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

Answer 1

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.

Answer 2

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

Answer 3

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.