We’ve discussed the core interfaces and their implementations but the thing that keeps the collection framework polymorphic (compatible) is the Collection interface. The Collection interface provides a generic, general-purpose API when our program needs a collection of elements and doesn’t care about what type of collection it is.

Implementing classes may implement collections methods and add restrictions to them, like a Set does to only contain unique elements. Also, implementing classes or extending interfaces do not need to implement all methods and instead will throw an UnsupportOperationException when a Collection method is not implemented.

We’ve seen add() and remove() be used but some other methods Collection defines are:

• addAll() - receives a Collection argument and adds all the elements.
• isEmpty() - return true if the collection is empty, false otherwise.
• iterator() - returns an Iterator over the collection.
• size() - returns the number of elements in the collection.
• stream() - returns a Stream over the elements in the collection.
• toArray() - returns an array with all elements in the collection.

Let’s see how we can use some these methods and iterate

Collection<Integer> collection = new ArrayList<>();
collection.add(4);
collection.add(8);

boolean isEmpty = collection.isEmpty(); // false
int collectionSize = collection.size(); // 2

Integer[] collectionArray = collection.toArray(new Integer[0]);

In the example above we:

• Created an Integer Collection with an ArrayList implementation.
• Called add() to add elements to the end of the Collection.
• Called isEmpty() to check if collection has elements.
• Called size() to get the number of elements in collection.
• Called toArray() to transform our collection to an array. Note the new Integer[0] argument that specifies the type of array we want returned.

We can iterate through a Collection with an enhanced for-loop as we’ve seen with the other core interfaces.

Let’s practice working with Collection.