The Collection.java Java example source code
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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. A collection
* represents a group of objects, known as its <i>elements. Some
* collections allow duplicate elements and others do not. Some are ordered
* and others unordered. The JDK does not provide any <i>direct
* implementations of this interface: it provides implementations of more
* specific subinterfaces like <tt>Set and List. This interface
* is typically used to pass collections around and manipulate them where
* maximum generality is desired.
*
* <p>Bags or multisets (unordered collections that may contain
* duplicate elements) should implement this interface directly.
*
* <p>All general-purpose Collection implementation classes (which
* typically implement <tt>Collection 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, 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
* 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 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 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>
* 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 or ClassCastException. 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 if and only if this collection
* contains at least one element <tt>e such that
* <tt>(o==null ? e==null : o.equals(e))." This specification should
* <i>not be construed to imply that invoking Collection.contains
* with a non-null argument <tt>o will cause o.equals(e) to be
* invoked for any element <tt>e. Implementations are free to implement
* optimizations whereby the <tt>equals 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 {
// Query Operations
/**
* Returns the number of elements in this collection. If this collection
* contains more than <tt>Integer.MAX_VALUE elements, returns
* <tt>Integer.MAX_VALUE.
*
* @return the number of elements in this collection
*/
int size();
/**
* Returns <tt>true if this collection contains no elements.
*
* @return <tt>true if this collection contains no elements
*/
boolean isEmpty();
/**
* Returns <tt>true if this collection contains the specified element.
* More formally, returns <tt>true if and only if this collection
* contains at least one element <tt>e such that
* <tt>(o==null ? e==null : o.equals(e)).
*
* @param o element whose presence in this collection is to be tested
* @return <tt>true 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)
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* (<a href="#optional-restrictions">optional)
*/
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 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. (This is useful in determining the length of this
* collection <i>only if the caller knows that this collection does
* not contain any <tt>null 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 x 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:
*
* <pre>
* String[] y = x.toArray(new String[0]);</pre>
*
* Note that <tt>toArray(new Object[0]) is identical in function to
* <tt>toArray().
*
* @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 if this collection changed as a
* result of the call. (Returns <tt>false 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 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 throw
* an exception (rather than returning <tt>false). 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 if this collection changed as a result of the
* call
* @throws UnsupportedOperationException if the <tt>add 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 such that
* <tt>(o==null ? e==null : o.equals(e)), if
* this collection contains one or more such elements. Returns
* <tt>true 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 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)
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* (<a href="#optional-restrictions">optional)
* @throws UnsupportedOperationException if the <tt>remove operation
* is not supported by this collection
*/
boolean remove(Object o);
// Bulk Operations
/**
* Returns <tt>true 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 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)
* @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),
* 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 if this collection changed as a result of the call
* @throws UnsupportedOperationException if the <tt>addAll 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 if this collection changed as a result of the
* call
* @throws UnsupportedOperationException if the <tt>removeAll 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)
* @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),
* 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 if this collection changed as a result of the call
* @throws UnsupportedOperationException if the <tt>retainAll 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)
* @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),
* 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 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 interface adds no stipulations to the
* general contract for the <tt>Object.equals, programmers who
* implement the <tt>Collection interface "directly" (in other words,
* create a class that is a <tt>Collection but is not a Set
* or a <tt>List) must exercise care if they choose to override the
* <tt>Object.equals. It is not necessary to do so, and the simplest
* course of action is to rely on <tt>Object's implementation, but
* the implementor may wish to implement a "value comparison" in place of
* the default "reference comparison." (The <tt>List and
* <tt>Set interfaces mandate such value comparisons.)
*
* The general contract for the <tt>Object.equals method states that
* equals must be symmetric (in other words, <tt>a.equals(b) if and
* only if <tt>b.equals(a)). The contracts for List.equals
* and <tt>Set.equals state that lists are only equal to other lists,
* and sets to other sets. Thus, a custom <tt>equals method for a
* collection class that implements neither the <tt>List nor
* <tt>Set interface must return false 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 and
* <tt>List interfaces.)
*
* @param o object to be compared for equality with this collection
* @return <tt>true 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 interface adds no stipulations to the general
* contract for the <tt>Object.hashCode method, programmers should
* take note that any class that overrides the <tt>Object.equals
* method must also override the <tt>Object.hashCode method in order
* to satisfy the general contract for the <tt>Object.hashCode method.
* In particular, <tt>c1.equals(c2) implies that
* <tt>c1.hashCode()==c2.hashCode().
*
* @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>late-binding.
* 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>late-binding spliterator
* from the collections's {@code Iterator}. The spliterator inherits the
* <em>fail-fast 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. (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. (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);
}
}
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