Interface - java.util.Set
Created by : Mr Dk.
2019 / 11 / 12 22:14
Nanjing, Jiangsu, China
Definition
public interface Set<E> extends Collection<E> {
}
包含无重复元素的集合
- 不包含两个元素
e1和e2使得e1.equals(e2) - 最多只能有一个
null
需要注意可变对象放在集合中的情况
- 尤其是对象的变化导致
equals()函数的行为也发生变化的情况 - 集合的行为将会变得不确定
某些 Set 的具体实现可能不允许 null 元素存在
/**
* A collection that contains no duplicate elements. More formally, sets
* contain no pair of elements <code>e1</code> and <code>e2</code> such that
* <code>e1.equals(e2)</code>, and at most one null element. As implied by
* its name, this interface models the mathematical <i>set</i> abstraction.
*
* <p>The <tt>Set</tt> interface places additional stipulations, beyond those
* inherited from the <tt>Collection</tt> interface, on the contracts of all
* constructors and on the contracts of the <tt>add</tt>, <tt>equals</tt> and
* <tt>hashCode</tt> methods. Declarations for other inherited methods are
* also included here for convenience. (The specifications accompanying these
* declarations have been tailored to the <tt>Set</tt> interface, but they do
* not contain any additional stipulations.)
*
* <p>The additional stipulation on constructors is, not surprisingly,
* that all constructors must create a set that contains no duplicate elements
* (as defined above).
*
* <p>Note: Great care must be exercised if mutable objects are used as set
* elements. The behavior of a set is not specified if the value of an object
* is changed in a manner that affects <tt>equals</tt> comparisons while the
* object is an element in the set. A special case of this prohibition is
* that it is not permissible for a set to contain itself as an element.
*
* <p>Some set implementations have restrictions on the elements that
* they may contain. 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</tt> or <tt>ClassCastException</tt>. 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 set 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>This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <E> the type of elements maintained by this set
*
* @author Josh Bloch
* @author Neal Gafter
* @see Collection
* @see List
* @see SortedSet
* @see HashSet
* @see TreeSet
* @see AbstractSet
* @see Collections#singleton(java.lang.Object)
* @see Collections#EMPTY_SET
* @since 1.2
*/
查询操作
/**
* Returns the number of elements in this set (its cardinality). If this
* set contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of elements in this set (its cardinality)
*/
int size();
/**
* Returns <tt>true</tt> if this set contains no elements.
*
* @return <tt>true</tt> if this set contains no elements
*/
boolean isEmpty();
/**
* Returns <tt>true</tt> if this set contains the specified element.
* More formally, returns <tt>true</tt> if and only if this set
* contains an element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this set is to be tested
* @return <tt>true</tt> if this set contains the specified element
* @throws ClassCastException if the type of the specified element
* is incompatible with this set
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* set does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean contains(Object o);
/**
* Returns an iterator over the elements in this set. The elements are
* returned in no particular order (unless this set is an instance of some
* class that provides a guarantee).
*
* @return an iterator over the elements in this set
*/
Iterator<E> iterator();
/**
* Returns an array containing all of the elements in this set.
* If this set 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 set. (In other words, this method must
* allocate a new array even if this set 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 the elements in this set
*/
Object[] toArray();
/**
* Returns an array containing all of the elements in this set; the
* runtime type of the returned array is that of the specified array.
* If the set 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 set.
*
* <p>If this set fits in the specified array with room to spare
* (i.e., the array has more elements than this set), the element in
* the array immediately following the end of the set is set to
* <tt>null</tt>. (This is useful in determining the length of this
* set <i>only</i> if the caller knows that this set does not contain
* any null elements.)
*
* <p>If this set 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 set known to contain only strings.
* The following code can be used to dump the set 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 a the array into which the elements of this set 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 the elements in this set
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in this
* set
* @throws NullPointerException if the specified array is null
*/
<T> T[] toArray(T[] a);
修改操作
/**
* Adds the specified element to this set if it is not already present
* (optional operation). More formally, adds the specified element
* <tt>e</tt> to this set if the set contains no element <tt>e2</tt>
* such that
* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns <tt>false</tt>. In combination with the
* restriction on constructors, this ensures that sets never contain
* duplicate elements.
*
* <p>The stipulation above does not imply that sets must accept all
* elements; sets may refuse to add any particular element, including
* <tt>null</tt>, and throw an exception, as described in the
* specification for {@link Collection#add Collection.add}.
* Individual set implementations should clearly document any
* restrictions on the elements that they may contain.
