infix2postfix.java

import java.util.*;
public class InfixtoPostfix
{
 
public static int precedence(char ch)
{
 if(ch=='+' || ch=='-')
 return 1;
 
 else if(ch=='*' || ch=='/')
 return 2;                       // * and / divide have higher precedence.
 
  return 0;
}
 
public static String convertToPostfix(String exp)
{
 Stack<Character> operators = new Stack<>();
 Stack<String> postFix = new Stack<>();
 
 for(int i=0;i<exp.length();i++)
 {
  char ch=exp.charAt(i);         // current character.
 
  if(ch=='(')
   operators.push(ch);
 
  else if((ch>='a' && ch<='z') || (ch>='A' && ch<='Z'))
   postFix.push(ch+"");
 
  else if(ch==')')
  {
   while(operators.peek()!= '(')
   {
    // STEP 5 of Algorithm.    
    char op = operators.pop();
 
    String first = postFix.pop();          // get the two operands.
    String second = postFix.pop();
 
    String new_postFix = second+first+op;  // add them in reverse order.
 
    postFix.push(new_postFix);     // push the result to postFix stack again.
   }
 
   operators.pop();     // pop '(' from stack.   
  }
 
      // We do the same thing if we get an operator as the current character.
 
  else if(ch=='+' || ch=='-' || ch== '*' || ch== '/')
  {
  // check precedence of each operator with top of the stack and process them.
   while(operators.size()>0 && operators.peek()!='(' && precedence(ch) <= precedence(operators.peek()))
   {
   char op = operators.pop();
 
   String first = postFix.pop();
   String second = postFix.pop();
 
   String new_postFix = second+first+op;
 
   postFix.push(new_postFix);
   }
 
  operators.push(ch);
  }
 }
 
 // If the operator stack still has some elements in it process them too Repeat Step 5.
 while(operators.size()>0)
 {
  char op = operators.pop();
 
  String first = postFix.pop();
  String second = postFix.pop();
 
  String new_postFix = second+first+op;
 
  postFix.push(new_postFix);
  }
 
  return postFix.pop();         // return resultant Postfix.
}
 
public static void main(String args[])
{
  // We pass Uppercase Infix
  String infixExpression = "A*(B-C)/D+E";
  System.out.println("The Infix Expression is: "+infixExpression);
  String result = convertToPostfix(infixExpression);
  System.out.println("The Postfix of the given Infix Expression is: "+result);
 
  System.out.println();
 
  //We also check for Lowercase Infix
  infixExpression = "a*(b-c+d)/e";
  System.out.println("The Infix Expression is: "+infixExpression);
  result = convertToPostfix(infixExpression);
  System.out.println("The Postfix of the given Infix Expression is: "+result);
 
}
}