Given an expression like add( sub(3, 2), add(0, 1) ), evaluate their results.

ExpressionEvaluation.java

import java.util.Stack;

public class ExpressionEvaluation {
    public static void main(String[] args) {
        String expression = "add( 1, 2)";
        int result = evaluateExpression(expression);
        System.out.println(result);
        expression = "add(sub(2, 1), 3)";
        System.out.println(evaluateExpression(expression));
        expression = "add(3, sub(22, 20))";
        System.out.println(evaluateExpression(expression));
        expression = "sub( add(18, 20), sub(22, 20))";
        System.out.println(evaluateExpression(expression));
        expression = "sub( add(add(10,8), 20), sub(22, 20))";
        System.out.println(evaluateExpression(expression));
        expression = "add( abc, 2)";
        System.out.println(evaluateExpression(expression));
        expression = "add(-10, sub(22, 20))";
        System.out.println(evaluateExpression(expression));
        expression = "add(-10, sub(2+2, 20))";
        System.out.println(evaluateExpression(expression));
    }

    private static int evaluateExpression(String expression) {
        char[] expChars = expression.toCharArray();
        String token = "";
        Stack<String> stack = new Stack<>();
        for (int i = 0; i < expChars.length; i++) {
            char ch = expChars[i];
            if (Character.isAlphabetic(ch) || Character.isDigit(ch) || ch == '-' || ch == '+') {
                token += ch;
            } else {
                if (Character.isSpaceChar(ch) || ch == '(' || ch == ')' || ch == ',') {
                    if (!token.isEmpty()) {
                        stack.push(token);
                    }
                    token = "";
                }
                if (ch == ')') {
                    //Get operation and operands
                    int op1 = 0, op2 = 0;
                    String operand = "";
                    try{
                        operand = stack.pop();
                        op2 = Integer.parseInt(operand);
                        operand = stack.pop();
                        op1 = Integer.parseInt(operand);
                    } catch (NumberFormatException nfe) {
                        System.out.println("Bad input " + operand + " at " + expression.indexOf(operand));
                        return -1;
                    }
                    String operation = stack.pop();
                    stack.push(Integer.toString(getResult(operation, op1, op2)));
                }
            }
        }
        return Integer.parseInt(stack.pop());
    }

    private static int getResult(String operation, int op1, int op2) {
        int result = 0;
        if (operation.equals("add")) {
            result = op1 + op2;
        } else if (operation.equals("sub")) {
            result = op1 - op2;
        }
        return result;
    }
}