*
* @param e element to be added to this set
* @return <tt>true</tt> if this set did not already contain the specified
* element
* @throws UnsupportedOperationException if the <tt>add</tt> operation
* is not supported by this set
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this set
* @throws NullPointerException if the specified element is null and this
* set does not permit null elements
* @throws IllegalArgumentException if some property of the specified element
* prevents it from being added to this set
*/
boolean add(E e);
/**
* Removes the specified element from this set if it is present
* (optional operation). More formally, removes an element <tt>e</tt>
* such that
* <tt>(o==null ? e==null : o.equals(e))</tt>, if
* this set contains such an element. Returns <tt>true</tt> if this set
* contained the element (or equivalently, if this set changed as a
* result of the call). (This set will not contain the element once the
* call returns.)
*
* @param o object to be removed from this set, if present
* @return <tt>true</tt> if this set contained the specified element
* @throws ClassCastException if the type of the specified element
* is incompatible with this set
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* set does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws UnsupportedOperationException if the <tt>remove</tt> operation
* is not supported by this set
*/
boolean remove(Object o);
容量操作
/**
* Returns <tt>true</tt> if this set contains all of the elements of the
* specified collection. If the specified collection is also a set, this
* method returns <tt>true</tt> if it is a <i>subset</i> of this set.
*
* @param c collection to be checked for containment in this set
* @return <tt>true</tt> if this set contains all of the elements of the
* specified collection
* @throws ClassCastException if the types of one or more elements
* in the specified collection are incompatible with this
* set
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified collection contains one
* or more null elements and this set does not permit null
* elements
* (<a href="Collection.html#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 set if
* they're not already present (optional operation). If the specified
* collection is also a set, the <tt>addAll</tt> operation effectively
* modifies this set so that its value is the <i>union</i> of the two
* sets. The behavior of this operation is undefined if the specified
* collection is modified while the operation is in progress.
*
* @param c collection containing elements to be added to this set
* @return <tt>true</tt> if this set changed as a result of the call
*
* @throws UnsupportedOperationException if the <tt>addAll</tt> operation
* is not supported by this set
* @throws ClassCastException if the class of an element of the
* specified collection prevents it from being added to this set
* @throws NullPointerException if the specified collection contains one
* or more null elements and this set 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 set
* @see #add(Object)
*/
boolean addAll(Collection<? extends E> c);
/**
* Retains only the elements in this set that are contained in the
* specified collection (optional operation). In other words, removes
* from this set all of its elements that are not contained in the
* specified collection. If the specified collection is also a set, this
* operation effectively modifies this set so that its value is the
* <i>intersection</i> of the two sets.
*
* @param c collection containing elements to be retained in this set
* @return <tt>true</tt> if this set changed as a result of the call
* @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
* is not supported by this set
* @throws ClassCastException if the class of an element of this set
* is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this set contains a null element and the
* specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see #remove(Object)
*/
boolean retainAll(Collection<?> c);
/**
* Removes from this set all of its elements that are contained in the
* specified collection (optional operation). If the specified
* collection is also a set, this operation effectively modifies this
* set so that its value is the <i>asymmetric set difference</i> of
* the two sets.
*
* @param c collection containing elements to be removed from this set
* @return <tt>true</tt> if this set changed as a result of the call
* @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
* is not supported by this set
* @throws ClassCastException if the class of an element of this set
* is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this set contains a null element and the
* specified collection does not permit null elements
* (<a href="Collection.html#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 from this set (optional operation).
* The set will be empty after this call returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> method
* is not supported by this set
*/
void clear();
比较操作
/**
* Compares the specified object with this set for equality. Returns
* <tt>true</tt> if the specified object is also a set, the two sets
* have the same size, and every member of the specified set is
* contained in this set (or equivalently, every member of this set is
* contained in the specified set). This definition ensures that the
* equals method works properly across different implementations of the
* set interface.
*
* @param o object to be compared for equality with this set
* @return <tt>true</tt> if the specified object is equal to this set
*/
boolean equals(Object o);
/**
* Returns the hash code value for this set. The hash code of a set is
* defined to be the sum of the hash codes of the elements in the set,
* where the hash code of a <tt>null</tt> element is defined to be zero.
* This ensures that <tt>s1.equals(s2)</tt> implies that
* <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
* and <tt>s2</tt>, as required by the general contract of
* {@link Object#hashCode}.
*
* @return the hash code value for this set
* @see Object#equals(Object)
* @see Set#equals(Object)
*/
int hashCode();