Implementation of type parameter in HashMap vs ArrayList

Question

HashMap<K,V> stores data as array of entry. i.e

Entry<K,V>[] table

and type of key and value inside Entry are K and V respectively.

But..

ArrayList<E> stores data as array of object. i.e.

Object[] elementData

So wondering why ArrayList doesn't use array of type E ??

Disagree that this is a duplicate; the question it is marked a duplicate of doesn't include anything about why HashMap is different. — Jules, Apr 21 '14 at 04:20

score 4 · Accepted Answer · answered Apr 20 '14 at 16:25

Short answer: try implementing it yourself and see what happens. Long answer: because Java uses type erasure, and you need to know the type at run time in order to create a new array, new E[size] won't compile, so it can't do this without some kind of hack. You don't need to know generic parameters to create a new array, however, as they're erased so you don't have to know them at runtime.

Implementation of type parameter in HashMap vs ArrayList

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