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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

import java.util.function.Predicate;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

/**
 * The root interface in the <i>collection hierarchy</i>. A collection
 * represents a group of objects, known as its <i>elements</i>. Some
 * collections allow duplicate elements and others do not. Some are ordered
 * and others unordered. The JDK does not provide any <i>direct</i>
 * implementations of this interface: it provides implementations of more
 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface
 * is typically used to pass collections around and manipulate them where
 * maximum generality is desired.
 *
 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
 * duplicate elements) should implement this interface directly.
 *
 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
 * typically implement <tt>Collection</tt> indirectly through one of its
 * subinterfaces) should provide two "standard" constructors: a void (no
 * arguments) constructor, which creates an empty collection, and a
 * constructor with a single argument of type <tt>Collection</tt>, which
 * creates a new collection with the same elements as its argument. In
 * effect, the latter constructor allows the user to copy any collection,
 * producing an equivalent collection of the desired implementation type.
 * There is no way to enforce this convention (as interfaces cannot contain
 * constructors) but all of the general-purpose <tt>Collection</tt>
 * implementations in the Java platform libraries comply.
 *
 * <p>The "destructive" methods contained in this interface, that is, the
 * methods that modify the collection on which they operate, are specified to
 * throw <tt>UnsupportedOperationException</tt> if this collection does not
 * support the operation. If this is the case, these methods may, but are not
 * required to, throw an <tt>UnsupportedOperationException</tt> if the
 * invocation would have no effect on the collection. For example, invoking
 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
 * but is not required to, throw the exception if the collection to be added
 * is empty.
 *
 * <p><a name="optional-restrictions">
 * Some collection implementations have restrictions on the elements that
 * they may contain.</a> For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements. Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter. More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the collection may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>It is up to each collection to determine its own synchronization
 * policy. In the absence of a stronger guarantee by the
 * implementation, undefined behavior may result from the invocation
 * of any method on a collection that is being mutated by another
 * thread; this includes direct invocations, passing the collection to
 * a method that might perform invocations, and using an existing
 * iterator to examine the collection.
 *
 * <p>Many methods in Collections Framework interfaces are defined in
 * terms of the {@link Object#equals(Object) equals} method. For example,
 * the specification for the {@link #contains(Object) contains(Object o)}
 * method says: "returns <tt>true</tt> if and only if this collection
 * contains at least one element <tt>e</tt> such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
 * invoked for any element <tt>e</tt>. Implementations are free to implement
 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
 * example, by first comparing the hash codes of the two elements. (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.) More generally, implementations of
 * the various Collections Framework interfaces are free to take advantage of
 * the specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * <p>Some collection operations which perform recursive traversal of the
 * collection may fail with an exception for self-referential instances where
 * the collection directly or indirectly contains itself. This includes the
 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
 * methods. Implementations may optionally handle the self-referential scenario,
 * however most current implementations do not do so.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @implSpec
 * The default method implementations (inherited or otherwise) do not apply any
 * synchronization protocol. If a {@code Collection} implementation has a
 * specific synchronization protocol, then it must override default
 * implementations to apply that protocol.
 *
 * @param <E> the type of elements in this collection
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Set
 * @see List
 * @see Map
 * @see SortedSet
 * @see SortedMap
 * @see HashSet
 * @see TreeSet
 * @see ArrayList
 * @see LinkedList
 * @see Vector
 * @see Collections
 * @see Arrays
 * @see AbstractCollection
 * @since 1.2
 */

public interface Collection<E> extends Iterable<E> {
 // Query Operations

 /**
 * Returns the number of elements in this collection. If this collection
 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
 * <tt>Integer.MAX_VALUE</tt>.
 *
 * @return the number of elements in this collection
 */
 int size();

 /**
 * Returns <tt>true</tt> if this collection contains no elements.
 *
 * @return <tt>true</tt> if this collection contains no elements
 */
 boolean isEmpty();

 /**
 * Returns <tt>true</tt> if this collection contains the specified element.
 * More formally, returns <tt>true</tt> if and only if this collection
 * contains at least one element <tt>e</tt> such that
 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
 *
 * @param o element whose presence in this collection is to be tested
 * @return <tt>true</tt> if this collection contains the specified
 * element
 * @throws ClassCastException if the type of the specified element
 * is incompatible with this collection
 * (<a href="#optional-restrictions">optional</a>)
 * @throws NullPointerException if the specified element is null and this
 * collection does not permit null elements
 * (<a href="#optional-restrictions">optional</a>)
 */
 boolean contains(Object o);

