Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false 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.
This implementation always throws an UnsupportedOperationException.
e | element whose presence in this collection is to be ensured |
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.)
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 UnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).
c | collection containing elements to be added to this collection |
add(Object)
Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.
This implementation iterates over this collection, removing each element using the Iterator.remove operation. Most implementations will probably choose to override this method for efficiency.
Note that this implementation will throw an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection is non-empty.
Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).
This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
o | element whose presence in this collection is to be tested |
Returns true if this collection contains all of the elements in the specified collection.
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 true is returned, otherwise false.
c | collection to be checked for containment in this collection |
contains(Object)
Returns true if this collection contains no elements.
This implementation returns size() == 0.
Returns an iterator over the elements contained in this collection.
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).
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 UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection contains the specified object.
o | element to be removed from this collection, if present |
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.
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 remove method.
Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements in common with the specified collection.
c | collection containing elements to be removed from this collection |
remove(Object), contains(Object)
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.
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 remove method.
Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements not present in the specified collection.
c | collection containing elements to be retained in this collection |
remove(Object), contains(Object)
Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
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.
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.
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. The size method 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 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 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 String:
String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to
toArray().
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. The size method 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);
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. |
Diagram: Collection List