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Editorial Team
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Asked: 2026-06-15T00:41:40+00:00

I know the contract says if two objects are equal then they should return

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I know the contract says “if two objects are equal then they should return the same hash code”. That is so that those objects can be place in the same hash bucket and to improve hash code related collection functions know. Then again why it says “if two objects have same hash code those should not always be equals”. I mean if it is true in the contract we should say “if two objects are equals they may return same hash code but which is not mandatory”

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  1. Editorial Team

    I mean if it is true in the contract we should say “if two objects are equals they may return same
    hash code but which is not mandatory”

    No, it should not. This is because, whenever an object is searched let’s say in a HashMap or a HashSet, then first it is searched on the basis of hashCode(NOTE : – hashCode() is not used for search in case of ArrayList, or LinkedList. They are not hash based collections), and then if the two objects have same hashcodes, it moves to the equals method to compare the objects themselves.

    Now suppose if the above statement was true, the first test itself would fail for those objects. That is, if two equal objects are allowed to have different hashcode, then while searching for a particular hashCode, it won’t return the correct result, and thus the test will not proceed to equals method, and declare those objects to be unequal even though you expected them to be equal.

    Now lets move to the second statement: –

    if two objects have same hash code those should not always be equals”

    Let’s understand like this: – Since hashCode generated for each object is of type int, so you can generate a maximum 2 ^ 32 unique hashcodes. So, imagine what would happen if you want to store more objects than that. In that case, there has to be a collison for two different objects. So, you would have no other way than to assign same hashCodes to two different objects. And hence the above statement is justified.

    So, two things are quite clear from the above explanation: –

    • Two same objects must have same hashCodes.
    • Two different objects can have same hashCodes.

    More details on this topic in the below link (Nothing can give better explanation than this): –

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