ArrayListpublic class ArrayList extends AbstractList implements RandomAccess, List, 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,
iterator, and listIterator operations run in constant
time. The add operation 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
{@link Collections#synchronizedList 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 {@link 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. |
| Fields Summary |
|---|
| private static final long | serialVersionUID | | private transient Object[] | elementDataThe array buffer into which the elements of the ArrayList are stored.
The capacity of the ArrayList is the length of this array buffer. | | private int | sizeThe size of the ArrayList (the number of elements it contains). |
| Constructors Summary |
|---|
public ArrayList(int initialCapacity)Constructs an empty list with the specified initial capacity.
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
| public ArrayList()Constructs an empty list with an initial capacity of ten.
this(10);
| public ArrayList(Collection c)Constructs a list containing the elements of the specified
collection, in the order they are returned by the collection's
iterator.
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
|
| Methods Summary |
|---|
| private void | RangeCheck(int index)Checks if the given index is in range. If not, throws an appropriate
runtime exception. This method does *not* check if the index is
negative: It is always used immediately prior to an array access,
which throws an ArrayIndexOutOfBoundsException if index is negative.
if (index >= size)
throw new IndexOutOfBoundsException(
"Index: "+index+", Size: "+size);
| | public boolean | add(E e)Appends the specified element to the end of this list.
ensureCapacity(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
| | 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).
if (index > size || index < 0)
throw new IndexOutOfBoundsException(
"Index: "+index+", Size: "+size);
ensureCapacity(size+1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
| | public boolean | addAll(java.util.Collection 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.)
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacity(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
| | public boolean | addAll(int index, java.util.Collection 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.
if (index > size || index < 0)
throw new IndexOutOfBoundsException(
"Index: " + index + ", Size: " + size);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacity(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
| | public void | clear()Removes all of the elements from this list. The list will
be empty after this call returns.
modCount++;
// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
| | public java.lang.Object | clone()Returns a shallow copy of this ArrayList instance. (The
elements themselves are not copied.)
try {
ArrayList<E> v = (ArrayList<E>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
| | public boolean | contains(java.lang.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
(o==null ? e==null : o.equals(e)).
return indexOf(o) >= 0;
| | 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.
modCount++;
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
| | private void | fastRemove(int index)
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
| | public E | get(int index)Returns the element at the specified position in this list.
RangeCheck(index);
return (E) elementData[index];
| | public int | indexOf(java.lang.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
(o==null ? get(i)==null : o.equals(get(i))),
or -1 if there is no such index.
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
| | public boolean | isEmpty()Returns true if this list contains no elements.
return size == 0;
| | public int | lastIndexOf(java.lang.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
(o==null ? get(i)==null : o.equals(get(i))),
or -1 if there is no such index.
if (o == null) {
for (int i = size-1; i >= 0; i--)
if (elementData[i]==null)
return i;
} else {
for (int i = size-1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
| | private void | readObject(java.io.ObjectInputStream s)Reconstitute the ArrayList instance from a stream (that is,
deserialize it).
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in array length and allocate array
int arrayLength = s.readInt();
Object[] a = elementData = new Object[arrayLength];
// Read in all elements in the proper order.
for (int i=0; i<size; i++)
a[i] = s.readObject();
| | 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).
RangeCheck(index);
modCount++;
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
return oldValue;
| | public boolean | remove(java.lang.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
(o==null ? get(i)==null : o.equals(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).
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
| | 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.)
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// Let gc do its work
int newSize = size - (toIndex-fromIndex);
while (size != newSize)
elementData[--size] = null;
| | public E | set(int index, E element)Replaces the element at the specified position in this list with
the specified element.
RangeCheck(index);
E oldValue = (E) elementData[index];
elementData[index] = element;
return oldValue;
| | public int | size()Returns the number of elements in this list.
return size;
| | public java.lang.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.
return Arrays.copyOf(elementData, size);
| | public 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.)
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
| | 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.
modCount++;
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);
}
| | private void | writeObject(java.io.ObjectOutputStream s)Save the state of the ArrayList instance to a stream (that
is, serialize it).
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out array length
s.writeInt(elementData.length);
// Write out all elements in the proper order.
for (int i=0; i<size; i++)
s.writeObject(elementData[i]);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
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