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ArrayList Size

layout: default title: ArrayList - Size vs. Capacity

The size() of an ArrayList is the number of elements in the list, not the length of the internal array.

ArrayList Capacity

The length of the internal array is the ArrayList's capacity. * The default capacity in Oracle's ArrayList is 10.

When the internal array is full, the ArrayList will grow the capacity by creating a new internal array and copying all references from the old array to the new one. * The size of the new array in Oracle's implementation is capacity + capacity/2 + 1.

Practice Exercise

ArrayList does not provide a method for reading the capacity.

Setting an ArrayList's Initial Capacity

ArrayList has a one-arg constructor to set the internal capacity.

ArrayList<Character> alphabet = new ArrayList(26);

This is useful when we know a minimum for how many elements the ArrayList will hold.

Changing the ArrayList Capacity

We can change the capacity of the internal array with two methods.

  • ensureCapacity(int) - ensure, if necessary, that the array can hold at least the number of elements specified. 
    alphabet.ensureCapacity(128);

This ensures we have at least 128 spaces in the array. * trimToSize() - shrink the underlying array to the actual size of the ArrayList.

Practice Exercise

When we say "change the capacity" or "shrink" the underlying array, we mean create a new array of the given size and copy references to the new array.

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