Question

In Java language  

Write a static method called "evaluate" that takes a string as a parameter. The string will contain a postfix expression, consisting only of integer operands and the arithmetic operators +, -, *, and / (representing addition, subtraction, multiplication, and division respectively). All operations should be performed as integer operations. You may assume that the input string contains a properly-formed postfix expression. The method should return the integer that the expression evaluates to. The method MUST use a stack to perform the evaluation. Specifically, you should use the Stack class from the java.util package.

Also, in the same file, write a program that prompts the user to enter a postfix expression. Your program should evaluate the expression and output the result. Your program should call the "evaluate" method described above. Here are some examples:

     Postfix expression: 1 2 +     Result: 3     Postfix expression: 1 2 + 3 4 - *     Result: -3     Postfix expression: 3 4 5 + + 1 -     Result: 11

Answer 1

import java.util.Scanner;
import java.util.Stack;

public class PostfixExpressionSolver {
    // Method to evaluate value of a postfix expression 
    static int evaluatePostfix(String exp) {
        //create a stack 
        Stack stack = new Stack();
        String tokens[] = exp.split("\\s+");
        // Scan all characters one by one 
        for (int i = 0; i < tokens.length; i++) {

            if (tokens[i].trim().length() == 0) continue;

            char c = tokens[i].trim().charAt(0);

            // If the scanned character is an operand (number here), 
            // push it to the stack. 
            if (Character.isDigit(c))
                stack.push(c - '0');

                //  If the scanned character is an operator, pop two
                // elements from stack apply the operator
            else {
                int val1 = stack.pop();
                int val2 = stack.pop();

                switch (c) {
                    case '+':
                        stack.push(val2 + val1);
                        break;

                    case '-':
                        stack.push(val2 - val1);
                        break;

                    case '/':
                        stack.push(val2 / val1);
                        break;

                    case '*':
                        stack.push(val2 * val1);
                        break;
                }
            }
        }
        return stack.pop();
    }

    public static void main(String[] args) {
        String expression;
        Scanner scanner = new Scanner(System.in);
        System.out.print("Postfix expression: ");
        expression = scanner.nextLine();
        System.out.println("Result: " + evaluatePostfix(expression));
    }
}

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