As I've seen that kind of questions a few times already, maybe one more example to what's illegal even if it might feel okay.
Consider this code:
if(somethingIsTrue()) {
String message = "Everything is fine";
} else {
String message = "We have an error";
}
System.out.println(message);
That's invalid code. Because neither of the variables named message is visible outside of their respective scope - which would be the surrounding brackets {} in this case.
You might say: "But a variable named message is defined either way - so message is defined after the if".
But you'd be wrong.
Java has no free() or delete operators, so it has to rely on tracking variable scope to find out when variables are no longer used (together with references to these variables of cause).
It's especially bad if you thought you did something good. I've seen this kind of error after "optimizing" code like this:
if(somethingIsTrue()) {
String message = "Everything is fine";
System.out.println(message);
} else {
String message = "We have an error";
System.out.println(message);
}
"Oh, there's duplicated code, let's pull that common line out" -> and there it it.
The most common way to deal with this kind of scope-trouble would be to pre-assign the else-values to the variable names in the outside scope and then reassign in if:
String message = "We have an error";
if(somethingIsTrue()) {
message = "Everything is fine";
}
System.out.println(message);