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);
}}