Is there any case when I should use ensureCapacity() on ArrayList externally?

Question

The below code ensures a capacity of 11 internally,

ArrayList list = new ArrayList(11);

so why/when should I use public method ensureCapacity() externally?

list.ensureCapacity(11);

And if there is no use why it is kept public?

public void ensureCapacity(int minCapacity) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        // any size if not default element table
        ? 0
        // larger than default for default empty table. It's already
        // supposed to be at default size.
        : DEFAULT_CAPACITY;

    if (minCapacity > minExpand) {
        ensureExplicitCapacity(minCapacity);
    }
}

Answer 1

Calling new ArrayList(n) initializes the ArrayList with capacity n.

However, there may be a case where you now want to add a large number of elements to this already created ArrayList and the ArrayList is full. Lets say we want to add x new elements.

It would be more efficient to call list.ensureCapacity(n + x) and then add the elements, than it would be to just add the elements and let the List dynamically resize.

Consider this crude example:

ArrayList<Integer> exampleList = new ArrayList<>(200);

Integer[] exampleArray = new Integer[800];

exampleList.ensureCapacity(1000);

Collections.addAll(exampleList, exampleArray);

Answer 2

You might need to use it when you don't want to resize the arrylist automatically and want to keep control to yourself. e.g. you created an ArrayList of 5 size. If you add 6th element it will create a space for 5+5 i.e 10 elements. If you dont want your arryList to double the size you can use list.ensureCapacity to tell how many elements you need.

This is fine for small data but assume your arrayList grows to 5K.

As per java documentation https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html :

An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.