Class ArrayList<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractList<E>
java.util.ArrayList<E>
Type Parameters:
E - the type of elements in this list
All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess, SequencedCollection<E>
Direct Known Subclasses:
AttributeList, RoleList, RoleUnresolvedList
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable
Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector, except that it is unsynchronized.)

The size, isEmpty, get, set, getFirst, getLast, removeLast, iterator, listIterator, and reversed operations run in constant time. The add, and addLast operations runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.

Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

   List list = Collections.synchronizedList(new ArrayList(...));

The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a 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.

Since:
1.2
See Also:

  • Field Summary

    Fields declared in class java.util.AbstractList

    modCount

  • Constructor Summary

    Constructors
    Constructor
    Description
    ArrayList()
    Constructs an empty list with an initial capacity of ten.
    ArrayList(int initialCapacity)
    Constructs an empty list with the specified initial capacity.
    ArrayList(Collection<? extends E> c)
    Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    add(int index, E element)
    Inserts the specified element at the specified position in this list.
    boolean
    add(E e)
    Appends the specified element to the end of this list.
    boolean
    addAll(int index, Collection<? extends E> c)
    Inserts all of the elements in the specified collection into this list, starting at the specified position.
    boolean
    addAll(Collection<? extends E> c)
    Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator.
    void
    addFirst(E element)
    Adds an element as the first element of this collection (optional operation).
    void
    addLast(E element)
    Adds an element as the last element of this collection (optional operation).
    void
    clear()
    Removes all of the elements from this list.
    Object
    clone()
    Returns a shallow copy of this ArrayList instance.
    boolean
    contains(Object o)
    Returns true if this list contains the specified element.
    void
    ensureCapacity(int minCapacity)
    Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
    boolean
    equals(Object o)
    Compares the specified object with this list for equality.
    void
    forEach(Consumer<? super E> action)
    Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
    E
    get(int index)
    Returns the element at the specified position in this list.
    E
    getFirst()
    Gets the first element of this collection.
    E
    getLast()
    Gets the last element of this collection.
    int
    hashCode()
    Returns the hash code value for this list.
    int
    indexOf(Object o)
    Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
    boolean
    isEmpty()
    Returns true if this list contains no elements.
    Iterator<E>
    iterator()
    Returns an iterator over the elements in this list in proper sequence.
    int
    lastIndexOf(Object o)
    Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
    ListIterator<E>
    listIterator()
    Returns a list iterator over the elements in this list (in proper sequence).
    ListIterator<E>
    listIterator(int index)
    Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.
    E
    remove(int index)
    Removes the element at the specified position in this list.
    boolean
    remove(Object o)
    Removes the first occurrence of the specified element from this list, if it is present.
    boolean
    removeAll(Collection<?> c)
    Removes from this list all of its elements that are contained in the specified collection.
    E
    removeFirst()
    Removes and returns the first element of this collection (optional operation).
    boolean
    removeIf(Predicate<? super E> filter)
    Removes all of the elements of this collection that satisfy the given predicate (optional operation).
    E
    removeLast()
    Removes and returns the last element of this collection (optional operation).
    protected void
    removeRange(int fromIndex, int toIndex)
    Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive.
    boolean
    retainAll(Collection<?> c)
    Retains only the elements in this list that are contained in the specified collection.
    E
    set(int index, E element)
    Replaces the element at the specified position in this list with the specified element.
    int
    size()
    Returns the number of elements in this list.
    Spliterator<E>
    spliterator()
    Creates a late-binding and fail-fast Spliterator over the elements in this list.
    List<E>
    subList(int fromIndex, int toIndex)
    Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.
    Object[]
    toArray()
    Returns an array containing all of the elements in this list in proper sequence (from first to last element).
    <T> T[]
    toArray(T[] a)
    Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
    void
    trimToSize()
    Trims the capacity of this ArrayList instance to be the list's current size.

    Methods declared in class java.util.AbstractList

    equals, hashCode

    Methods declared in class java.util.AbstractCollection

    containsAll, toString

    Methods declared in class java.lang.Object

    finalize, getClass, notify, notifyAll, wait, wait, wait

    Methods declared in interface java.util.Collection

    parallelStream, stream, toArray

    Methods declared in interface java.util.List

    containsAll, replaceAll, reversed, sort

  • Constructor Details

    • ArrayList

      public ArrayList(int initialCapacity)
      Constructs an empty list with the specified initial capacity.
      Parameters:
      initialCapacity - the initial capacity of the list
      Throws:
      IllegalArgumentException - if the specified initial capacity is negative

    • ArrayList

      public ArrayList()
      Constructs an empty list with an initial capacity of ten.

