What is the meaning of the information that I get by printing an Object in Java?

Question

Lets say i have this code :

Integer[] a= new Integer[5];
System.Out.println(((Object)a).toString());

the output is get is

[Integer@89fbe3

what is the meaning of 89fbe3 ? is this some kind of address ? hash code? is it unique for each object? , and if so- if its a multi-threaded program , is it still unique ?

thanks !

score 4 · Answer 1 · edited Jan 14 '11 at 21:36

4

It's the result of System.identityHashCode(Object x);

which is the default implementation of every object's hashCode()...

from the Object javadoc:

getClass().getName() + '@' + Integer.toHexString(hashCode())

edited Jan 14 '11 at 21:36

Mark Peters

80,126
17
159
190

answered Jan 14 '11 at 21:28

david van brink

3,604
1
22
17

This covers all of the bases. `[I` is actually what you get when you do `new int[0].getClass().getName()`. +1. – Mark Peters Jan 14 '11 at 21:32
ope! & thanks for the format fixup. :) – david van brink Jan 14 '11 at 21:45

Mark Peters · Answer 2 · 2011-01-14T21:30:49.363

3

The 89fbe3 is a hex version of the hash code. The [I means an array of ints (I'm surprised you get that with an Integer[], are you sure it wasn't an int[]?)

Some others:

[L<typename>;: an array of reference type "typename" (e.g. [Ljava.lang.Integer)
[J: an array of longs
[B: an array of bytes

etc.

edited Jan 14 '11 at 21:30

answered Jan 14 '11 at 21:25

Mark Peters

80,126
17
159
190

score 2 · Answer 3 · answered Jan 14 '11 at 21:26

2

It is the identity hash code of the object (you can think of it as the address of the object), along with some type information.

[ = array I = Integer

answered Jan 14 '11 at 21:26

time4tea

2,169
3
16
21

score 1 · Answer 4 · answered Jan 14 '11 at 21:29

1

I think that while technically all the answers are correct, the real answer is "NO". This number has no meaning and you can make absolutely no assumptions about it.

answered Jan 14 '11 at 21:29

MK.

33,605
18
74
111

So is there any way to determine between two arrays in runtime? – RanZilber Jan 14 '11 at 21:31
The number is the hash code in hex, which for a method that doesn't override hash code is the (somewhat) unique object ID. This can be used to see if two objects are the same instance. – Steve Kuo Jan 14 '11 at 21:34
@RanZilber I think you accidentally the question. – MK. Jan 14 '11 at 21:34
It's good for checking whether the two variables you assumed to be referring to the same instance, actually do. – biziclop Jan 14 '11 at 21:34
@Steve Kuo '==' can be used to see if 2 objects are the same instance. equals() can be used to see if two objects are "the same" according to their creator. I don't know what the default toString() can be used for. – MK. Jan 14 '11 at 21:35
@biziclop what is wrong with '=='??? – MK. Jan 14 '11 at 21:37
I think this is a correct answer as well. If you need to absolutely identify an Object you need to retain a reference to it and use `==` to see if it's the same as another Object. Just beware of memory leaks taking that route. – Mark Peters Jan 14 '11 at 21:40
@MK I was thinking about printing the default object string into a debug log file for manual observation, not using it in actual code. – biziclop Jan 14 '11 at 22:13

score -1 · Accepted Answer · answered Jan 14 '11 at 21:25

-1

It's the memory address of the object which is what the default toString() implemented in the Object class does. It is also the default hashCode().

answered Jan 14 '11 at 21:25

Jesus Ramos

22,940
10
58
88

I was going to downvote but it's not *technically* wrong. But it's misleading. It's only because arrays use the default hashCode that it's the memory address. – Mark Peters Jan 14 '11 at 21:28
1

It is not (necessarily) the memory address. It's up to the JVM. (I suspect that it's never the memory address, actually.) – david van brink Jan 14 '11 at 21:29
that's why I said it was the default toString() and hashCode() defined in the base Object type. Because he can obviously override any of those methods to produce different results. – Jesus Ramos Jan 14 '11 at 21:29
So , it is true only for arrays? – RanZilber Jan 14 '11 at 21:29
2

@RanZilber: No, it's true for any object whose class hierarchy has not somewhere overridden the default `hashCode()` or `toString()` method. – Mark Peters Jan 14 '11 at 21:33
@Jesus no, he can't override the methods. Not for array classes anyway. – Sean Patrick Floyd Jan 14 '11 at 21:33
1

Look at the example in the question. It's an odd number. Clearly that isn't the actual memory address. – Tom Hawtin - tackline Jan 15 '11 at 05:55

What is the meaning of the information that I get by printing an Object in Java?

5 Answers5

Linked