The Collection Interface

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Trail: Collections
Lesson: Interfaces

## The Collection Interface

> 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 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.

> Suppose, for example, that you have a `Collection<String> c`, which may be a `List`, a `Set`, or another kind of `Collection`. This idiom creates a new `ArrayList` (an implementation of the `List` interface), initially containing all the elements in `c`.

> >     List<String> list = new ArrayList<String>(c);

> The following shows the `Collection` interface.

> >     public interface Collection<E> extends Iterable<E> {
> >         //Basic operations
> >         int size();
> >         boolean isEmpty();
> >         boolean contains(Object element);
> >         boolean add(E element);         //optional
> >         boolean remove(Object element); //optional
> >         Iterator iterator();

> >         //Bulk operations
> >         boolean containsAll(Collection<?> c);
> >         boolean addAll(Collection<? extends E> c); //optional
> >         boolean removeAll(Collection<?> c);        //optional
> >         boolean retainAll(Collection<?> c);        //optional
> >         void clear(); //optional

> >         //Array operations
> >         Object[] toArray();
> >         <T> T[] toArray(T[] a);
> >     }

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

### Traversing Collections

> There are two ways to traverse collections: (1)with the `for-each` construct and (2) by using `Iterator`s.

> #### for-each Construct

> The `for-each` construct allows you to concisely traverse a collection or array using a `for` loop — see The for Statement. The following code uses the `for-each` construct to print out each element of a collection on a separate line.

> >     for (Object o : collection)
> >         System.out.println(o);

> #### Iterators

> An `Iterator` is an object that enables you to traverse through a collection and to remove elements from the collection selectively, if desired. You get an `Iterator` for a collection by calling its `iterator` method. The following is the `Iterator` interface.

> >     public interface Iterator<E> {
> >         boolean hasNext();
> >         E next();
> >         void remove(); //optional
> >     }

> The `hasNext` method returns `true` if the iteration has more elements, and the `next` method returns the next element in the iteration. The `remove` method removes the last element that was returned by `next` from the underlying `Collection`. The `remove` method may be called only once per call to `next` and throws an exception if this rule is violated.

> Note that `Iterator.remove` is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

> Use `Iterator` instead of the `for-each` construct when you need to:

> - Remove the current element. The `for-each` construct hides the iterator, so you cannot call `remove`. Therefore, the `for-each` construct is not usable for filtering.
> - Replace elements in a list or array as you traverse it.
> - Iterate over multiple collections in parallel.
> The following method shows you how to use an `Iterator` to filter an arbitrary `Collection` — that is, traverse the collection removing specific elements.

> >     static void filter(Collection c) {
> >         for (Iterator i = c.iterator(); i.hasNext(); )
> >             if (!cond(i.next()))
> >                 i.remove();
> >     }

> This simple piece of code is polymorphic, which means that it works for any `Collection` regardless of implementation. This example demonstrates how easy it is to write a polymorphic algorithm using the Java Collections Framework.

### Collection Interface Bulk Operations

> Bulk operations perform an operation on an entire `Collection`. You could implement these shorthand operations using the basic operations, though in most cases such implementations would be less efficient. The following are the bulk operations:
> - `containsAll` — returns `true` if the target `Collection` contains all the elements in the specified `Collection`.
> - `addAll` — adds all the elements in the specified `Collection` to the target `Collection`.
> - `removeAll` — removes from the target `Collection` all its elements that are also contained in the specified `Collection`.
> - `retainAll` — removes from the target `Collection` all its elements that are not also contained in the specified `Collection`. That is, it retains only those elements in the target `Collection` that are also contained in the specified `Collection`.
> - `clear` — removes all elements from the `Collection`.
> The `addAll`, `removeAll`, and `retainAll` methods all return `true` if the target `Collection` was modified in the process of executing the operation.

> As a simple example of the power of bulk operations, consider the following idiom to remove all instances of a specified element, `e`, from a `Collection`, `c`.

> >     c.removeAll(Collections.singleton(e));

> More specifically, suppose you want to remove all the `null` elements from a `Collection`.

> >     c.removeAll(Collections.singleton(null));

> This idiom uses `Collections.singleton`, which is a static factory method that returns an immutable `Set` containing only the specified element.

### Collection Interface Array Operations

> The `toArray` methods are provided as a bridge between collections and older APIs that expect arrays on input. The array operations allow the contents of a `Collection` to be translated into an array. The simple form with no arguments creates a new array of `Object`. The more complex form allows the caller to provide an array or to choose the runtime type of the output array.

> For example, suppose that `c` is a `Collection`. The following snippet dumps the contents of `c` into a newly allocated array of `Object` whose length is identical to the number of elements in `c`.

> >     Object[] a = c.toArray();

> Suppose that `c` is known to contain only strings (perhaps because `c` is of type `Collection<String>`). The following snippet dumps the contents of `c` into a newly allocated array of `String` whose length is identical to the number of elements in `c`.

> >     String[] a = c.toArray(new String[0]);

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