Java: ensureCapacity() method to increase the size of an ArrayList explicitly

The default size of an ArrayList in Java is 10. When you create a new instance of ArrayList without specifying an initial capacity, it will create an internal array of size 10 to hold the elements you add to the list. 

In addition to increasing the size of the List automatically, we can also increase it explicitly using the ensureCapacity() method of the ArrayList class. This method allows us to set the minimum capacity of the ArrayList to the specified value. If the current capacity of the ArrayList is less than the specified value, then the capacity of the ArrayList is increased to the specified value.

Here is an example of how to use the ensureCapacity() method to increase the size of an ArrayList explicitly:

In this example, we create a new ArrayList and print its initial capacity (which is 0). Then, we use the ensureCapacity() method to reserve memory for at least 10 elements. Finally, we print the new capacity of the ArrayList (which is still 0, because we haven't added any elements yet).

Note that if you try to add more elements than the current capacity of the ArrayList, it will be automatically resized to accommodate the new elements. So, while ensureCapacity() can be used to increase the capacity of an ArrayList explicitly, it's not always necessary. The ArrayList class already provides a default growth behavior that ensures efficient use of memory and avoids excessive copying of the underlying array.

What happens after ArrayList exceeds its default capacity of 10?

After an ArrayList exceeds its default capacity of 10 (or the capacity set by the user), it automatically increases its capacity to accommodate new elements. When the ArrayList needs to increase its capacity, it creates a new array with a larger capacity and copies the elements from the old array to the new array.

The new capacity of the ArrayList depends on its current capacity and a growth factor, which is typically 1.5. This means that the new capacity is typically 50% larger than the old capacity. For example, if the current capacity is 10 and the growth factor is 1.5, then the new capacity would be 15.

The process of increasing the capacity of the ArrayList is called "resizing", and it can be a relatively expensive operation, especially when the ArrayList is large. Therefore, it's best to set the initial capacity of the ArrayList to a value that's close to the expected size of the list, to minimize the number of times the ArrayList needs to resize.

How to increase the growth factor in ArrayList?

In Java, the growth factor for ArrayList is determined by the formula:

newCapacity = oldCapacity + (oldCapacity >> 1)

This means that the new capacity of the ArrayList is 1.5 times the old capacity, rounded up to the nearest power of 2.

If you want to increase the growth factor of ArrayList, you can create a custom implementation of ArrayList that uses a different formula for calculating the new capacity. For example, you can use a growth factor of 2, which would double the capacity of the ArrayList each time it needs to grow:

This implementation overrides the ensureCapacity method of ArrayList and uses a growth factor of 2 to calculate the new capacity. When the ArrayList needs to grow, it doubles its current capacity until it reaches the required minimum capacity. This can be more efficient than the default growth factor of 1.5 for large ArrayLists.