Java Implementation to evaluate a postfix expression using a stack

Main.java

import java.util.Stack;

public class Main {

    public static void main(String[] args) {
        // the driver code

        String expression1 = "22*3+";
        //should print answer as 7
        evaluatePostfix(expression1);

        String expression2 = "32*2+";
        //should print answer as 8
        evaluatePostfix(expression2);

        String expression3 = "3*2+";
        //should print Not a valid expression
        evaluatePostfix(expression3);


    }

    //simple function to evaluate when given operands and the operator
    //can be extended to more operators
    static int evaluate(int operand1, int operand2, char operator) {

        switch (operator) {
            case '+':
                System.out.println(operand1 + " + " + operand2);
                return operand1 + operand2;
            case '-':
                System.out.println(operand1 + " - " + operand2);
                return operand1 - operand2;
            case '/':
                System.out.println(operand1 + " / " + operand2);
                return operand1 / operand2;
            case '*':
                System.out.println(operand1 + " * " + operand2);
                return operand1 * operand2;
            default:
                return 0;
        }
    }

    static void evaluatePostfix(String expression) {

        System.out.println("Evaluating expression " + expression);
        Stack<Character> stack = new Stack<Character>();
        char[] array = expression.toCharArray();

        int i = 0;
        while (i < array.length) {
            //if its a digit put it in the stack
            if (Character.isDigit(array[i])) {
                stack.push(array[i]);
            } else {
                //always stack has to be of size of 2 , if not the expression is not valid
                if (stack.size() != 2) {
                    System.out.println("postfix expression is not valid");
                    return;
                }

                //get the operands by popping the stack
                int operand1 = Integer.parseInt(String.valueOf(stack.pop()));
                int operand2 = Integer.parseInt(String.valueOf(stack.pop()));

                //evaluate the operands using the operator
                int result = evaluate(operand1, operand2, array[i]);

                //push the result back to the stack
                stack.push(Character.forDigit(result, 10));
            }
            i++;
        }
        //print the result 
        System.out.println("result is " + stack.pop() + '\n');
    }
}

another way for the code but work for only one digite

// Java program to evaluate value of a postfix
// expression having multiple digit operands
import java.util.Stack;
 
class Test1
{
    // Method to evaluate value of a postfix expression
    static int evaluatePostfix(String exp)
    {
        //create a stack
        Stack<Integer> stack = new Stack<>();
         
        // Scan all characters one by one
        for(int i = 0; i < exp.length(); i++)
        {
            char c = exp.charAt(i);
             
            if(c == ' ')
            continue;
             
            // If the scanned character is an operand
            // (number here),extract the number
            // Push it to the stack.
            else if(Character.isDigit(c))
            {
                int n = 0;
                 
                //extract the characters and store it in num
                while(Character.isDigit(c))
                {
                    n = n*10 + (int)(c-'0');
                    i++;
                    c = exp.charAt(i);
                }
                
 
                //push the number in stack
                stack.push(n);L
            }
             
            // 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();
    }
     
    // Driver program to test above functions
    public static void main(String[] args)
    {
        String exp = "100 200 + 2 / 5 * 7 +";
        System.out.println(evaluatePostfix(exp));
    }
}