    • ArrayList

      public ArrayList(Collection<? extends E> c)
      Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
      Parameters:
      c - the collection whose elements are to be placed into this list
      Throws:
      NullPointerException - if the specified collection is null

  • Method Details

    • trimToSize

      public void trimToSize()
      Trims the capacity of this ArrayList instance to be the list's current size. An application can use this operation to minimize the storage of an ArrayList instance.

    • ensureCapacity

      public void ensureCapacity(int minCapacity)
      Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
      Parameters:
      minCapacity - the desired minimum capacity

    • size

      public int size()
      Returns the number of elements in this list.
      Specified by:
      size in interface Collection<E>
      Specified by:
      size in interface List<E>
      Returns:
      the number of elements in this list

    • isEmpty

      public boolean isEmpty()
      Returns true if this list contains no elements.
      Specified by:
      isEmpty in interface Collection<E>
      Specified by:
      isEmpty in interface List<E>
      Overrides:
      isEmpty in class AbstractCollection<E>
      Returns:
      true if this list contains no elements

    • contains

      public boolean contains(Object o)
      Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that Objects.equals(o, e).
      Specified by:
      contains in interface Collection<E>
      Specified by:
      contains in interface List<E>
      Overrides:
      contains in class AbstractCollection<E>
      Parameters:
      o - element whose presence in this list is to be tested
      Returns:
      true if this list contains the specified element

    • indexOf

      public int indexOf(Object o)
      Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that Objects.equals(o, get(i)), or -1 if there is no such index.
      Specified by:
      indexOf in interface List<E>
      Overrides:
      indexOf in class AbstractList<E>
      Parameters:
      o - element to search for
      Returns:
      the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element

    • lastIndexOf

      public int lastIndexOf(Object o)
      Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index i such that Objects.equals(o, get(i)), or -1 if there is no such index.
      Specified by:
      lastIndexOf in interface List<E>
      Overrides:
      lastIndexOf in class AbstractList<E>
      Parameters:
      o - element to search for
      Returns:
      the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element

    • clone

      public Object clone()
      Returns a shallow copy of this ArrayList instance. (The elements themselves are not copied.)
      Overrides:
      clone in class Object
      Returns:
      a clone of this ArrayList instance
      See Also:

    • toArray

      public Object[] toArray()
      Returns an array containing all of the elements in this list in proper sequence (from first to last element).

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

      This method acts as bridge between array-based and collection-based APIs.

      Specified by:
      toArray in interface Collection<E>
      Specified by:
      toArray in interface List<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Returns:
      an array containing all of the elements in this list in proper sequence
      See Also:

    • toArray

      public <T> T[] toArray(T[] a)
      Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list 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 list.

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

      Specified by:
      toArray in interface Collection<E>
      Specified by:
      toArray in interface List<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Type Parameters:
      T - the component type of the array to contain the collection
      Parameters:
      a - the array into which the elements of the list 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 the elements of the list
      Throws:
      ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this list
      NullPointerException - if the specified array is null

    • get

      public E get(int index)
      Returns the element at the specified position in this list.
      Specified by:
      get in interface List<E>
      Specified by:
      get in class AbstractList<E>
      Parameters:
      index - index of the element to return
      Returns:
      the element at the specified position in this list
      Throws:
      IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

    • getFirst

      public E getFirst()
      Gets the first element of this collection.
      Specified by:
      getFirst in interface List<E>
      Specified by:
      getFirst in interface SequencedCollection<E>
      Returns:
      the retrieved element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • getLast

      public E getLast()
      Gets the last element of this collection.
      Specified by:
      getLast in interface List<E>
      Specified by:
      getLast in interface SequencedCollection<E>
      Returns:
      the retrieved element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • set

      public E set(int index, E element)
      Replaces the element at the specified position in this list with the specified element.
      Specified by:
      set in interface List<E>
      Overrides:
      set in class AbstractList<E>
      Parameters:
      index - index of the element to replace
      element - element to be stored at the specified position
      Returns:
      the element previously at the specified position
      Throws:
      IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

    • add

      public boolean add(E e)
      Appends the specified element to the end of this list.
      Specified by:
      add in interface Collection<E>
      Specified by:
      add in interface List<E>
      Overrides:
      add in class AbstractList<E>
      Parameters:
      e - element to be appended to this list
      Returns:
      true (as specified by Collection.add(E))

