How to write a generic isEmpty method which can check for null, empty?

Question

I am writing a utility method which can check for empty and null string, or collection or an object or any general types -

public static boolean isEmpty(Object obj) {
    if (obj == null)
        return true;
    if (obj instanceof Collection)
        return ((Collection<?>) obj).size() == 0;

    // is below line expensive?
    final String s = String.valueOf(obj).trim();

    return s.length() == 0 || s.equalsIgnoreCase("null");
}

How can I make my above method efficient, since above isEmpty method will be called multiple times from the application which is very performance critical?

I am suspecting below line will be expensive because of heavy toString methods and it will create temporary garbage as well that might cause GC and slow down the performance?

final String s = String.valueOf(obj).trim();

Update:-

I have separated isEmpty method for each type now. Below is what I got after simplifying the above isEmpty method.

public static boolean isEmpty(Object obj) {
    if (obj == null) {
        return true;
    }
    return false;
}

public static boolean isEmpty(Collection<?> value) {
    if (value == null || value.isEmpty()) {
        return true;
    }
    return false;
}

public static boolean isEmpty(String value) {
    if (value == null || value.isEmpty()) {
        return true;
    }
    return false;
}

Update 2:-

If I need to check for map null or empty, should I keep both collection isEmpty and Map isEmpty method both or Collection isEmpty method will be fine for that?

public static void main(String[] args) {

    Map<String, String> hello = new HashMap<String, String>();
    System.out.println(isEmpty(hello));

    Map<String, HashMap<Integer, String>> primary = new HashMap<String, HashMap<Integer, String>>();
    System.out.println(isEmpty(primary));

}

public static boolean isEmpty(Collection<?> value) {
    return value == null || value.isEmpty();
}

public static boolean isEmpty(Map<?, ?> value) {
    return value == null || value.isEmpty();
}

Answer 1

This sounds like a bad design to me. Null is null, empty is empty, if it's a string it's a string, and so on. Don't try to jam everything up in one method. It's bad for maintainability and readability.

if (str == null || str.isEmpty())
    ...

and

if (coll == null || coll.isEmpty())

are both perfectly fine.

Personally however, I try to never ever equate null with an empty string or empty collection. I think it's a bad practice. A null collection is no collection at all, an empty collection is in fact still a collection. You can avoid many if (coll == null) checks by keeping a collection non-null. If you're worried about memory consumption, use use Collections.emptySet et al.

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That being said, if you still want to go in this direction, I'd suggest you use plain method overloading and create one isEmpty(Collection<?> coll) and one isEmpty(String str) to avoid instanceof and casting.

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Regarding your edit:

Don't do for instance

if (value == null || value.isEmpty()) {
    return true;
}
return false;

just do

return value == null || value.isEmpty();