CODEWITHSUNDEEP
java
user1495220
user1495220

Reputation: 111

Are there any constraints on the hash map data type

Are there any constraint on the key type in the hash map and hash table?----Interview Question. I think yes we can customize it as needed.

Upvotes: 2

Views: 3535

Answers (2)

Adrian Shum
Adrian Shum

Reputation: 40056

Several things to consider:

  1. Just about the "Type", you cannot use primitive type. This is language constraint of Java. e.g. HashMap<int, Foo> is not valid, you need to use HashMap<Integer, Foo>

  2. Base on the way HashMap work, key should have a meaningful implementation of hashCode() and equals(). How it is "meaningful" depends on your need. It may be possible that the default implementation in Object already serve your need, but you need to aware of it.

  3. Once an object instance is put into the Map as key, its hashCode() and equals() should stay consistent. You should never put to a map, and change the state of the object instance as Key and cause hashCode()/equals() returns different value. The easiest way to ensure it is of course use an immutable object as key. However it is still fine that you use mutable object, but in your code, you ensure changing state of keys are not happening.

Upvotes: 1

Blake
Blake

Reputation: 591

Technically, no. Generally, you want to use an object that implements equals() and hashCode() although that is not strictly necessary. If you don't, then it will use the base implementations defined by Object which compare object identity. A lot of times, that is not appropriate but sometimes it's fine.

Technically the key doesn't need to be immutable as long as the values used in the equals() and hashCode() implementations are immutable. For example, if your class Foo uses a string "foo" as part of its has then that value "foo" must not change. That's because hash maps put the keys into buckets based on the hashCode() value for efficiency reasons. If the hashCode suddenly changes, the hash map is unaware and the key will now live in the wrong bucket and you'll run into nasty bugs because it's then possible to have "duplicate" objects in your map. Hope that makes sense.

Upvotes: 2

Related Questions