    • add

      public void add(int index, E element)
      Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).
      Specified by:
      add in interface List<E>
      Overrides:
      add in class AbstractList<E>
      Parameters:
      index - index at which the specified element is to be inserted
      element - element to be inserted
      Throws:
      IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())

    • addFirst

      public void addFirst(E element)
      Adds an element as the first element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the first element in encounter order.
      Specified by:
      addFirst in interface List<E>
      Specified by:
      addFirst in interface SequencedCollection<E>
      Parameters:
      element - the element to be added
      Since:
      21

    • addLast

      public void addLast(E element)
      Adds an element as the last element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the last element in encounter order.
      Specified by:
      addLast in interface List<E>
      Specified by:
      addLast in interface SequencedCollection<E>
      Parameters:
      element - the element to be added.
      Since:
      21

    • remove

      public E remove(int index)
      Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices).
      Specified by:
      remove in interface List<E>
      Overrides:
      remove in class AbstractList<E>
      Parameters:
      index - the index of the element to be removed
      Returns:
      the element that was removed from the list
      Throws:
      IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

    • removeFirst

      public E removeFirst()
      Removes and returns the first element of this collection (optional operation).
      Specified by:
      removeFirst in interface List<E>
      Specified by:
      removeFirst in interface SequencedCollection<E>
      Returns:
      the removed element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • removeLast

      public E removeLast()
      Removes and returns the last element of this collection (optional operation).
      Specified by:
      removeLast in interface List<E>
      Specified by:
      removeLast in interface SequencedCollection<E>
      Returns:
      the removed element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • equals

      public boolean equals(Object o)
      Compares the specified object with this list for equality. Returns true if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal. (Two elements e1 and e2 are equal if (e1==null ? e2==null : e1.equals(e2)).) In other words, two lists are defined to be equal if they contain the same elements in the same order.
      Specified by:
      equals in interface Collection<E>
      Specified by:
      equals in interface List<E>
      Overrides:
      equals in class AbstractList<E>
      Parameters:
      o - the object to be compared for equality with this list
      Returns:
      true if the specified object is equal to this list
      See Also:

    • hashCode

      public int hashCode()
      Returns the hash code value for this list.
      Specified by:
      hashCode in interface Collection<E>
      Specified by:
      hashCode in interface List<E>
      Overrides:
      hashCode in class AbstractList<E>
      Returns:
      the hash code value for this list
      See Also:

    • remove

      public boolean remove(Object o)
      Removes the first occurrence of the specified element from this list, if it is present. If the list does not contain the element, it is unchanged. More formally, removes the element with the lowest index i such that Objects.equals(o, get(i)) (if such an element exists). Returns true if this list contained the specified element (or equivalently, if this list changed as a result of the call).
      Specified by:
      remove in interface Collection<E>
      Specified by:
      remove in interface List<E>
      Overrides:
      remove in class AbstractCollection<E>
      Parameters:
      o - element to be removed from this list, if present
      Returns:
      true if this list contained the specified element

    • clear

      public void clear()
      Removes all of the elements from this list. The list will be empty after this call returns.
      Specified by:
      clear in interface Collection<E>
      Specified by:
      clear in interface List<E>
      Overrides:
      clear in class AbstractList<E>

    • addAll

      public boolean addAll(Collection<? extends E> c)
      Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator. 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 list, and this list is nonempty.)
      Specified by:
      addAll in interface Collection<E>
      Specified by:
      addAll in interface List<E>
      Overrides:
      addAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be added to this list
      Returns:
      true if this list changed as a result of the call
      Throws:
      NullPointerException - if the specified collection is null
      See Also:

    • addAll

      public boolean addAll(int index, Collection<? extends E> c)
      Inserts all of the elements in the specified collection into this list, starting at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in the list in the order that they are returned by the specified collection's iterator.
      Specified by:
      addAll in interface List<E>
      Overrides:
      addAll in class AbstractList<E>
      Parameters:
      index - index at which to insert the first element from the specified collection
      c - collection containing elements to be added to this list
      Returns:
      true if this list changed as a result of the call
      Throws:
      IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())
      NullPointerException - if the specified collection is null

    • removeRange

      protected void removeRange(int fromIndex, int toIndex)
      Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens the list by (toIndex - fromIndex) elements. (If toIndex==fromIndex, this operation has no effect.)
      Overrides:
      removeRange in class AbstractList<E>
      Parameters:
      fromIndex - index of first element to be removed
      toIndex - index after last element to be removed
      Throws:
      IndexOutOfBoundsException - if fromIndex or toIndex is out of range (fromIndex < 0 || toIndex > size() || toIndex < fromIndex)

