given a postfix mathematical expression, evaluate it

EvalatePostfix.java

package test;

import java.util.Stack;

public class EvaluatePostfix {
	
	public static int solution(String postfix){
		
		Stack<Integer> stack = new Stack<Integer>();
		int sum = 0;
		int val1 = 0;
		int val2 = 0;
		
		for(int i =0; i<postfix.length(); i++){
			if(Character.isDigit(postfix.charAt(i))){
				stack.push(postfix.charAt(i) -'0');
				//System.out.println(stack.peek());
			}
			else{
				//System.out.println(stack.peek());
				val1 = stack.pop();
				val2 = stack.pop();
		
				switch(postfix.charAt(i)){
				
				case '+':
					stack.push(val1 + val2);
					sum += val2;
					break;
				case '-':
					stack.push(val1 - val2) ;
					sum -= val2;
					break;
				case '/':
					stack.push(val1 / val2) ;
					sum /= val2;
					break;
				case '*':
					stack.push(val1 * val2) ;
					sum *= val2;
					break;
				}
				//System.out.println(sum);
			}
			//System.out.println(stack.pop());
		}
		
		return stack.pop();
		
		
	}
	
	public static void main(String[]args){
		String test = "231*+9-";
		//solution(test);
		System.out.println(solution(test));
	}

}

// below we are able to solve with more than 1 didigts, eg "10 20 30 * +"

package test;

import java.util.Stack;

public class EvaluatePostfix {
	
	public static int solution(String postfix){
		
		Stack<Integer> stack = new Stack<Integer>();
		int sum = 0;
		int val1 = 0;
		int val2 = 0;
		String[] str = postfix.split(" ");
		
		
		for(int i =0; i<str.length; i++){
			if(Character.isDigit(str[i].charAt(0))){
				System.out.println(str[i]);
				stack.push(Integer.parseInt(str[i]));
				//System.out.println(stack.peek());
			}
			else{
				//System.out.println(stack.peek());
				val1 = stack.pop();
				val2 = stack.pop();
		
				switch(str[i].charAt(0)){
				
				case '+':
					stack.push(val1 + val2);
					sum += val2;
					break;
				case '-':
					stack.push(val1 - val2) ;
					sum -= val2;
					break;
				case '/':
					stack.push(val1 / val2) ;
					sum /= val2;
					break;
				case '*':
					stack.push(val1 * val2) ;
					sum *= val2;
					break;
				}
				//System.out.println(sum);
			}
			//System.out.println(stack.pop());
		}
		
		return stack.pop();
		
		
	}
	
	public static void main(String[]args){
		
		String test = "10 20 30 * +";
		//solution(test);
		
		System.out.println(solution(test));
//		String man = "200";
//		int ch =Integer.parseInt(man);
		//System.out.println(Char);
	}

}