New to java - Functional way for if else check

Below is traditional way of doing this. Can you please suggest functional way/replacement of using Optional & Predicates for code below

// Traditional if else check

if (operator == "add") {
result = a + b;
} else if (operator == "multiply") {
result = a * b;
} else if (operator == "subtract") {
result = a - b;
} else {
result = 0;
}
}```

The method have to return result 

Thanks