Postfixeval | Stack Java

package lat04;
import java.util.Stack;

public class PostfixEval {
    private String postfixExpression;
    private Stack<Integer> operandStack;

    public PostfixEval() {
        postfixExpression = new String();
        operandStack = new Stack<>();
    }
    public void setPostfixExp(String postfixExp) {
        postfixExpression = postfixExp;
    }
    public int evaluate(){
        for (int i = 0; i < postfixExpression.length(); i++){
            char ch = postfixExpression.charAt(i); // get the current char

            if (Character.isDigit(ch))
                operandStack.push(ch - '0'); // put integer on stack
            else if (isOperator(ch)) {
                int right = getOperand(); // pop right operand
                int left = getOperand();  // pop left operand
                int result = compute(left, right, ch);
                operandStack.push(result); // push result
            }
            else if (!Character.isWhitespace(ch)) {    // other  chars can be whitespace
                throw new ArithmeticException("PostfixEval: Improper char");
            }
        }

        int exprValue = ((Integer) operandStack.pop()).intValue(); // pop expr value
        if (!operandStack.isEmpty()) // stack should be empty
            throw new ArithmeticException("PostfixEval: Too many operands");
        return exprValue;
    }
    private boolean isOperator(char ch) {
        return ch == '+' || ch == '-' || ch == '*' || ch == '%' || ch == '/' || ch == '^';
    }
    private int getOperand() {
        if(operandStack.isEmpty())
            throw new ArithmeticException("PostfixEval: Too many operators");
        return operandStack.pop();
    }
    public int compute(int left, int right, char op) {
        int value = 0;
        // evaluate "left op right"
        switch (op) {
            case '+':
                value = left + right; break;
            case '-':
                value = left - right; break;
            case '*':
                value = left * right; break;
            case '^':
                value = (int)Math.pow(left, right);
                break;
            case '%':
                if (right == 0) {
                    throw new ArithmeticException("PostfixEval: divide by 0");
                }
                value = left % right;
                break;
            case '/':
                if (right == 0) {
                    throw new ArithmeticException("PostfixEval: divide by 0");
                }
                value = left / right;
                break;
        }
        return value;
    }
}