Java hashCode() – How Does the hashCode() Method Work?

The value returned by hashCode() is by no means guaranteed to be the memory address of the object. I'm not sure of the implementation in the Object class, but keep in mind most classes will override hashCode() such that two instances that are semantically equivalent (but are not the same instance) will hash to the same value. This is especially important if the classes may be used within another data structure, such as Set, that relies on hashCode being consistent with equals.

There is no hashCode() that uniquely identifies an instance of an object no matter what. If you want a hashcode based on the underlying pointer (e.g. in Sun's implementation), use System.identityHashCode() - this will delegate to the default hashCode method regardless of whether it has been overridden.

Nevertheless, even System.identityHashCode() can return the same hash for multiple objects. See the comments for an explanation, but here is an example program that continuously generates objects until it finds two with the same System.identityHashCode(). When I run it, it quickly finds two System.identityHashCode()s that match, on average after adding about 86,000 Long wrapper objects (and Integer wrappers for the key) to a map.

public static void main(String[] args) {
    Map<Integer,Long> map = new HashMap<>();
    Random generator = new Random();
    Collection<Integer> counts = new LinkedList<>();

    Long object = generator.nextLong();
    // We use the identityHashCode as the key into the map
    // This makes it easier to check if any other objects
    // have the same key.
    int hash = System.identityHashCode(object);
    while (!map.containsKey(hash)) {
        map.put(hash, object);
        object = generator.nextLong();
        hash = System.identityHashCode(object);
    }
    System.out.println("Identical maps for size:  " + map.size());
    System.out.println("First object value: " + object);
    System.out.println("Second object value: " + map.get(hash));
    System.out.println("First object identityHash:  " + System.identityHashCode(object));
    System.out.println("Second object identityHash: " + System.identityHashCode(map.get(hash)));
}

Example output:

Identical maps for size:  105822
First object value: 7446391633043190962
Second object value: -8143651927768852586
First object identityHash:  2134400190
Second object identityHash: 2134400190

I read in a book that hashCode() shows a memory area which helps (e.g. HashSets) to locate appropriate objects in memory.

No, that's a completely bogus description of the purpose of hashCode. It's used to find potentially equal objects in an efficient manner. It's got nothing to do with the location of the object in memory.

The idea is that if you've got something like a HashMap, you want to find a matching key quickly when you do a lookup. So you first check the requested key's hash code, and then you can really efficiently find all the keys in your map with that hash code. You can then check each of those (and only those) candidate keys for equality against the requested key.

See the Wikipedia article on hash tables for more information.