It is s class very much quite similar to an array. It can store any number of dis-similar type. Every collection we have as a property capacity. Which describes the number of elements collection can hold it. By default if a collection is created using default constructor the initial capacity will be Zero which becomes four. After adding the first value and keeps on doubling the capacity. Whenever the requirement comes we can also create a collection by specifying own capacity pass another parameter to it. Here also whenever the requirement comes the capacity doubles.