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package java.util;
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.
Author: Josh Bloch, Neal Gafter Type parameters: - <E> – the type of elements maintained by this set
See Also: Since: 1.2
/**
* This class provides a skeletal implementation of the {@code Set}
* interface to minimize the effort required to implement this
* interface. <p>
*
* 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 {@code Set}
* interface (for instance, the add method must not permit addition of
* multiple instances of an object to a set).<p>
*
* Note that this class does not override any of the implementations from
* the {@code AbstractCollection} class. It merely adds implementations
* for {@code equals} and {@code hashCode}.<p>
*
* This class is a member of the
* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
*
* @param <E> the type of elements maintained by this set
*
* @author Josh Bloch
* @author Neal Gafter
* @see Collection
* @see AbstractCollection
* @see Set
* @since 1.2
*/
public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
Sole constructor. (For invocation by subclass constructors, typically
implicit.)
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractSet() {
}
// Comparison and hashing
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).
Params: - o – object to be compared for equality with this set
Returns: true if the specified object is equal to this set
/**
* Compares the specified object with this set for equality. Returns
* {@code 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 {@code equals} method works
* properly across different implementations of the {@code Set}
* interface.<p>
*
* This implementation first checks if the specified object is this
* set; if so it returns {@code 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
* {@code containsAll((Collection) o)}.
*
* @param o object to be compared for equality with this set
* @return {@code true} if the specified object is equal to this set
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection<?> c = (Collection<?>) o;
if (c.size() != size())
return false;
try {
return containsAll(c);
} catch (ClassCastException | NullPointerException unused) {
return false;
}
}
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.
See Also: Returns: the hash code value for this set
/**
* 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 {@code null} element is defined to be zero.
* This ensures that {@code s1.equals(s2)} implies that
* {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
* and {@code s2}, as required by the general contract of
* {@link Object#hashCode}.
*
* <p>This implementation iterates over the set, calling the
* {@code hashCode} method on each element in the set, and adding up
* the results.
*
* @return the hash code value for this set
* @see Object#equals(Object)
* @see Set#equals(Object)
*/
public int hashCode() {
int h = 0;
Iterator<E> i = iterator();
while (i.hasNext()) {
E obj = i.next();
if (obj != null)
h += obj.hashCode();
}
return h;
}
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.
Params: - c – collection containing elements to be removed from this set
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: Returns: true if this set changed as a result of the call
/**
* 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.
*
* <p>This implementation determines which is the smaller of this set
* and the specified collection, by invoking the {@code 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 {@code 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 {@code remove} method.
*
* <p>Note that this implementation will throw an
* {@code UnsupportedOperationException} if the iterator returned by the
* {@code iterator} method does not implement the {@code remove} method.
*
* @param c collection containing elements to be removed from this set
* @return {@code true} if this set changed as a result of the call
* @throws UnsupportedOperationException if the {@code removeAll} 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)
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
boolean modified = false;
if (size() > c.size()) {
for (Object e : c)
modified |= remove(e);
} else {
for (Iterator<?> i = iterator(); i.hasNext(); ) {
if (c.contains(i.next())) {
i.remove();
modified = true;
}
}
}
return modified;
}
}