I have a problem with my code... it's a simple code and the error I think in ' = ' button or sum button
import javax.swing.*;


import java.awt.event.*;
import java.awt.*;


public class Frame extends JFrame implements ActionListener{
JButton b0,b1,b2,b3,before,b5,b6,b7,b8,b9,C,X,O,sum;
JLabel Ans;
JTextField Prob;
public Frame(){
this.setTitle("Calcolater");
this.setBounds(250, 200, 500, 450);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE );
this.setLayout(null);
this.setVisible(true);

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");
C=new JButton("C");
X=new JButton("X");
O=new JButton("/");
sum=new JButton("=");
Ans=new JLabel();
Prob=new JTextField();

this.add(b0).setBounds(10, 10, 45, 45);
this.add(b1).setBounds(10+50, 10, 45, 45);
this.add(b2).setBounds(10+50+50, 10, 45, 45);
this.add(b3).setBounds(10+50+50+50, 10, 45, 45);
this.add(b4).setBounds(10, 10+50, 45, 45);
this.add(b5).setBounds(10+50, 10+50, 45, 45);
this.add(b6).setBounds(10+50+50, 10+50, 45, 45);
this.add(b7).setBounds(10+50+50+50, 10+50, 45, 45);
this.add(b8).setBounds(10, 10+50+50, 45, 45);
this.add(b9).setBounds(10+50, 10+50+50, 45, 45);
this.add(C).setBounds(10+50+50+25, 10+50+50, 45, 45);
this.add(X).setBounds(10+25, 10+50+50+50, 45, 45);
this.add(O).setBounds(10+50+25, 10+50+50+50, 45, 45);
this.add(sum).setBounds(10+50, 10+50+50+50+50, 45, 45);
this.add(Ans).setBounds(300, 10, 100, 10);
this.add(Prob).setBounds(300, 10+50, 100, 25);

b0.addActionListener(this);
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);
C.addActionListener(this);
X.addActionListener(this);
O.addActionListener(this);
sum.addActionListener(this);

}
@Override
public void actionPerformed(ActionEvent a) {

if(a.getSource()==b0){
Prob.setText(Prob.getText()+"0");
}
else if(a.getSource()==b1){
Prob.setText(Prob.getText()+"1");
}
else if(a.getSource()==b2){
Prob.setText(Prob.getText()+"2");
}
else if(a.getSource()==b3){
Prob.setText(Prob.getText()+"3");
}
else if(a.getSource()==b4){
Prob.setText(Prob.getText()+"4");
}
else if(a.getSource()==b5){
Prob.setText(Prob.getText()+"5");
}
else if(a.getSource()==b6){
Prob.setText(Prob.getText()+"6");
}
else if(a.getSource()==b7){
Prob.setText(Prob.getText()+"7");
}
else if(a.getSource()==b8){
Prob.setText(Prob.getText()+"8");
}
else if(a.getSource()==b9){
Prob.setText(Prob.getText()+"9");
}
else if(a.getSource()==C){
Prob.setText("");
Ans.setText("");
}
else if(a.getSource()==X){
Prob.setText(Prob.getText()+" X ");
}
else if(a.getSource()==O){
Prob.setText(Prob.getText()+" / ");
}
else if(a.getSource()==sum){
String s=Prob.getText();int Sum=1,v=0;
for(int m=0;m<s.length();m++){
if(s.substring(m, m+2)==" X "){
Sum*=Integer.valueOf(s.substring(v, m-1));
v+=m+2;
}
else if(s.substring(m, m+2)==" / "){
Sum/=Integer.valueOf(s.substring(v, m-1));
v+=m+2;
}
}
Ans.setText(String.valueOf(Sum));
}
}

public static void main(String [] args) {
new Frame();
}

}