 /**
 * Returns an iterator over the elements in this collection. There are no
 * guarantees concerning the order in which the elements are returned
 * (unless this collection is an instance of some class that provides a
 * guarantee).
 *
 * @return an <tt>Iterator</tt> over the elements in this collection
 */
 Iterator<E> iterator();

 /**
 * Returns an array containing all of the elements in this collection.
 * If this collection makes any guarantees as to what order its elements
 * are returned by its iterator, this method must return the elements in
 * the same order.
 *
 * <p>The returned array will be "safe" in that no references to it are
 * maintained by this collection. (In other words, this method must
 * allocate a new array even if this collection is backed by an array).
 * The caller is thus free to modify the returned array.
 *
 * <p>This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this collection
 */
 Object[] toArray();

 /**
 * Returns an array containing all of the elements in this collection;
 * the runtime type of the returned array is that of the specified array.
 * If the collection fits in the specified array, it is returned therein.
 * Otherwise, a new array is allocated with the runtime type of the
 * specified array and the size of this collection.
 *
 * <p>If this collection fits in the specified array with room to spare
 * (i.e., the array has more elements than this collection), the element
 * in the array immediately following the end of the collection is set to
 * <tt>null</tt>. (This is useful in determining the length of this
 * collection <i>only</i> if the caller knows that this collection does
 * not contain any <tt>null</tt> elements.)
 *
 * <p>If this collection makes any guarantees as to what order its elements
 * are returned by its iterator, this method must return the elements in
 * the same order.
 *
 * <p>Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs. Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
 * The following code can be used to dump the collection into a newly
 * allocated array of <tt>String</tt>:
 *
 * <pre>
 * String[] y = x.toArray(new String[0]);</pre>
 *
 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 * <tt>toArray()</tt>.
 *
 * @param <T> the runtime type of the array to contain the collection
 * @param a the array into which the elements of this collection are to be
 * stored, if it is big enough; otherwise, a new array of the same
 * runtime type is allocated for this purpose.
 * @return an array containing all of the elements in this collection
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in
 * this collection
 * @throws NullPointerException if the specified array is null
 */
 <T> T[] toArray(T[] a);

 // Modification Operations

 /**
 * Ensures that this collection contains the specified element (optional
 * operation). Returns <tt>true</tt> if this collection changed as a
 * result of the call. (Returns <tt>false</tt> if this collection does
 * not permit duplicates and already contains the specified element.)<p>
 *
 * Collections that support this operation may place limitations on what
 * elements may be added to this collection. In particular, some
 * collections will refuse to add <tt>null</tt> elements, and others will
 * impose restrictions on the type of elements that may be added.
 * Collection classes should clearly specify in their documentation any
 * restrictions on what elements may be added.<p>
 *
 * If a collection refuses to add a particular element for any reason
 * other than that it already contains the element, it <i>must</i> throw
 * an exception (rather than returning <tt>false</tt>). This preserves
 * the invariant that a collection always contains the specified element
 * after this call returns.
 *
 * @param e element whose presence in this collection is to be ensured
 * @return <tt>true</tt> if this collection changed as a result of the
 * call
 * @throws UnsupportedOperationException if the <tt>add</tt> operation
 * is not supported by this collection
 * @throws ClassCastException if the class of the specified element
 * prevents it from being added to this collection
 * @throws NullPointerException if the specified element is null and this
 * collection does not permit null elements
 * @throws IllegalArgumentException if some property of the element
 * prevents it from being added to this collection
 * @throws IllegalStateException if the element cannot be added at this
 * time due to insertion restrictions
 */
 boolean add(E e);

 /**
 * Removes a single instance of the specified element from this
 * collection, if it is present (optional operation). More formally,
 * removes an element <tt>e</tt> such that
 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
 * this collection contains one or more such elements. Returns
 * <tt>true</tt> if this collection contained the specified element (or
 * equivalently, if this collection changed as a result of the call).
 *
 * @param o element to be removed from this collection, if present
 * @return <tt>true</tt> if an element was removed as a result of this call
 * @throws ClassCastException if the type of the specified element
 * is incompatible with this collection
 * (<a href="#optional-restrictions">optional</a>)
 * @throws NullPointerException if the specified element is null and this
 * collection does not permit null elements
 * (<a href="#optional-restrictions">optional</a>)
 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 * is not supported by this collection
 */
 boolean remove(Object o);


