Why does the first if block keep executing?

Question

Java

Good evening! Why does the first if block continue to execute in the code below, if if expression[i] is equal to one of the characters in the condition, it must transfer control to the second block at the next iteration of the loop?

Input data - ["1", "2", "+", "4"]

void parse(){
        String firstNumber = "";
        String secondNumber = "";

        for(int i = 0, j = 0; i < expression.length; i++){
            if((expression[i] != "+" && expression[i] != "-" && expression[i] != "*" && expression[i] != "/") && j == 0){
                firstNumber += expression[i];
                System.out.println(firstNumber);
                System.out.println(j);
            }else if(j == 1){
                secondNumber += expression[i];
                System.out.println(secondNumber);
                System.out.println(j);
            }else{
                j = j + 1;
            }
        }

Answer 1

Because strings in Java don't compare like that. This is described almost on the first pages of any textbook.