Interface HCollection
- All Known Subinterfaces:
HSet
Bags or multisets (unordered collections that may contain duplicate elements) should implement this interface directly.
All general-purpose Collection implementation classes (which typically implement 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 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 Collection implementations in the Java platform libraries comply.
The "destructive" methods contained in this interface, that is, the methods
that modify the collection on which they operate, are specified to throw
UnsupportedOperationException if this collection does not support
the operation. If this is the case, these methods may, but are not required
to, throw an UnsupportedOperationException if the invocation would
have no effect on the collection. For example, invoking the
addAll(HCollection)
method on an unmodifiable collection may, but is
not required to, throw the exception if the collection to be added is empty.
Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException. 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.
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Method Summary
Modifier and TypeMethodDescriptionboolean
Ensures that this collection contains the specified element (optional operation).boolean
addAll
(HCollection coll) Adds all of the elements in the specified collection to this collection (optional operation).void
clear()
Removes all of the elements from this collection (optional operation).boolean
Returnstrue
if this collection contains the specified element.boolean
containsAll
(HCollection coll) Returnstrue
if this collection contains all of the elements in the specified collection.boolean
Compares the specified object with this collection for equality.int
hashCode()
Returns the hash code value for this collection.boolean
isEmpty()
Returnstrue
if this collection contains no elements.iterator()
Returns an iterator over the elements in this collection.boolean
Removes a single instance of the specified element from this collection, if it is present (optional operation).boolean
removeAll
(HCollection coll) Removes all of this collection's elements that are also contained in the specified collection (optional operation).boolean
retainAll
(HCollection coll) Retains only the elements in this collection that are contained in the specified collection (optional operation).int
size()
Returns the number of elements in this collection.Object[]
toArray()
Returns an array containing all of the elements in this collection.Object[]
Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
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Method Details
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size
int size()Returns the number of elements in this collection. If this collection contains more thanInteger.MAX_VALUE
elements, returnsInteger.MAX_VALUE
.- Returns:
- the number of elements in this collection
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isEmpty
boolean isEmpty()Returnstrue
if this collection contains no elements.- Returns:
true
if this collection contains no elements
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contains
Returnstrue
if this collection contains the specified element. More formally, returnstrue
if and only if this collection contains at least one elemente
such thatObjects.equals(o, e)
.- Parameters:
o
- element whose presence in this collection is to be tested- Returns:
true
if this collection contains the specified element
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iterator
HIterator iterator()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).- Returns:
- an
Iterator
over the elements in this collection
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toArray
Object[] toArray()Returns an array containing all of the elements in this collection. If the 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 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.
This method acts as bridge between array-based and collection-based APIs.
- Returns:
- an array containing all of the elements in this collection
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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.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 any null elements.)
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.
Like the 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
Suppose l is a List known to contain only strings. The following code can be used to dump the list into a newly allocated array of String:
String[] x = (String[]) v.toArray(new String[0]);
Note that
toArray(new Object[0])
is identical in function totoArray()
.- Parameters:
arrayTarget
- 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:
NullPointerException
- if the specified array is null.
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add
Ensures that this collection contains the specified element (optional operation). Returnstrue
if this collection changed as a result of the call. (Returnsfalse
if 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
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.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.- Parameters:
obj
- element whose presence in this collection is to be ensured- Returns:
true
if this collection changed as a result of the call
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remove
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an elemente
such thatObjects.equals(o, e)
, if this collection contains one or more such elements. Returnstrue
if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).- Parameters:
obj
- element to be removed from this collection, if present- Returns:
true
if an element was removed as a result of this call
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containsAll
Returnstrue
if this collection contains all of the elements in the specified collection.- Parameters:
coll
- collection to be checked for containment in this collection- Returns:
true
if this collection contains all of the elements in the specified collection- Throws:
NullPointerException
- if the specified collection is null.- See Also:
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addAll
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.)- Parameters:
coll
- collection containing elements to be added to this collection- Returns:
true
if this collection changed as a result of the call- Throws:
NullPointerException
- if the specified collection is null.- See Also:
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removeAll
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.- Parameters:
coll
- collection containing elements to be removed from this collection- Returns:
true
if this collection changed as a result of the call- Throws:
NullPointerException
- if the specified collection is null.- See Also:
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retainAll
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.- Parameters:
coll
- collection containing elements to be retained in this collection- Returns:
true
if this collection changed as a result of the call- Throws:
NullPointerException
- if the specified collection is null.- See Also:
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clear
void clear()Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns. -
equals
Compares the specified object with this collection for equality.While the
Collection
interface adds no stipulations to the general contract for theObject.equals
, programmers who implement theCollection
interface "directly" (in other words, create a class that is aCollection
but is not aSet
or aList
) must exercise care if they choose to override theObject.equals
. It is not necessary to do so, and the simplest course of action is to rely onObject
's implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (TheList
andSet
interfaces mandate such value comparisons.)The general contract for the
Object.equals
method states that equals must be symmetric (in other words,a.equals(b)
if and only ifb.equals(a)
). The contracts forList.equals
andSet.equals
state that lists are only equal to other lists, and sets to other sets. Thus, a customequals
method for a collection class that implements neither theList
norSet
interface must returnfalse
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 theSet
andList
interfaces.)- Overrides:
equals
in classObject
- Parameters:
o
- object to be compared for equality with this collection- Returns:
true
if the specified object is equal to this collection- See Also:
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Object.equals(Object)
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invalid @see
Set#equals(Object)
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invalid @see
List#equals(Object)
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hashCode
int hashCode()Returns the hash code value for this collection. While theCollection
interface adds no stipulations to the general contract for theObject.hashCode
method, programmers should take note that any class that overrides theObject.equals
method must also override theObject.hashCode
method in order to satisfy the general contract for theObject.hashCode
method. In particular,c1.equals(c2)
implies thatc1.hashCode()==c2.hashCode()
.
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