Collection Interface Basic Operations in Java

A Collection represents a group of objects known as its elements. The Collection interface is used to pass around collections of objects where maximum generality is desired. For example, by convention all general-purpose collection implementations have a constructor that takes a Collection argument. This constructor, known as a conversion constructor, initializes the new collection to contain all of the elements in the specified collection, whatever the given collection's subinterface or implementation type. In other words, it allows you to convert the collection's type.

For example if you have a Collectionc, which may be a List, a Set, or another kind of Collection. This idiom creates a new ArrayList, initially containing all the elements in c.

     List list = new ArrayList(c);

     List list = new ArrayList<>(c);

The Collection interface contains methods that perform basic operations, such as int size(), boolean isEmpty(), boolean contains(Object element), boolean add(E element), boolean remove(Object element), and Iterator iterator().

It also contains methods that operate on entire collections, such as boolean containsAll(Collection c), boolean addAll(Collection c), boolean removeAll(Collection c), boolean retainAll(Collection c), and void clear().

Additional methods for array operations (such as Object[ ] toArray() and T[ ] toArray(T[ ] a) exist as well.

In JDK 8 and later, the Collection interface also exposes methods Stream stream() and Stream parallelStream(), for obtaining sequential or parallel streams from the underlying collection. (See the lesson entitled Aggregate Operations for more information about using streams.)

The Collection interface does about what you'd expect given that a Collection represents a group of objects. It has methods that tell you how many elements are in the collection (size, isEmpty), methods that check whether a given object is in the collection (contains), methods that add and remove an element from the collection (add, remove), and methods that provide an iterator over the collection (iterator).

The add method is defined generally enough so that it makes sense for collections that allow duplicates as well as those that don't. It guarantees that the Collection will contain the specified element after the call completes, and returns true if the Collection changes as a result of the call. Similarly, the remove method is designed to remove a single instance of the specified element from the Collection, assuming that it contains the element to start with, and to return true if the Collection was modified as a result.

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