Module java.base
Package java.util

Class AbstractCollection<E>

java.lang.Object
java.util.AbstractCollection<E>
Type Parameters:
E - the type of elements in this collection
All Implemented Interfaces:
Iterable<E>, Collection<E>
Direct Known Subclasses:
AbstractList, AbstractQueue, AbstractSet, ArrayDeque, ConcurrentLinkedDeque
public abstract class AbstractCollection<E> extends Object implements Collection<E>
This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface.

To implement an unmodifiable collection, the programmer needs only to extend this class and provide implementations for the iterator and size methods. (The iterator returned by the iterator method must implement hasNext and next.)

To implement a modifiable collection, the programmer must additionally override this class's add method (which otherwise throws an UnsupportedOperationException), and the iterator returned by the iterator method must additionally implement its remove method.

The programmer should generally provide a void (no argument) and Collection constructor, as per the recommendation in the Collection interface specification.

The documentation for each non-abstract method in this class describes its implementation in detail. Each of these methods may be overridden if the collection being implemented admits a more efficient implementation.

This class is a member of the Java Collections Framework.

Since:
1.2
See Also:

  • Constructor Summary

    Constructors
    Modifier
    Constructor
    Description
    protected
    AbstractCollection()
    Sole constructor.

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    add(E e)
    Ensures that this collection contains the specified element (optional operation).
    boolean
    addAll(Collection<? extends E> c)
    Adds all of the elements in the specified collection to this collection (optional operation).
    void
    clear()
    Removes all of the elements from this collection (optional operation).
    boolean
    contains(Object o)
    Returns true if this collection contains the specified element.
    boolean
    containsAll(Collection<?> c)
    Returns true if this collection contains all of the elements in the specified collection.
    boolean
    isEmpty()
    Returns true if this collection contains no elements.
    abstract Iterator<E>
    iterator()
    Returns an iterator over the elements contained in this collection.
    boolean
    remove(Object o)
    Removes a single instance of the specified element from this collection, if it is present (optional operation).
    boolean
    removeAll(Collection<?> c)
    Removes all of this collection's elements that are also contained in the specified collection (optional operation).
    boolean
    retainAll(Collection<?> c)
    Retains only the elements in this collection that are contained in the specified collection (optional operation).
    Object[]
    toArray()
    Returns an array containing all of the elements in this collection.
    <T> T[]
    toArray(T[] a)
    Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
    String
    toString()
    Returns a string representation of this collection.

    Methods declared in class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

    Methods declared in interface java.util.Collection

    equals, hashCode, parallelStream, removeIf, size, spliterator, stream, toArray

    Methods declared in interface java.lang.Iterable

    forEach

  • Constructor Details

    • AbstractCollection

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

  • Method Details

    • iterator

      public abstract Iterator<E> iterator()
      Returns an iterator over the elements contained in this collection.
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Returns:
      an iterator over the elements contained in this collection

    • isEmpty

      public boolean isEmpty()
      Returns true if this collection contains no elements.
      Specified by:
      isEmpty in interface Collection<E>
      Implementation Requirements:
      This implementation returns size() == 0.
      Returns:
      true if this collection contains no elements

    • contains

      public boolean contains(Object o)
      Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that Objects.equals(o, e).
      Specified by:
      contains in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
      Parameters:
      o - element whose presence in this collection is to be tested
      Returns:
      true if this collection contains the specified element
      Throws:
      ClassCastException - if the type of the specified element is incompatible with this collection (optional)
      NullPointerException - if the specified element is null and this collection does not permit null elements (optional)

    • toArray

      public Object[] toArray()
      Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's runtime component type is Object.

      The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

      Specified by:
      toArray in interface Collection<E>
      Implementation Requirements:
      This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0. The length of the returned array is equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.

      This method is equivalent to: 

       
 List<E> list = new ArrayList<E>(size());
 for (E e : this)
     list.add(e);
 return list.toArray();
      Returns:
      an array, whose runtime component type is Object, containing all of the elements in this collection

    • toArray

      public <T> T[] toArray(T[] a)
      Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection 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 collection.

      If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.)

      If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

      Specified by:
      toArray in interface Collection<E>
      Implementation Requirements:
      This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index 0. If the number of elements returned by the iterator is too large to fit into the specified array, then the elements are returned in a newly allocated array with length equal to the number of elements returned by the iterator, even if the size of this collection changes during iteration, as might happen if the collection permits concurrent modification during iteration. The size method is called only as an optimization hint; the correct result is returned even if the iterator returns a different number of elements.

