Using a char array as Hashtable key

Question

I am working with Hashtable in my java program. I just surprised by seeing the abnormal behavior of Hastable. Below is my code (This is not my final code, i simply created a new simple project with the code which is running abnormal)

    Hashtable<char[], char[]> h1 = new Hashtable<char[], char[]>();
    char[] key = Integer.toString(12).toCharArray();
    char[] val = Integer.toString(21).toCharArray();
    h1.put(key, val);
    System.out.println(h1.containsKey(Integer.toString(12).toCharArray()));// Should print true, since 12 is there in Hashtable

possible duplicate of Using a byte array as HashMap key (Java) — MK.
– MK., Commented Apr 14, 2012 at 21:28
You can't even use StringBuffer/Builder as Key in Hashmap stackoverflow.com/questions/23790423/… — Kanagavelu Sugumar
– Kanagavelu Sugumar, Commented Aug 20, 2019 at 12:27

Louis Wasserman · Accepted Answer · 2012-04-14 21:26:56Z

8

You can't use arrays like this as map keys, because arrays have the default, referential-equality-based Object implementations of equals and hashCode. Using String as the key instead would make your program work as desired.

answered Apr 14, 2012 at 21:26

Louis Wasserman

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3 Comments

user Over a year ago

But I've used arrays of integers. I didn't face any problems.

Voo Over a year ago

@crucified You did, maybe you just didn't notice it.. But since the basic statement ("arrays have referential equality") is true for any array, that's just how it is.

Louis Wasserman Over a year ago

@Voo is right. int[] a = {1}; int[] b = {1}; System.out.println(a == b) prints false.

Matt Ball · Accepted Answer · 2012-04-14 21:26:31Z

4

Array equality is based on referential equality ("Are these two references to the same object?") not deep equality ("Are these two objects semantically identical?"). Look:

char[] one = Integer.toString(12).toCharArray();
char[] two = Integer.toString(12).toCharArray();
System.out.println(one == two); // false

http://ideone.com/YwEjV

answered Apr 14, 2012 at 21:26

Matt Ball

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2 Comments

ravi Over a year ago

They aren't referring to the same object. In my above code, i used Integer.toString(). But in my actual code, i am working with char[]. So what can be done now?

Matt Ball Over a year ago

Use Strings, not char[]s, as map keys. Also, you probably should be using HashMap instead of the legacy Hashtable. ...Also it would improve the code clarity to write "12".toCharArray() or new char[]{'1', '2'} instead of Integer.toString(12).toCharArray().

MK. · Accepted Answer · 2012-04-14 21:30:31Z

2

if a and b are 2 arrays than a.equals(b) if a == b. So hashCode of a == hashCode of b if a == b. Since this is not the case here, it will not be found in the hashtable. Using arrays as hashtable keys is a bad idea. Also using any mutable object as a hashtable key is a bad idea.

answered Apr 14, 2012 at 21:30

MK.

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2 Comments

Matt Ball Over a year ago

Your logic is not completely sound, since (in practice) the .equals() and .hashCode() method implementations are independent.

MK. Over a year ago

Part of the contract of equals and hashCode is to be consistent between each other. I would assume that Java follows it's own contract.

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