Calculator that converts infix to postfix, then solves.

PostfixCalculator.java

import java.util.Stack;
import java.lang.*;


public class PostfixCalculator {

    private static boolean isOperator(char c) {
    return c == '+' || c == '-' || c == '*' || c == '/' || c == '^'
            || c == '(' || c == ')';
    }

    private static boolean isLowerPrecedence(char op1, char op2) {

        switch (op1) {
            case '+':
            case '-':
                return !(op2 == '+' || op2 == '-');

            case '*':
            case '/':
                return op2 == '^' || op2 == '(';

            case '^':
                return op2 == '(';

            case '(':
                return true;

            default:
                return false;
        }
    }

    public static String convertToPostfix(String infix) {

        Stack<Character> stack = new Stack<Character>();
        StringBuffer postfix = new StringBuffer(infix.length());
        char c;

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

            if (!isOperator(c)) {
                postfix.append(c + " ");
            }

            else {
                if (c == ')') {

                    while (!stack.isEmpty() && stack.peek() != '(') {
                        postfix.append(stack.pop() + " ");
                    }
                    if (!stack.isEmpty()) {
                        stack.pop();
                    }
                }

                else {
                    if (!stack.isEmpty() && !isLowerPrecedence(c, stack.peek())) {
                        stack.push(c);
                    }
                    else {
                        while (!stack.isEmpty() && isLowerPrecedence(c, stack.peek())) {
                            Character pop = stack.pop();
                            if (c != '(') {
                                postfix.append(pop + " ");
                            } else {
                              c = pop;
                            }
                        }
                        stack.push(c);
                    }

                }
            }
        }

        while (!stack.isEmpty()) {
          postfix.append(stack.pop() + " ");
        }
        return postfix.toString();
    }

    public static void main(String[] args) {
    // 3*2-(4+5)+1

        long startTime = 0L;
        long endTime = 0L;

        startTime = System.nanoTime();
        System.out.println(calculator(convertToPostfix(args[0]).split(" ")));
        endTime = System.nanoTime();
        System.out.println("Operation took " + (endTime - startTime) + " nanoseconds");
    }

    public static double calculator(String[] strArr) {
        Stack<Double> operands = new Stack<Double>();

        for(String str : strArr) {
            if (str.trim().equals("")) {
                continue;
            }

            switch (str) {
                case "+":
                case "-":
                case "*":
                case "/":
                    double right = operands.pop();
                    double left = operands.pop();
                    double value = 0;
                    switch(str) {
                        case "+":
                            value = left + right;
                            break;
                        case "-":
                            value = left - right;
                            break;
                        case "*":
                            value = left * right;
                            break;
                        case "/":
                            value = left / right;
                            break;
                        default:
                            break;
                    }
                    operands.push(value);
                    break;
                default:
                    operands.push(Double.parseDouble(str));
                    break;  
            }
        }
        return operands.pop();
    }
}