What is the best practice for writing methods that are opposite of each other when returning a boolean?

Question

I am learning about conditionals and want to know if this is a bad convention for similar situations. Should I define isOdd() without using isEven()?

    static boolean isEven(int i) {
        return i % 2 == 0;
    }
    
    static boolean isOdd(int i) {
        return !isEven(i);  
    }

Answer 1

No, that's absolutely fine, and code reuse is positively encouraged - if you actually need both methods, of course. Is it really too hard for the caller to use !isEven(...)? Sometimes it can be worth it for the sake of caller readability, but I'd encourage careful consideration first.

However, what I would discourage is the approach of isEven using an if statement where the body just returns true or false. That can always be replaced by just returning the condition:

static boolean isEven(int i) {
    return i % 2 == 0;
}