Here is an example of converting infix expression to its postfix. The code accepts the expression from the user and display its postfix expression.

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<String> stack = new Stack<String>();

    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);


  } 
}