Question

Please use Java language in an easy way with comments! Thanks!

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 (For this problem, you must write a static "evaluate" method).

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

The java program for your query is given below:

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import java.util.Stack;
import java.util.Scanner;
public class Main
{
   // Method to evaluate value of a postfix expression
   static int evaluate(String exp){
         
       Stack<Integer> stack = new Stack<>(); //create a stack class object with Integer data type in it   
      
         
       for(int i=0 ; i<exp.length() ; i++){ // Scan all characters one by one
           char c = exp.charAt(i); //taking character at position i
          
          
          
           if(Character.isDigit(c)) // If the scanned character is an operand (number here),
           stack.push(c - '0'); // push it to the stack.
          
          
           // If the scanned character is an operator, pop two
           // elements from stack apply the operator
           else{
           int val1 = stack.pop(); //pop first element
               int val2 = stack.pop(); //pop second element
              
               switch(c){ //checking whether the operator is +,-,*,/   
                   case '+':
                   stack.push(val2+val1); //when + , add both elements add to stack
                   break;
                  
                   case '-':
                   stack.push(val2- val1); //when - , subtract both elements add to stack
                   break;
                  
                   case '/':   
                   stack.push(val2/val1); //when / , divide both elements add to stack
                   break;
                  
                   case '*':
                   stack.push(val2*val1); //when * , multiply both elements add to stack
                   break;
           }
           }
       }
       return stack.pop();   //pop from the stack and return
   }
  
   // Driver program to test above functions
   public static void main(String[] args)
   {
       System.out.print("Postfix Expression:"); //prompt the user for Postfix Expression
       Scanner input = new Scanner(System.in); //defining scanner class object
       String expression = input.nextLine(); //taking input of expression with scanner object
       System.out.println("Result: "+evaluate(expression)); //Print Result
   }
}

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Providing screenshot of the code for further reference:

Output of the program :