    • removeAll

      public boolean removeAll(Collection<?> c)
      Removes from this list all of its elements that are contained in the specified collection.
      Specified by:
      removeAll in interface Collection<E>
      Specified by:
      removeAll in interface List<E>
      Overrides:
      removeAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be removed from this list
      Returns:
      true if this list changed as a result of the call
      Throws:
      ClassCastException - if the class of an element of this list is incompatible with the specified collection (optional)
      NullPointerException - if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • retainAll

      public boolean retainAll(Collection<?> c)
      Retains only the elements in this list that are contained in the specified collection. In other words, removes from this list all of its elements that are not contained in the specified collection.
      Specified by:
      retainAll in interface Collection<E>
      Specified by:
      retainAll in interface List<E>
      Overrides:
      retainAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be retained in this list
      Returns:
      true if this list changed as a result of the call
      Throws:
      ClassCastException - if the class of an element of this list is incompatible with the specified collection (optional)
      NullPointerException - if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • listIterator

      public ListIterator<E> listIterator(int index)
      Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list. The specified index indicates the first element that would be returned by an initial call to next. An initial call to previous would return the element with the specified index minus one.

      The returned list iterator is fail-fast.

      Specified by:
      listIterator in interface List<E>
      Overrides:
      listIterator in class AbstractList<E>
      Parameters:
      index - index of the first element to be returned from the list iterator (by a call to next)
      Returns:
      a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list
      Throws:
      IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())

    • listIterator

      public ListIterator<E> listIterator()
      Returns a list iterator over the elements in this list (in proper sequence).

      The returned list iterator is fail-fast.

      Specified by:
      listIterator in interface List<E>
      Overrides:
      listIterator in class AbstractList<E>
      Returns:
      a list iterator over the elements in this list (in proper sequence)
      See Also:

    • iterator

      public Iterator<E> iterator()
      Returns an iterator over the elements in this list in proper sequence.

      The returned iterator is fail-fast.

      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in interface List<E>
      Overrides:
      iterator in class AbstractList<E>
      Returns:
      an iterator over the elements in this list in proper sequence

    • subList

      public List<E> subList(int fromIndex, int toIndex)
      Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. The returned list supports all of the optional list operations.

      This method eliminates the need for explicit range operations (of the sort that commonly exist for arrays). Any operation that expects a list can be used as a range operation by passing a subList view instead of a whole list. For example, the following idiom removes a range of elements from a list: 

            list.subList(from, to).clear();
      Similar idioms may be constructed for indexOf(Object) and lastIndexOf(Object), and all of the algorithms in the Collections class can be applied to a subList.

      The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

      Specified by:
      subList in interface List<E>
      Overrides:
      subList in class AbstractList<E>
      Parameters:
      fromIndex - low endpoint (inclusive) of the subList
      toIndex - high endpoint (exclusive) of the subList
      Returns:
      a view of the specified range within this list
      Throws:
      IndexOutOfBoundsException - if an endpoint index value is out of range (fromIndex < 0 || toIndex > size)
      IllegalArgumentException - if the endpoint indices are out of order (fromIndex > toIndex)

    • forEach

      public void forEach(Consumer<? super E> action)
      Description copied from interface: Iterable
      Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

      The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

      Specified by:
      forEach in interface Iterable<E>
      Parameters:
      action - The action to be performed for each element
      Throws:
      NullPointerException - if the specified action is null

    • spliterator

      public Spliterator<E> spliterator()
      Creates a late-binding and fail-fast Spliterator over the elements in this list.

      The Spliterator reports Spliterator.SIZED, Spliterator.SUBSIZED, and Spliterator.ORDERED. Overriding implementations should document the reporting of additional characteristic values.

      Specified by:
      spliterator in interface Collection<E>
      Specified by:
      spliterator in interface Iterable<E>
      Specified by:
      spliterator in interface List<E>
      Returns:
      a Spliterator over the elements in this list
      Since:
      1.8

    • removeIf

      public boolean removeIf(Predicate<? super E> filter)
      Description copied from interface: Collection
      Removes all of the elements of this collection that satisfy the given predicate (optional operation). Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
      Specified by:
      removeIf in interface Collection<E>
      Parameters:
      filter - a predicate which returns true for elements to be removed
      Returns:
      true if any elements were removed
      Throws:
      NullPointerException - if the specified filter is null