 // Bulk Operations

 /**
 * Returns <tt>true</tt> if this collection contains all of the elements
 * in the specified collection.
 *
 * @param c collection to be checked for containment in this collection
 * @return <tt>true</tt> if this collection contains all of the elements
 * in the specified collection
 * @throws ClassCastException if the types of one or more elements
 * in the specified collection are incompatible with this
 * collection
 * (<a href="#optional-restrictions">optional</a>)
 * @throws NullPointerException if the specified collection contains one
 * or more null elements and this collection does not permit null
 * elements
 * (<a href="#optional-restrictions">optional</a>),
 * or if the specified collection is null.
 * @see #contains(Object)
 */
 boolean containsAll(Collection<?> c);

 /**
 * Adds all of the elements in the specified collection to this collection
 * (optional operation). The behavior of this operation is undefined if
 * the specified collection is modified while the operation is in progress.
 * (This implies that the behavior of this call is undefined if the
 * specified collection is this collection, and this collection is
 * nonempty.)
 *
 * @param c collection containing elements to be added to this collection
 * @return <tt>true</tt> if this collection changed as a result of the call
 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
 * is not supported by this collection
 * @throws ClassCastException if the class of an element of the specified
 * collection prevents it from being added to this collection
 * @throws NullPointerException if the specified collection contains a
 * null element and this collection does not permit null elements,
 * or if the specified collection is null
 * @throws IllegalArgumentException if some property of an element of the
 * specified collection prevents it from being added to this
 * collection
 * @throws IllegalStateException if not all the elements can be added at
 * this time due to insertion restrictions
 * @see #add(Object)
 */
 boolean addAll(Collection<? extends E> c);

 /**
 * Removes all of this collection's elements that are also contained in the
 * specified collection (optional operation). After this call returns,
 * this collection will contain no elements in common with the specified
 * collection.
 *
 * @param c collection containing elements to be removed from this collection
 * @return <tt>true</tt> if this collection changed as a result of the
 * call
 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
 * is not supported by this collection
 * @throws ClassCastException if the types of one or more elements
 * in this collection are incompatible with the specified
 * collection
 * (<a href="#optional-restrictions">optional</a>)
 * @throws NullPointerException if this collection contains one or more
 * null elements and the specified collection does not support
 * null elements
 * (<a href="#optional-restrictions">optional</a>),
 * or if the specified collection is null
 * @see #remove(Object)
 * @see #contains(Object)
 */
 boolean removeAll(Collection<?> c);

 /**
 * Removes all of the elements of this collection that satisfy the given
 * predicate. Errors or runtime exceptions thrown during iteration or by
 * the predicate are relayed to the caller.
 *
 * @implSpec
 * The default implementation traverses all elements of the collection using
 * its {@link #iterator}. Each matching element is removed using
 * {@link Iterator#remove()}. If the collection's iterator does not
 * support removal then an {@code UnsupportedOperationException} will be
 * thrown on the first matching element.
 *
 * @param filter a predicate which returns {@code true} for elements to be
 * removed
 * @return {@code true} if any elements were removed
 * @throws NullPointerException if the specified filter is null
 * @throws UnsupportedOperationException if elements cannot be removed
 * from this collection. Implementations may throw this exception if a
 * matching element cannot be removed or if, in general, removal is not
 * supported.
 * @since 1.8
 */
 default boolean removeIf(Predicate<? super E> filter) {
 Objects.requireNonNull(filter);
 boolean removed = false;
 final Iterator<E> each = iterator();
 while (each.hasNext()) {
 if (filter.test(each.next())) {
 each.remove();
 removed = true;
 }
 }
 return removed;
 }

 /**
 * Retains only the elements in this collection that are contained in the
 * specified collection (optional operation). In other words, removes from
 * this collection all of its elements that are not contained in the
 * specified collection.
 *
 * @param c collection containing elements to be retained in this collection
 * @return <tt>true</tt> if this collection changed as a result of the call
 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
 * is not supported by this collection
 * @throws ClassCastException if the types of one or more elements
 * in this collection are incompatible with the specified
 * collection
 * (<a href="#optional-restrictions">optional</a>)
 * @throws NullPointerException if this collection contains one or more
 * null elements and the specified collection does not permit null
 * elements
 * (<a href="#optional-restrictions">optional</a>),
 * or if the specified collection is null
 * @see #remove(Object)
 * @see #contains(Object)
 */
 boolean retainAll(Collection<?> c);

 /**
 * Removes all of the elements from this collection (optional operation).
 * The collection will be empty after this method returns.
 *
 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 * is not supported by this collection
 */
 void clear();


