Case Study: Evaluating Expressions

package demo;
import java.util.Stack;

public class EvaluateExpression {

    public static void main(String[] args) {
        // Check number of arguments passed
        if(args.length != 1) {
            System.out.println("Usage: java EvaluateExpression \"expression\"");
            System.exit(1);
        }

        try {
            System.out.println(evaluateExpression(args[0]));
        }
        catch(Exception ex) {
            System.out.println("Wrong expression: " + args[0]);
        }
    }

    /** Evaluate an expression */
    public static int evaluateExpression(String expression) {
        // Create operandStack to store operands
        Stack<Integer> operandStack = new Stack<>();

        // Create operatorStack to store operators
        Stack<Character> operatorStack = new Stack<>();

        // Insert blanks around (, ), +, -, /, and *
        expression = insertBlanks(expression);

        // Extract operands and operators
        String[] tokens = expression.split(" ");

        // Phase 1: Scan tokens
        for(String token: tokens) {
            if(token.length() == 0) // Blank space
                continue; // Back to the while loop to extract the next token
            else if(token.charAt(0) == '+' || token.charAt(0) == '-') {
                // Process all +, -, *, / in the top of the operator stack
                while(!operatorStack.isEmpty() && (operatorStack.peek() == '+' || operatorStack.peek() == '-' || operatorStack.peek() == '*' || operatorStack.peek() == '/')) {
                    processAnOperator(operandStack, operatorStack);
                }

                // Push the + or - operator into the operator stack
                operatorStack.push(token.charAt(0));
            }
            else if(token.charAt(0) == '*' || token.charAt(0) == '/') {
                // Process all *, / in the top of the operator stack
                while(!operatorStack.isEmpty() && (operatorStack.peek() == '*' || operatorStack.peek() == '/')) {
                    processAnOperator(operandStack, operatorStack);
                }

                // Push the * or / operator into the operator stack
                operatorStack.push(token.charAt(0));
            }
            else if(token.trim().charAt(0) == '(') {
                operatorStack.push('('); // Push '(' to stack
            }
            else if(token.trim().charAt(0) == ')') {
                // Process all the operators in the stack until seeing '('
                while(operatorStack.peek() != '(') {
                    processAnOperator(operandStack, operatorStack);
                }

                operatorStack.pop(); // Pop the '(' symbol from the stack
            }
            else {
                // Push an operand to the stack
                operandStack.push(Integer.valueOf(token));
            }
        }

        // Phase 2: Process all the remaining operators in the stack
        while(!operatorStack.isEmpty()) {
            processAnOperator(operandStack, operatorStack);
        }

        // Return the result
        return operandStack.pop();
    }

    /** Process one operator: Take an operator from operatorStack and apply it on the operands in the operandStack */
    public static void processAnOperator(Stack<Integer> operandStack, Stack<Character> operatorStack) {
        char op = operatorStack.pop();
        int op1 = operandStack.pop();
        int op2 = operandStack.pop();
        if(op == '+')
            operandStack.push(op2 + op1);
        else if(op == '-')
            operandStack.push(op2 - op1);
        else if(op == '*')
            operandStack.push(op2 * op1);
        else if(op == '/')
            operandStack.push(op2 / op1);
    }

    public static String insertBlanks(String s) {
        String result = "";

        for(int i = 0; i < s.length(); i++) {
            if(s.charAt(i) == '(' || s.charAt(i) == ')' || s.charAt(i) == '+' || s.charAt(i) == '-' || s.charAt(i) == '*' || s.charAt(i) == '/')
                result += " " + s.charAt(i) + " ";
            else
                result += s.charAt(i);
        }

        return result;
    }

}