Why two objects with the same hashcode are not necessarily equals?

Question

Currently I drilled into the JSE source code for fun. From some tutorials I found a principle two equals objects (i.e. as to object a and b a.equals(b) returns true) must have the same hashcode, on the other hand, two objects with the same hashcode are not necessarily equals. According to the HashTable source code of Java API (http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/Hashtable.java), e.g. refer to line 254, we can almost derive equals => same hashcode, but I still can not fully understand why two objects with the same hashcode are not necessarily equals. Anyone can give more reasoning on this? Thanks in advance

Note! This question is more interested in why two objects with the same hash code are not necessarily equals

There are more possible `String`s (for example) than there are possible `int`s. Eventually you'll run out of unique hashcodes, even with an otherwise perfect hashing algorithm. — resueman, Oct 12 '16 at 17:04
you and your twin brother have the same birthday day but are different persons.... — ΦXocę 웃 Пepeúpa ツ, Oct 12 '16 at 17:21

score 1 · Accepted Answer · answered Oct 12 '16 at 17:06

1

That's a simple matter of counting. A hash code is an integer so has 32 bits. Take now a Long for example. Since it is 64 bits long, there are much more values than 2³². Hence there must be a lot of values having the same hash code.

answered Oct 12 '16 at 17:06

Henry

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Why two objects with the same hashcode are not necessarily equals?

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