Exhaustiveness

When a method returns data, Java needs to know that no matter what happens in the method there will be some return line reached.

int compute(int x) {
    if (x < 0) {
        return 5;
    }
    // Error! No return if x >= 0
}

int compute(int x) {
    // Both "branches" have returns, so all is well
    if (x < 0) {
        return 5;
    }
    else {
        return 1;
    }
}

We call this property, whether in every situation a method will return a value, "exhaustiveness." If there could be cases where no return statement is reached, that is "non-exhaustive" and Java won't accept your code.