 // Comparison and hashing

 /**
 * Compares the specified object with this collection for equality. <p>
 *
 * While the <tt>Collection</tt> interface adds no stipulations to the
 * general contract for the <tt>Object.equals</tt>, programmers who
 * implement the <tt>Collection</tt> interface "directly" (in other words,
 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
 * or a <tt>List</tt>) must exercise care if they choose to override the
 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
 * course of action is to rely on <tt>Object</tt>'s implementation, but
 * the implementor may wish to implement a "value comparison" in place of
 * the default "reference comparison." (The <tt>List</tt> and
 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
 *
 * The general contract for the <tt>Object.equals</tt> method states that
 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
 * collection class that implements neither the <tt>List</tt> nor
 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
 * is compared to any list or set. (By the same logic, it is not possible
 * to write a class that correctly implements both the <tt>Set</tt> and
 * <tt>List</tt> interfaces.)
 *
 * @param o object to be compared for equality with this collection
 * @return <tt>true</tt> if the specified object is equal to this
 * collection
 *
 * @see Object#equals(Object)
 * @see Set#equals(Object)
 * @see List#equals(Object)
 */
 boolean equals(Object o);

 /**
 * Returns the hash code value for this collection. While the
 * <tt>Collection</tt> interface adds no stipulations to the general
 * contract for the <tt>Object.hashCode</tt> method, programmers should
 * take note that any class that overrides the <tt>Object.equals</tt>
 * method must also override the <tt>Object.hashCode</tt> method in order
 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
 * In particular, <tt>c1.equals(c2)</tt> implies that
 * <tt>c1.hashCode()==c2.hashCode()</tt>.
 *
 * @return the hash code value for this collection
 *
 * @see Object#hashCode()
 * @see Object#equals(Object)
 */
 int hashCode();

 /**
 * Creates a {@link Spliterator} over the elements in this collection.
 *
 * Implementations should document characteristic values reported by the
 * spliterator. Such characteristic values are not required to be reported
 * if the spliterator reports {@link Spliterator#SIZED} and this collection
 * contains no elements.
 *
 * <p>The default implementation should be overridden by subclasses that
 * can return a more efficient spliterator. In order to
 * preserve expected laziness behavior for the {@link #stream()} and
 * {@link #parallelStream()}} methods, spliterators should either have the
 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
 * <em><a href="Spliterator.html#binding">late-binding</a></em>.
 * If none of these is practical, the overriding class should describe the
 * spliterator's documented policy of binding and structural interference,
 * and should override the {@link #stream()} and {@link #parallelStream()}
 * methods to create streams using a {@code Supplier} of the spliterator,
 * as in:
 * <pre>{@code
 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
 * }</pre>
 * <p>These requirements ensure that streams produced by the
 * {@link #stream()} and {@link #parallelStream()} methods will reflect the
 * contents of the collection as of initiation of the terminal stream
 * operation.
 *
 * @implSpec
 * The default implementation creates a
 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
 * from the collections's {@code Iterator}. The spliterator inherits the
 * <em>fail-fast</em> properties of the collection's iterator.
 * <p>
 * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
 *
 * @implNote
 * The created {@code Spliterator} additionally reports
 * {@link Spliterator#SUBSIZED}.
 *
 * <p>If a spliterator covers no elements then the reporting of additional
 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
 * does not aid clients to control, specialize or simplify computation.
 * However, this does enable shared use of an immutable and empty
 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
 * empty collections, and enables clients to determine if such a spliterator
 * covers no elements.
 *
 * @return a {@code Spliterator} over the elements in this collection
 * @since 1.8
 */
 @Override
 default Spliterator<E> spliterator() {
 return Spliterators.spliterator(this, 0);
 }

 /**
 * Returns a sequential {@code Stream} with this collection as its source.
 *
 * <p>This method should be overridden when the {@link #spliterator()}
 * method cannot return a spliterator that is {@code IMMUTABLE},
 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 * for details.)
 *
 * @implSpec
 * The default implementation creates a sequential {@code Stream} from the
 * collection's {@code Spliterator}.
 *
 * @return a sequential {@code Stream} over the elements in this collection
 * @since 1.8
 */
 default Stream<E> stream() {
 return StreamSupport.stream(spliterator(), false);
 }

 /**
 * Returns a possibly parallel {@code Stream} with this collection as its
 * source. It is allowable for this method to return a sequential stream.
 *
 * <p>This method should be overridden when the {@link #spliterator()}
 * method cannot return a spliterator that is {@code IMMUTABLE},
 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 * for details.)
 *
 * @implSpec
 * The default implementation creates a parallel {@code Stream} from the
 * collection's {@code Spliterator}.
 *
 * @return a possibly parallel {@code Stream} over the elements in this
 * collection
 * @since 1.8
 */
 default Stream<E> parallelStream() {
 return StreamSupport.stream(spliterator(), true);
 }
}