Java String hashCode of null string

问题:

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.

回答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.

回答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.

回答3:

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.

回答4:

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.

回答5:

Its returning hashCode of an Object not an class.