What is the default implementation of `hashCode`?

Question

If one does not override the hashCode method, what is the default implementation of hashCode ?

About default implementation you can read [this](http://blogs.tedneward.com/CommentView,guid,eca26c5e-307c-4b7c-931b-2eaf5b176e98.aspx) — CAMOBAP, Feb 28 '13 at 08:39
default implementation is JVM specific, but in general it returns `return Objects.hash(this.field1, this.field2, this.field3, etc.);` — NoName, Jun 24 '17 at 06:19

score 57 · Accepted Answer · edited Apr 02 '17 at 02:19

57

Then this class inherits hashCode from one of its ancestors. If non of them overrides it, then Object.hashCode is used.

From the docs:

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

So default implementation is JVM-specific

edited Apr 02 '17 at 02:19

Ry-

218,210
55
464
476

answered Feb 28 '13 at 08:28

default locale

13,035
13
56
62

Thank you, yes. What does the implementation of `Object.hashCode() look` like? – John Threepwood Feb 28 '13 at 08:29
1

@JohnThreepwood it's an implementation detail, you need to address your JVM docs for that. – default locale Feb 28 '13 at 08:32
3

http://stackoverflow.com/a/32454673/6785908 – so-random-dude Mar 11 '17 at 23:27

score 16 · Answer 2 · answered Feb 28 '13 at 08:35

By default, methods that are not overriden are inherited from Object.

If you look at that method's documentation, the return values are "[...] distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer [...])". The method in java.lang.Object is declared as native, which means the implementation is provided by the JVM and may vary depending on your runtime environment.

A small example:

Object o1 = new Object();
Object o2 = new Object();
System.out.println(o1.hashCode());
System.out.println(o2.hashCode());

prints (using my jdk6):

1660187542
516992923

A Hex representation of the hashCode() value is used in the default implementation of toString() by the way: Running System.out.println(o1) prints something like

java.lang.Object@7a5e1077

score 7 · Answer 3 · edited May 23 '17 at 12:10

7

Object.hashcode() is a native method.

public native int hashCode();

That means it's implemented in platform specific code and is exposed as a native method.

code for the same will be a compiled code and not available withing JDK

this existing question might provide more info.

edited May 23 '17 at 12:10

Community

1
1

answered Feb 28 '13 at 08:34

TheWhiteRabbit

15,480
4
33
57

What is the default implementation of `hashCode`?

3 Answers3

Linked