Shunting-yard algorithm

ShuntingYardAlgo.java

import java.util.HashMap;
import java.util.Map;
import java.util.Stack;

/**
 * @author humayun
 *
 * https://en.wikipedia.org/wiki/Shunting-yard_algorithm
 *
 */
public class ShuntingYardAlgorithm {
    public static void main(String[] args) {
        String infix = "3+4*2/(1-5)";
        String output = new ShuntingYardAlgorithm().shuntingYard(infix);
        System.out.println(output);

    }

    private enum Operator {
        ADD(1), SUBTRACT(2), MULTIPLY(3), DIVIDE(4);
        final int precedence;
        Operator(int p) {
            precedence = p;
        }
    }

    private Map<String, Operator> operatorMap = new HashMap<String, Operator>() {{
        put("+", Operator.ADD);
        put("-", Operator.SUBTRACT);
        put("*", Operator.MULTIPLY);
        put("/", Operator.DIVIDE);
    }};

    private boolean isHigherPrec(String op, String sub) {
        return (operatorMap.containsKey(sub) &&
                operatorMap.get(sub).precedence >= operatorMap.get(op).precedence);
    }

    public String shuntingYard(String infix) {
        StringBuilder output = new StringBuilder();
        Stack<String> stack = new Stack<String>();

        for (String token : infix.split("")) {
            //operator
            if (operatorMap.containsKey(token)) {
                while ( ! stack.isEmpty() && isHigherPrec(token, stack.peek())) {
                    output.append(stack.pop()).append(' ');
                }
                stack.push(token);
            }
            //left parenthesis
            else if (token.equals("(")) {
                stack.push(token);
            }
            //right parenthesis
            else if (token.equals(")")) {
                while ( ! stack.peek().equals("(")) {
                    output.append(stack.pop()).append(' ');
                }
                stack.pop();
            }
            //digit
            else {
                output.append(token).append(' ');
            }
        }

        while ( ! stack.isEmpty()) {
            output.append(stack.pop()).append(' ');
        }

        return output.toString();
    }
}