if(str.equals("mul"))
                {
                        v1=Integer.parseInt(t1.getText());
                        OP='*';
                        t1.setText("");
                }
                else if(str.equals("div"))
                {
                        v1=Integer.parseInt(t1.getText());
                        OP='/';
                        t1.setText("");
                }
                else if(str.equals("mod"))
                {
                        v1=Integer.parseInt(t1.getText());
                        OP='%';
                        t1.setText("");
                }
                if(str.equals("EQ"))
                {
                        v2=Integer.parseInt(t1.getText());
                        if(OP=='+')
                                result=v1+v2;
                        else if(OP=='-')
                                result=v1-v2;
                        else if(OP=='*')
                                result=v1*v2;
                        else if(OP=='/')
                                result=v1/v2;
                        else if(OP=='%')
                                result=v1%v2;
                        t1.setText(""+result);
                }      
                if(str.equals("clear"))
                {
                        t1.setText("");
                }
        }
}