- Type Parameters:
- E- the type of elements in this collection
- All Implemented Interfaces:
- Iterable<E>,- Collection<E>
- Direct Known Subclasses:
- AbstractList,- AbstractQueue,- AbstractSet,- ArrayDeque,- ConcurrentLinkedDeque
Collection
 interface, to minimize the effort required to implement this interface. 
 To implement an unmodifiable collection, the programmer needs only to
 extend this class and provide implementations for the iterator and
 size methods.  (The iterator returned by the iterator
 method must implement hasNext and next.)
 To implement a modifiable collection, the programmer must additionally
 override this class's add method (which otherwise throws an
 UnsupportedOperationException), and the iterator returned by the
 iterator method must additionally implement its remove
 method.
 The programmer should generally provide a void (no argument) and
 Collection constructor, as per the recommendation in the
 Collection interface specification.
The documentation for each non-abstract method in this class describes its implementation in detail. Each of these methods may be overridden if the collection being implemented admits a more efficient implementation.
This class is a member of the Java Collections Framework.
- Since:
- 1.2
- See Also:
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Constructor SummaryConstructors
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Method SummaryModifier and TypeMethodDescriptionbooleanEnsures that this collection contains the specified element (optional operation).booleanaddAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).voidclear()Removes all of the elements from this collection (optional operation).booleanReturnstrueif this collection contains the specified element.booleancontainsAll(Collection<?> c) Returnstrueif this collection contains all of the elements in the specified collection.booleanisEmpty()Returnstrueif this collection contains no elements.iterator()Returns an iterator over the elements contained in this collection.booleanRemoves a single instance of the specified element from this collection, if it is present (optional operation).booleanremoveAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).booleanretainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).Object[]toArray()Returns an array containing all of the elements in this collection.<T> T[]toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.toString()Returns a string representation of this collection.Methods declared in class java.lang.Objectclone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, waitMethods declared in interface java.util.Collectionequals, hashCode, parallelStream, removeIf, size, spliterator, stream, toArray
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Constructor Details- 
AbstractCollectionprotected AbstractCollection()Sole constructor. (For invocation by subclass constructors, typically implicit.)
 
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Method Details- 
iteratorReturns an iterator over the elements contained in this collection.
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isEmptypublic boolean isEmpty()Returnstrueif this collection contains no elements.- Specified by:
- isEmptyin interface- Collection<E>
- Implementation Requirements:
- This implementation returns size() == 0.
- Returns:
- trueif this collection contains no elements
 
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containsReturnstrueif this collection contains the specified element. More formally, returnstrueif and only if this collection contains at least one elementesuch thatObjects.equals(o, e).- Specified by:
- containsin interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
- Parameters:
- o- element whose presence in this collection is to be tested
- Returns:
- trueif this collection contains the specified element
- Throws:
- ClassCastException- if the type of the specified element is incompatible with this collection (optional)
- NullPointerException- if the specified element is null and this collection does not permit null elements (optional)
 
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toArrayReturns 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. The returned array's runtime component type isObject.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. - Specified by:
- toArrayin interface- Collection<E>
- Implementation Requirements:
- This implementation returns an array containing all the elements
 returned by this collection's iterator, in the same order, stored in
 consecutive elements of the array, starting with index 0. The length of the returned array is equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. Thesizemethod is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.This method is equivalent to: List<E> list = new ArrayList<E>(size()); for (E e : this) list.add(e); return list.toArray();
- Returns:
- an array, whose runtime component
 type is Object, containing all of the elements in this collection
 
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toArraypublic <T> T[] toArray(T[] a) 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.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 null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain anynullelements.)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. - Specified by:
- toArrayin interface- Collection<E>
- Implementation Requirements:
- This implementation returns an array containing all the elements
 returned by this collection's iterator in the same order, stored in
 consecutive elements of the array, starting with index 0. If the number of elements returned by the iterator is too large to fit into the specified array, then the elements are returned in a newly allocated array with length equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. Thesizemethod is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.This method is equivalent to: List<E> list = new ArrayList<E>(size()); for (E e : this) list.add(e); return list.toArray(a);
- Type Parameters:
- T- the component type of the array to contain the collection
- Parameters:
- 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.
- Returns:
- an array containing all of the elements in this collection
- Throws:
- ArrayStoreException- if the runtime type of any element in this collection is not assignable to the runtime component type of the specified array
- NullPointerException- if the specified array is null
 
