Evaluate Reverse Polish Notation

String equals() ==

public class Solution {
    public int evalRPN(String[] tokens) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        Stack<Integer> stack = new Stack<Integer>();
        if(tokens == null || tokens.length == 0)
            return 0;
        for(int i = 0; i < tokens.length; i++){
            
            String str = tokens[i];
            if(str.equals("+")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                stack.push(num1 + num2);
            }
            else if(str.equals("-")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                stack.push(num2 - num1);
            }
            else if(str.equals("*")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                stack.push(num1 * num2);
            }
            else if(str.equals("/")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                stack.push(num2 / num1);
            }
            else
                stack.push(Integer.valueOf(str));
        }
        return stack.pop();
    }
}