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)](http://stackoverflow.com/questions/1058149/using-a-byte-array-as-hashmap-key-java) — MK., Apr 14 '12 at 21:28
You can't even use StringBuffer/Builder as Key in Hashmap https://stackoverflow.com/questions/23790423/string-vs-stringbuffer-as-hashmap-key/23790485#23790485 — Kanagavelu Sugumar, Aug 20 '19 at 12:27

score 8 · Accepted Answer · answered Apr 14 '12 at 21:26

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 '12 at 21:26

Louis Wasserman

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But I've used arrays of integers. I didn't face any problems. – user Apr 14 '12 at 22:15
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@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. – Voo Apr 14 '12 at 22:19
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@Voo is right. `int[] a = {1}; int[] b = {1}; System.out.println(a == b)` prints `false`. – Louis Wasserman Apr 14 '12 at 22:19

score 4 · Answer 2 · answered Apr 14 '12 at 21:26

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 '12 at 21:26

Matt Ball

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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? – ravi Apr 14 '12 at 21:28
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Use `String`s, 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()`. – Matt Ball Apr 14 '12 at 21:29

score 2 · Answer 3 · answered Apr 14 '12 at 21:30

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 '12 at 21:30

MK.

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Your logic is not _completely_ sound, since (in practice) the `.equals()` and `.hashCode()` method implementations are independent. – Matt Ball Apr 14 '12 at 21:36
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. – MK. Apr 14 '12 at 21:40

Using a char array as Hashtable key

3 Answers3