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addEnsures that this collection contains the specified element (optional operation). Returnstrueif this collection changed as a result of the call. (Returnsfalseif this collection does not permit duplicates and already contains the specified element.)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 nullelements, 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.If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns.- Specified by:
- addin interface- Collection<E>
- Implementation Requirements:
- This implementation always throws an
 UnsupportedOperationException.
- Parameters:
- e- element whose presence in this collection is to be ensured
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- UnsupportedOperationException- if the- addoperation is not supported by this collection
- ClassCastException- if the class of the specified element prevents it from being added to this collection
- NullPointerException- if the specified element is null and this collection does not permit null elements
- IllegalArgumentException- if some property of the element prevents it from being added to this collection
- IllegalStateException- if the element cannot be added at this time due to insertion restrictions
 
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removeRemoves a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an elementesuch thatObjects.equals(o, e), if this collection contains one or more such elements. Returnstrueif this collection contained the specified element (or equivalently, if this collection changed as a result of the call).- Specified by:
- removein interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over the collection looking for the
 specified element.  If it finds the element, it removes the element
 from the collection using the iterator's remove method.
 Note that this implementation throws an UnsupportedOperationExceptionif the iterator returned by this collection's iterator method does not implement theremovemethod and this collection contains the specified object.
- Parameters:
- o- element to be removed from this collection, if present
- Returns:
- trueif an element was removed as a result of this call
- Throws:
- UnsupportedOperationException- if the- removeoperation is not supported by this collection
- ClassCastException- if the type of the specified element is incompatible with this collection (optional)
- NullPointerException- if the specified element is null and this collection does not permit null elements (optional)
 
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containsAllReturnstrueif this collection contains all of the elements in the specified collection.- Specified by:
- containsAllin interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over the specified collection,
 checking each element returned by the iterator in turn to see
 if it's contained in this collection.  If all elements are so
 contained trueis returned, otherwisefalse.
- Parameters:
- c- collection to be checked for containment in this collection
- Returns:
- trueif 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 (optional)
- NullPointerException- if the specified collection contains one or more null elements and this collection does not permit null elements (optional) or if the specified collection is null.
- See Also:
 
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addAllAdds 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.) If the specified collection has a defined encounter order, processing of its elements generally occurs in that order.- Specified by:
- addAllin interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over the specified collection, and adds
 each object returned by the iterator to this collection, in turn.
 Note that this implementation will throw an UnsupportedOperationExceptionunlessaddis overridden (assuming the specified collection is non-empty).
- Parameters:
- c- collection containing elements to be added to this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- UnsupportedOperationException- if the- addAlloperation is not supported by this collection
- ClassCastException- if the class of an element of the specified collection prevents it from being added to this collection
- NullPointerException- if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null
- IllegalArgumentException- if some property of an element of the specified collection prevents it from being added to this collection
- IllegalStateException- if not all the elements can be added at this time due to insertion restrictions
- See Also:
 
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removeAllRemoves 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.- Specified by:
- removeAllin interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over this collection, checking each
 element returned by the iterator in turn to see if it's contained
 in the specified collection.  If it's so contained, it's removed from
 this collection with the iterator's removemethod.Note that this implementation will throw an UnsupportedOperationExceptionif the iterator returned by theiteratormethod does not implement theremovemethod and this collection contains one or more elements in common with the specified collection.
- Parameters:
- c- collection containing elements to be removed from this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- UnsupportedOperationException- if the- removeAllmethod is not supported by this collection
- ClassCastException- if the types of one or more elements in this collection are incompatible with the specified collection (optional)
- NullPointerException- if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null
- See Also:
 
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retainAllRetains 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.- Specified by:
- retainAllin interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over this collection, checking each
 element returned by the iterator in turn to see if it's contained
 in the specified collection.  If it's not so contained, it's removed
 from this collection with the iterator's removemethod.Note that this implementation will throw an UnsupportedOperationExceptionif the iterator returned by theiteratormethod does not implement theremovemethod and this collection contains one or more elements not present in the specified collection.
- Parameters:
- c- collection containing elements to be retained in this collection
- Returns:
- trueif this collection changed as a result of the call
- Throws:
- UnsupportedOperationException- if the- retainAlloperation is not supported by this collection
- ClassCastException- if the types of one or more elements in this collection are incompatible with the specified collection (optional)
- NullPointerException- if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null
- See Also:
 
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clearpublic void clear()Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.- Specified by:
- clearin interface- Collection<E>
- Implementation Requirements:
- This implementation iterates over this collection, removing each
 element using the Iterator.removeoperation. Most implementations will probably choose to override this method for efficiency.Note that this implementation will throw an UnsupportedOperationExceptionif the iterator returned by this collection'siteratormethod does not implement theremovemethod and this collection is non-empty.
- Throws:
- UnsupportedOperationException- if the- clearoperation is not supported by this collection
 
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toStringReturns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters", "(comma and space). Elements are converted to strings as byString.valueOf(Object).
 
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