CODEWITHSUNDEEP
javahashmaphashcode
user7805408
user7805408

Reputation:

HashMap have the key of integer[] type though its containsKey method returns false?

While solving a problem on hackerrank, I found that my output differed from the correct answer because of a logical mistake. I have recreated the logical mistake to explain the situation in a better way. 

HashMap<Integer[] , Integer> hm = new HashMap<>();

//both a and b have different hashcode
Integer[] a = {1, 1, 0, 0};
Integer[] b = {1, 1, 0, 0}; 

hm.put(a,1);

if (!hm.containsKey(b)) {
    //key does not exists so, create new one  
    hm.put(b, 1);
}
else {
    //key does exists so, put its value = value + 1
    hm.put(b, hm.get(b)+1); 
}

Here hm.containsKey(b) return false but if it returns true my output will match to the correct one. How do I make containsKey(b) return true since content of a and b are equal?

Upvotes: 0

Views: 175

Answers (1)

Eran
Eran

Reputation: 393936

You shouldn't use an array as the key of a HashMap, since arrays don't override equals and hashCode, so different array instances that contain the exact same elements are not considered identical by the HashMap.

Use List<Integer> keys instead.

Map<List<Integer>, Integer> hm = new HashMap<>();

List<Integer> a = List.of(1, 1, 0, 0);
List<Integer> b = List.of(1, 1, 0, 0);

hm.put(a,1);

if (!hm.containsKey(b)) {
    //key does not exists so, create new one  
    hm.put(b, 1);
}
else {
    //key does exists so, put its value = value + 1
    hm.put(b, hm.get(b)+1); 
}

Upvotes: 4

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