Infix to postfix Conversion

Algorithm:

for each expression token:
  if the token is a number:
    add it to output
  else if the token is a left paren:
    push it
  else if the token is a right paren:
    pop and add to output all tokens on stack up to left paren
    pop the left paren, but don't add to output
  else if the token is an operator:
    if stack is empty or the top is a left paren:
       push it
    else if the stack top is a lower precedence operator:
       push it
    else
       pop and add to output all operators of higher or equal precedence 
       push it

when there are no more tokens,
  pop and add to output all remaining operators on stack

Example:

7 - 3 + 5 - 2

token   stack     output
-----   -----     ------
        []
  7               7
  -     [-]
  3               7 3
  +     [+]       7 3 -
  5               7 3 - 5
  -     [-]       7 3 - 5 +
  2               7 3 - 5 + 2
 END    []        7 3 - 5 + 2 -

Code:

import java.util.*;

public class InfixToPostfix {
	
	// Translate's the expression to postfix
	public static String translate(String expression) {

		String output = "";
		char character = ' ';
		Stack<Character> stack = new Stack<Character>();

		for (int x = 0; x < expression.length(); x++) {
			character = expression.charAt(x);

			if (isOperator(character)) {
				while (!stack.empty()
						&& precedence(stack.peek()) >= precedence(character))
					output += stack.pop() + " ";
				stack.push(character);
			} else if (character == '(') {
				stack.push(character);
			} else if (character == ')') {
				while (!stack.peek().equals('('))
					output += stack.pop() + " ";
				stack.pop();
			} else {
					output += character;
			}
		}

		while (!stack.empty()) {
			output += stack.pop() + " ";
		}

		return output;
	}

	// Check priority on characters
	public static int precedence(char operator) {
		if (operator == '+' || operator == '-')
			return 1;
		else if (operator == '*' || operator == '/')
			return 2;
		else
			return 0;
	}

	public static boolean isOperator(char element) {
		if (element == '*' || element == '-' || element == '/'
				|| element == '+')
			return true;
		else
			return false;
	}
	
	public static void main(String args[])
	{
		System.out.println("Postfix conversion"+InfixToPostfix.translate("((2 * 3) - 5) / 4"));
		
	}
}