Converts infix expression into Postfix form

/* Week 5 :a) Write a Java program that:(b)Converts infix expression into Postfix form*/ 

import java.util.Scanner;

import java.util.Stack;

public class InfixToPostfix {

private String infix;

private String postfix = "";

public void convertString(String a){

String str = "";

infix = a;

Stack stack = new Stack();

for(int i = 0; i < infix.length(); i++){

str = infix.substring(i,i+1);

if(str.matches("[a-zA-Z]|\\d"))

postfix += str;

else if (isOperator(str)){

if (stack.isEmpty()){

stack.push(str);

}

else{

String stackTop = stack.peek();

while (getPrecedence(stackTop,str).equals(stackTop)&& !(stack.isEmpty())){

postfix += stack.pop();

if (!(stack.isEmpty()))

stackTop = stack.peek();

}

stack.push(str);

}

}

}

while(!(stack.isEmpty()))

postfix += stack.pop();

System.out.println("Postfix of the expression is: " + postfix);

}

private boolean isOperator(String ch){

String operators = "*/%+-";

if (operators.indexOf(ch) != -1)

return true;

else

return false;

}

private String getPrecedence(String op1, String op2){

String multiplicativeOps = "*/%";

String additiveOps = "+-";

if ((multiplicativeOps.indexOf(op1) != -1) && (additiveOps.indexOf(op2) != -1))

return op1;

else if ((multiplicativeOps.indexOf(op2) != -1) && (additiveOps.indexOf(op1) != -1))

return op2;

else if((multiplicativeOps.indexOf(op1) != -1) && (multiplicativeOps.indexOf(op2) != -1))

return op1;

else

return op1;

}

public static void main(String[] args) {

System.out.println("Enter an expression in the Infix form:");

Scanner scanner = new Scanner(System.in);

String expression = scanner.nextLine();

new InfixToPostfix().convertString(expression);

}

}