调整 ArrayDeque 的大小

Question

Quote: Default initial capacity of ArrayDeque is 16. It will increase at a power of 2 (24, 25, 26 and so on) when size exceeds capacity.

Does this mean it behaves similar to ArrayList ? Each time size exceeds capacity there is new array where older elements are copied to? Can I say internal implementation of ArrayDequeue and ArrayList is array (as their name says)? Just the resizing differs?

Answer 1

Yes ArrayDeque behaves similarly to ArrayList : Internally it uses an Object array. If the capacity is not enough, it creates a new, larger array, and copies items from the old array to the new.

The Java API specification does not require any particular resizing behavior. In fact the current implementation in OpenJDK doubles the size of the array if it's small (64), otherwise it grows by 50% :

    // Double capacity if small; else grow by 50%
    int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);

It seems that the"doubling" behavior is approximate: thanks to the "+2" after the first resize the capacity is 16+16+2 = 34. After the second resize it's 34+34+2 = 70. After that the array increases by 50% in every resize.