AbstractSet (Java SE 15 & JDK 15)
Description: declaration: module: java.base, package: java.util, class: AbstractSet
Keywords: java.util.AbstractSet class

Module java.base
Package java.util

# Class AbstractSet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>

Type Parameters:
  `E` - the type of elements maintained by this set

All Implemented Interfaces:
  `Iterable<E>`, `Collection<E>`, `Set<E>`

Direct Known Subclasses:
  `ConcurrentSkipListSet`, `CopyOnWriteArraySet`, `EnumSet`, `HashSet`, `TreeSet`

    public abstract class AbstractSet<E>
    extends AbstractCollection<E>
    implements Set<E>

This class provides a skeletal implementation of the `Set` interface to minimize the effort required to implement this interface.

The process of implementing a set by extending this class is identical to that of implementing a Collection by extending AbstractCollection, except that all of the methods and constructors in subclasses of this class must obey the additional constraints imposed by the `Set` interface (for instance, the add method must not permit addition of multiple instances of an object to a set).

Note that this class does not override any of the implementations from the `AbstractCollection` class. It merely adds implementations for `equals` and `hashCode`.

This class is a member of the Java Collections Framework.

Since:
  1.2
See Also:
  `Collection`, `AbstractCollection`, `Set`

- ## Constructor Summary

  Constructors
  Modifier | Constructor | Description
  --- | --- | ---
  `protected` | `AbstractSet()` | Sole constructor.

- ## Method Summary

  Modifier and Type | Method | Description
  --- | --- | ---
  `boolean` | `equals​(Object o)` | Compares the specified object with this set for equality.
  `int` | `hashCode()` | Returns the hash code value for this set.
  `boolean` | `removeAll​(Collection<?> c)` | Removes from this set all of its elements that are contained in the specified collection (optional operation).

  ### Methods declared in class java.util.AbstractCollection

  `add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, retainAll, toArray, toArray, toString`

  ### Methods declared in class java.lang.Object

  `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface java.util.Collection

  `parallelStream, removeIf, stream, toArray`

  ### Methods declared in interface java.lang.Iterable

  `forEach`

  ### Methods declared in interface java.util.Set

  `add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, retainAll, size, spliterator, toArray, toArray`

- ## Constructor Details

  - ### AbstractSet

    protected AbstractSet()
    Sole constructor. (For invocation by subclass constructors, typically implicit.)

- ## Method Details

  - ### equals

    public boolean equals​(Object o)
    Compares the specified object with this set for equality. Returns `true` if the given object is also a set, the two sets have the same size, and every member of the given set is contained in this set. This ensures that the `equals` method works properly across different implementations of the `Set` interface.

    This implementation first checks if the specified object is this set; if so it returns `true`. Then, it checks if the specified object is a set whose size is identical to the size of this set; if not, it returns false. If so, it returns `containsAll((Collection) o)`.

    Specified by:
      `equals` in interface `Collection<E>`
    Specified by:
      `equals` in interface `Set<E>`
    Overrides:
      `equals` in class `Object`
    Parameters:
      `o` - object to be compared for equality with this set
    Returns:
      `true` if the specified object is equal to this set
    See Also:
      `Object.hashCode()`, `HashMap`

  - ### hashCode

    public int hashCode()
    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 `null` element is defined to be zero. This ensures that `s1.equals(s2)` implies that `s1.hashCode()==s2.hashCode()` for any two sets `s1` and `s2`, as required by the general contract of `Object.hashCode()`.

    This implementation iterates over the set, calling the `hashCode` method on each element in the set, and adding up the results.

    Specified by:
      `hashCode` in interface `Collection<E>`
    Specified by:
      `hashCode` in interface `Set<E>`
    Overrides:
      `hashCode` in class `Object`
    Returns:
      the hash code value for this set
    See Also:
      `Object.equals(Object)`, `Set.equals(Object)`

  - ### removeAll

    public boolean removeAll​(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 asymmetric set difference of the two sets.

    This implementation determines which is the smaller of this set and the specified collection, by invoking the `size` method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's `remove` method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set'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.

    Specified by:
      `removeAll` in interface `Collection<E>`
    Specified by:
      `removeAll` in interface `Set<E>`
    Overrides:
      `removeAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be removed from this set
    Returns:
      `true` if this set changed as a result of the call
    Throws:
      `UnsupportedOperationException` - if the `removeAll` operation is not supported by this set
      `ClassCastException` - if the class of an element of this set is incompatible with the specified collection (optional)
      `NullPointerException` - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
    See Also:
      `AbstractCollection.remove(Object)`, `AbstractCollection.contains(Object)`