      This method is equivalent to: 

       
 List<E> list = new ArrayList<E>(size());
 for (E e : this)
     list.add(e);
 return list.toArray(a);
      Type Parameters:
      T - the component type of the array to contain the collection
      Parameters:
      a - the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
      Returns:
      an array containing all of the elements in this collection
      Throws:
      ArrayStoreException - if the runtime type of any element in this collection is not assignable to the runtime component type of the specified array
      NullPointerException - if the specified array is null

    • add

      public boolean add(E e)
      Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.)

      Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.

      If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns.

      Specified by:
      add in interface Collection<E>
      Implementation Requirements:
      This implementation always throws an UnsupportedOperationException.
      Parameters:
      e - element whose presence in this collection is to be ensured
      Returns:
      true if this collection changed as a result of the call
      Throws:
      UnsupportedOperationException - if the add operation is not supported by this collection
      ClassCastException - if the class of the specified element prevents it from being added to this collection
      NullPointerException - if the specified element is null and this collection does not permit null elements
      IllegalArgumentException - if some property of the element prevents it from being added to this collection
      IllegalStateException - if the element cannot be added at this time due to insertion restrictions

    • remove

      public boolean remove(Object o)
      Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that Objects.equals(o, e), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).
      Specified by:
      remove in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator's remove method.

      Note that this implementation throws an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection contains the specified object.

      Parameters:
      o - element to be removed from this collection, if present
      Returns:
      true if an element was removed as a result of this call
      Throws:
      UnsupportedOperationException - if the remove operation is not supported by this collection
      ClassCastException - if the type of the specified element is incompatible with this collection (optional)
      NullPointerException - if the specified element is null and this collection does not permit null elements (optional)

    • containsAll

      public boolean containsAll(Collection<?> c)
      Returns true if this collection contains all of the elements in the specified collection.
      Specified by:
      containsAll in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection. If all elements are so contained true is returned, otherwise false.
      Parameters:
      c - collection to be checked for containment in this collection
      Returns:
      true if this collection contains all of the elements in the specified collection
      Throws:
      ClassCastException - if the types of one or more elements in the specified collection are incompatible with this collection (optional)
      NullPointerException - if the specified collection contains one or more null elements and this collection does not permit null elements (optional), or if the specified collection is null.
      See Also:

    • addAll

      public boolean addAll(Collection<? extends E> c)
      Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.)
      Specified by:
      addAll in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.

      Note that this implementation will throw an UnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).

      Parameters:
      c - collection containing elements to be added to this collection
      Returns:
      true if this collection changed as a result of the call
      Throws:
      UnsupportedOperationException - if the addAll operation is not supported by this collection
      ClassCastException - if the class of an element of the specified collection prevents it from being added to this collection
      NullPointerException - if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null
      IllegalArgumentException - if some property of an element of the specified collection prevents it from being added to this collection
      IllegalStateException - if not all the elements can be added at this time due to insertion restrictions
      See Also:

    • removeAll

      public boolean removeAll(Collection<?> c)
      Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
      Specified by:
      removeAll in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's so contained, it's removed from this collection with the iterator'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 and this collection contains one or more elements in common with the specified collection.

      Parameters:
      c - collection containing elements to be removed from this collection
      Returns:
      true if this collection changed as a result of the call
      Throws:
      UnsupportedOperationException - if the removeAll method is not supported by this collection
      ClassCastException - if the types of one or more elements in this collection are incompatible with the specified collection (optional)
      NullPointerException - if this collection contains one or more null elements and the specified collection does not support null elements (optional), or if the specified collection is null
      See Also:

    • retainAll

      public boolean retainAll(Collection<?> c)
      Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
      Specified by:
      retainAll in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's not so contained, it's removed from this collection with the iterator'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 and this collection contains one or more elements not present in the specified collection.

      Parameters:
      c - collection containing elements to be retained in this collection
      Returns:
      true if this collection changed as a result of the call
      Throws:
      UnsupportedOperationException - if the retainAll operation is not supported by this collection
      ClassCastException - if the types of one or more elements in this collection are incompatible with the specified collection (optional)
      NullPointerException - if this collection contains one or more null elements and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • clear

      public void clear()
      Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns.
      Specified by:
      clear in interface Collection<E>
      Implementation Requirements:
      This implementation iterates over this collection, removing each element using the Iterator.remove operation. Most implementations will probably choose to override this method for efficiency.

      Note that this implementation will throw an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection is non-empty.

      Throws:
      UnsupportedOperationException - if the clear operation is not supported by this collection

    • toString

      public String toString()
      Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).
      Overrides:
      toString in class Object
      Returns:
      a string representation of this collection