java.util.LinkedHashSet Maven / Gradle / Ivy
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package java.util;
/**
* Hash table and linked list implementation of the Set interface,
* with predictable iteration order. This implementation differs from
* HashSet in that it maintains a doubly-linked list running through
* all of its entries. This linked list defines the iteration ordering,
* which is the order in which elements were inserted into the set
* (insertion-order). Note that insertion order is not affected
* if an element is re-inserted into the set. (An element e
* is reinserted into a set s if s.add(e) is invoked when
* s.contains(e) would return true immediately prior to
* the invocation.)
*
*
This implementation spares its clients from the unspecified, generally
* chaotic ordering provided by {@link HashSet}, without incurring the
* increased cost associated with {@link TreeSet}. It can be used to
* produce a copy of a set that has the same order as the original, regardless
* of the original set's implementation:
*
* void foo(Set s) {
* Set copy = new LinkedHashSet(s);
* ...
* }
*
* This technique is particularly useful if a module takes a set on input,
* copies it, and later returns results whose order is determined by that of
* the copy. (Clients generally appreciate having things returned in the same
* order they were presented.)
*
* This class provides all of the optional Set operations, and
* permits null elements. Like HashSet, it provides constant-time
* performance for the basic operations (add, contains and
* remove), assuming the hash function disperses elements
* properly among the buckets. Performance is likely to be just slightly
* below that of HashSet, due to the added expense of maintaining the
* linked list, with one exception: Iteration over a LinkedHashSet
* requires time proportional to the size of the set, regardless of
* its capacity. Iteration over a HashSet is likely to be more
* expensive, requiring time proportional to its capacity.
*
*
A linked hash set has two parameters that affect its performance:
* initial capacity and load factor. They are defined precisely
* as for HashSet. Note, however, that the penalty for choosing an
* excessively high value for initial capacity is less severe for this class
* than for HashSet, as iteration times for this class are unaffected
* by capacity.
*
*
Note that this implementation is not synchronized.
* If multiple threads access a linked hash set concurrently, and at least
* one of the threads modifies the set, it must be synchronized
* externally. This is typically accomplished by synchronizing on some
* object that naturally encapsulates the set.
*
* If no such object exists, the set should be "wrapped" using the
* {@link Collections#synchronizedSet Collections.synchronizedSet}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the set:
* Set s = Collections.synchronizedSet(new LinkedHashSet(...));
*
* The iterators returned by this class's iterator method are
* fail-fast: if the set is modified at any time after the iterator
* is created, in any way except through the iterator's own remove
* method, the iterator will throw a {@link ConcurrentModificationException}.
* Thus, in the face of concurrent modification, the iterator fails quickly
* and cleanly, rather than risking arbitrary, non-deterministic behavior at
* an undetermined time in the future.
*
*
Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw ConcurrentModificationException on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: the fail-fast behavior of iterators
* should be used only to detect bugs.
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @param the type of elements maintained by this set
*
* @author Josh Bloch
* @see Object#hashCode()
* @see Collection
* @see Set
* @see HashSet
* @see TreeSet
* @see Hashtable
* @since 1.4
*/
public class LinkedHashSet
extends HashSet
implements Set, Cloneable, java.io.Serializable {
private static final long serialVersionUID = -2851667679971038690L;
/**
* Constructs a new, empty linked hash set with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity of the linked hash set
* @param loadFactor the load factor of the linked hash set
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
public LinkedHashSet(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor, true);
}
/**
* Constructs a new, empty linked hash set with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity of the LinkedHashSet
* @throws IllegalArgumentException if the initial capacity is less
* than zero
*/
public LinkedHashSet(int initialCapacity) {
super(initialCapacity, .75f, true);
}
/**
* Constructs a new, empty linked hash set with the default initial
* capacity (16) and load factor (0.75).
*/
public LinkedHashSet() {
super(16, .75f, true);
}
/**
* Constructs a new linked hash set with the same elements as the
* specified collection. The linked hash set is created with an initial
* capacity sufficient to hold the elements in the specified collection
* and the default load factor (0.75).
*
* @param c the collection whose elements are to be placed into
* this set
* @throws NullPointerException if the specified collection is null
*/
public LinkedHashSet(Collection c) {
super(Math.max(2*c.size(), 11), .75f, true);
addAll(c);
}
}