in my reverse polish notation evaluation code's if statement is executing even if conditons are not met in java laguange but in c++ its doing fine

Question

        import java.util.*;
    
    
    class ReversePolishNotation{
        public static void main(String args[]){
            Scanner sc = new Scanner(System.in);
            Vector<String> vec = new Vector<String>();
            while(sc.hasNext())
                vec.add(sc.next());
            System.out.println(reversePolishNotation(vec));
        }
    
        static int reversePolishNotation(Vector<String> vec){
    
            Stack<Integer> stack = new Stack<Integer>();
            //String operators = "+-*/";        
            
            for(String i : vec){   
                         
                if(i != "+" && i != "-" && i != "*" && i != "/"){   
`
here this if statement is executing even if i = "+"
`            
                    stack.push(Integer.parseInt(i));
                }
                else{
                    int x = stack.pop();
                    int y = stack.pop();
    
                    switch(i){
                        case "+" : stack.push(y + x); break;
                        case "-" : stack.push(y - x); break;
                        case "*" : stack.push(y * x); break;
                        case "/" : stack.push(y / x); break;                                
                    }                
                }
            }        
            return stack.pop();
        }
    }

this is my code for reverse polish notation evaluation, there is something wrong in "if" condition , this condition is ok in C++ and not in java

here is the error coming and i dont know this is ok in c++ but not in java INPUT : 2 1 + 3 *

Exception in thread "main" java.lang.NumberFormatException: For input string: "+" at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) at java.base/java.lang.Integer.parseInt(Integer.java:642) at java.base/java.lang.Integer.parseInt(Integer.java:770) at ReversePolishNotation.reversePolishNotation(ReversePolishNotation.java:21) at ReversePolishNotation.main(ReversePolishNotation.java:10)