Java String hashCode of null string

Question

Only interesting, why method hashCode() in java.lang.String is not static? And in case of null return e.g. -1 ? Because frequently need do somethihg like:

String s;
.............
if (s==null) {
  return 0;}
else {
  return s.hashCode();
}

Thanks.

Solution 1

As others have noted hashCode is a method on Object and is non-static because it inherently relies (i.e. belongs to) an object/instance.

Note that Java 7 introduced the Objects class, which has the hashCode(Object) method, which does exactly what you want: return o.hashCode() if o is non-null or 0 otherwise.

This class also has other methods that deal with possibly-null values, such as equals(Object, Object), toString(Object) and a few others.

Solution 2

because if it was static "1".hashCode() and "2".hashCode() would have returned the same value, which is obviously wrong.

It is specific per instance, and influenced by it, therefore it cannot be static.

Solution 3

hashCode is used to get the hashCode of an object, in order to know in which bucket of a HashMap this object must be placed. It thus has to be an instance method of the object, and it must be called polymorphically.

null can be used as a key in a HashMap, but it's treated as a special case.

You seem to be using hashCode for a different purpose, so you have to handle is in a specific way.

Solution 4

Because the hash code of a String is a property of that String.

With the same train of thought you could make every method static.

Solution 5

Its returning hashCode of an Object not an class.

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Comments

[bernard paulus]

Wrong: if a static hashCode(String) was provided along the non-static String.hashCode(), it could be called like this String.hashCode("1"), or, if you ignore the warning, "2".hashCode("1"). Both calls would return the same value.

[amit]

@bernardpaulus But the questions is not about hashCode(String), it's about hashCode() (unparametrized).

[bernard paulus]

a no-parameter static String.hashCode() does not make much sense (should not even compile as it cannot override Object.hashCode() ). Plus the foo() notation is often used to indicate that foo is a function, without specifying the parameters.

[Helen Cui]

" return o.hashCode() if o is non-null or 0 otherwise." How can you call hashCode() on a null and get 0?

[Joachim Sauer]

@HelenCui: you can't call hashCode on null, obviously. That's why you call the static method on the class called Objects and pass the possibly-null value in: int hash = Objects.hashCode(aVariableThatMightBeNull). If the variable was null then hash will be 0, otherwise it will be the value returned by its hashCode() method.