Module org.apache.sis.util
Package org.apache.sis.util.collection

Class WeakHashSet<E>

Object
AbstractCollection<E>
AbstractSet<E>
WeakHashSet<E>
Type Parameters:
E - the type of elements in the set.
All Implemented Interfaces:
Iterable<E>, Collection<E>, Set<E>, Checked­Container<E>
public class WeakHashSet<E> extends AbstractSet<E> implements CheckedContainer<E>
A set of objects hold by weak references. An entry in a Weak­Hash­Set will automatically be removed when it is no longer in ordinary use. More precisely, the presence of an entry will not prevent the entry from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When an entry has been discarded it is effectively removed from the set, so this class behaves somewhat differently than other Set implementations.

If the elements stored in this set are arrays like int[], float[] or Object[], then the hash code computations and the comparisons are performed using the static hash­Code(a) and equals(a1, a2) methods defined in the Arrays class.

Optimizing memory use in factory implementations

The Weak­Hash­Set class has a get(Object) method that is not part of the Set interface. This get method retrieves an entry from this set that is equal to the supplied object. The unique(Object) method combines a get followed by a add operation if the specified object was not in the set. This is similar in spirit to the String​.intern() method. The following example shows a convenient way to use Weak­Hash­Set as an internal pool of immutable objects: 
class MyClass {
    private final WeakHashSet<Foo> pool = new WeakHashSet<Foo>(Foo.class);

    public Foo create(String definition) {
        Foo created = new Foo(definition);
        return pool.unique(created);
    }
}
Thus, Weak­Hash­Set can be used inside a factory to prevent creating duplicate immutable objects.

Thread safety

The same Weak­Hash­Set instance can be safely used by many threads without synchronization on the part of the caller. But if a sequence of two or more method calls need to appear atomic from other threads perspective, then the caller can synchronize on this.
Since:
0.3
See Also:

  • Constructor Details

    • WeakHashSet

      public WeakHashSet(Class<E> type)
      Creates a Weak­Hash­Set for elements of the specified type.
      Parameters:
      type - the type of the element to be included in this set.

  • Method Details

    • getElementType

      public Class<E> getElementType()
      Returns the type of elements in this set.
      Specified by:
      get­Element­Type in interface Checked­Container<E>
      Returns:
      the element type.

    • size

      public int size()
      Returns the count of element in this set.
      Specified by:
      size in interface Collection<E>
      Specified by:
      size in interface Set<E>
      Specified by:
      size in class Abstract­Collection<E>
      Returns:
      number of elements in this set.

    • add

      public boolean add(E element)
      Adds the specified element to this set if it is not already present. If this set already contains the specified element, the call leaves this set unchanged and returns false.
      Specified by:
      add in interface Collection<E>
      Specified by:
      add in interface Set<E>
      Overrides:
      add in class Abstract­Collection<E>
      Parameters:
      element - element to be added to this set.
      Returns:
      true if this set did not already contain the specified element.
      Throws:
      Null­Pointer­Exception - if the given object is null.

    • remove

      public boolean remove(Object element)
      Removes a single instance of the specified element from this set, if it is present Null values are considered never present.
      Specified by:
      remove in interface Collection<E>
      Specified by:
      remove in interface Set<E>
      Overrides:
      remove in class Abstract­Collection<E>
      Parameters:
      element - element to be removed from this set, if present. Can be null.
      Returns:
      true if the set contained the specified element.

    • get

      public E get(Object element)
      Returns an object equals to the specified object, if present. If this set doesn't contain any object equals to element, then this method returns null. Null values are considered never present.
      Parameters:
      element - the element to get.
      Returns:
      an element equals to the given one if already presents in the set, or null otherwise.
      See Also:

    • contains

      public boolean contains(Object element)
      Returns true if this set contains the specified element. Null values are considered never present.
      Specified by:
      contains in interface Collection<E>
      Specified by:
      contains in interface Set<E>
      Overrides:
      contains in class Abstract­Collection<E>
      Parameters:
      element - object to be checked for containment in this set. Can be null.
      Returns:
      true if this set contains the specified element.

    • unique

      public <T extends E> T unique(T element)
      Returns an object equals to element if such an object already exist in this Weak­Hash­Set. Otherwise, adds element to this Weak­Hash­Set. This method is functionally equivalents to the following code: 
      if (element != null) {
    T current = get(element);
    if (current != null) {
        return current;
    } else {
        add(element);
    }
}
return element;
      Type Parameters:
      T - the type of the element to get. Can be null.
      Parameters:
      element - the element to get or to add in the set if not already presents, or null if the given element was null.
      Returns:
      an element equals to the given one if already presents in the set, or the given object otherwise.

    • clear

      public void clear()
      Removes all of the elements from this set.
      Specified by:
      clear in interface Collection<E>
      Specified by:
      clear in interface Set<E>
      Overrides:
      clear in class Abstract­Collection<E>

    • toArray

      public E[] toArray()
      Returns a view of this set as an array. Elements will be in an arbitrary order. Note that this array contains strong references. Consequently, no object reclamation will occur as long as a reference to this array is hold.
      Specified by:
      to­Array in interface Collection<E>
      Specified by:
      to­Array in interface Set<E>
      Overrides:
      to­Array in class Abstract­Collection<E>
      Returns:
      all elements in this set.

    • iterator

      public Iterator<E> iterator()
      Returns an iterator over the elements contained in this collection. No element from this set will be garbage collected as long as a reference to the iterator is hold.
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in interface Set<E>
      Specified by:
      iterator in class Abstract­Collection<E>
      Returns:
      an iterator over all elements in this set.