标签：index return list ArrayList elementData List element 源码 size
- 基于数组实现，是一个动态数组，其容量能自动增长。
- ArrayList不是线程安全的，建议在单线程中使用，多线程可以选择Vector或CopyOnWriteArrayList。
- 实现了RandomAccess接口，可以通过下标序号进行快速访问。
- 实现了Cloneable接口，能被克隆。
- 实现了Serializable接口，支持序列化。
ArrayList源码分析
展示部分源码：
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
/**
* Default initial capacity. 默认初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* The array buffer into which the elements of the ArrayList are stored.The capacity of the ArrayList is the length of this array buffer. Any empty ArrayList with
* elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA will be expanded to DEFAULT_CAPACITY when the first element is added. 使用DEFAULT**的集合，添加第一个元素时，扩展到默认容量
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
* @serial
*/
private int size;
/**
* Constructs an empty list with the specified initial capacity. 有参构造函数
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity is negative
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
}
}
/**
* Constructs an empty list with an initial capacity of ten. 无参构造函数
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator. 有参构造函数，集合转成ArrayList
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
/**
* The maximum size of array to allocate. Some VMs reserve some header words in an array. Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit 最大数组大小
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* Returns the number of elements in this list.
* @return the number of elements in this list
*/
public int size() {
return size;
}
/**
* Returns <tt>true</tt> if this list contains no elements.
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
}
/**
* Returns <tt>true</tt> if this list contains the specified element. More formally, returns <tt>true</tt> if and only if this list contains
* at least one element <tt>e</tt> such that <tt>(o==null ? e==null : o.equals(e))</tt>.
* @param o element whose presence in this list is to be tested
* @return <tt>true</tt> if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/**
* 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 <tt>i</tt> such that <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, or -1 if there is no such index.
*/
public int indexOf(Object o) {
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;
}
/**
* 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 <tt>i</tt> such that <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, or -1 if there is no such index.
*/
public int lastIndexOf(Object o) {
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;
}
/**
* Returns a shallow copy of this <tt>ArrayList</tt> instance. (The elements themselves are not copied.) 浅拷贝
* @return a clone of this <tt>ArrayList</tt> instance
*/
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) 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(e);
}
}
/**
* Returns an array containing all of the elements in this list in proper sequence (from first to last element).
* @return an array containing all of the elements in this list in proper sequence
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
/**
* 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. 数组长度大于list长度，多余的会被赋值null
* @param 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.
* @return an array containing the elements of the list*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
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;
}
// Positional Access Operations
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
/**
* Returns the element at the specified position in this list.
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
/**
* Replaces the element at the specified position in this list with the specified element. set方法会返回旧值
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
/**
* 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). 在指定位置增加元素,后面元素顺次后移
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index] = element;
size++;
}
/**
* Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index, numMoved);
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
/**
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) {
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;
}
/**
* Removes all of the elements from this list. The list will be empty after this call returns. 清空集合
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
/**
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
/**
* Inserts all of the elements in the specified collection into this list, starting at the specified position.
*
* @param index index at which to insert the first element from the specified collection
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call*/
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(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;
}
/**
* Removes from this list all of the elements whose index is between {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.*/
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}/**
* Removes from this list all of its elements that are contained in the specified collection.
*
* @param c collection containing elements to be removed from this list
* @return {@code true} if this list changed as a result of the call
* @see Collection#contains(Object)
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
/**
* 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. 从列表中删除所有未包含在指定集合中的元素
*
* @param c collection containing elements to be retained in this list
* @return {@code true} if this list changed as a result of the call
* @see Collection#contains(Object)
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection, even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r, elementData, w, size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that is, serialize it).*/
private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
s.writeInt(size);
// 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();
}
}
/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, deserialize it).
*/
private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in capacity
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}
@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
final int expectedModCount = modCount;
Arrays.sort((E[]) elementData, 0, size, c);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
}
总结：
- ArrayList 实际上是通过一个数组去保存数据的。当我们使用无参构造函数构造ArrayList时，则ArrayList的默认容量大小是10。
- 当ArrayList容量不足以容纳全部元素时，ArrayList会重新设置容量：新的容量=“(原始容量x3)/2 + 1”;如果设置后的新容量还不够，则直接把新容量设置为传入的参数。容量最大不超过 Integer.MAX_VALUE - 8
- ArrayList基于数组实现，可以通过下标索引直接查找到指定位置的元素，因此查找效率高，但每次插入或删除元素，就要大量地移动元素，插入删除元素的效率低。
- Object[] toArray()方法。该方法有可能会抛出java.lang.ClassCastException异常。toArray()返回的是 Object[] 数组，Java不支持向下转型。(例如，将Object[]转换为的Integer[])
- System.arraycopy()方法。该方法被标记了native，调用了系统的C/C++代码，该函数实际上最终调用了C语言的memmove()函数，因此它可以保证同一个数组内元素的正确复制和移动，比一般的复制方法的实现效率要高很多，很适合用来批量处理数组
标签：index,return,list,ArrayList,elementData,List,element,源码,size 来源： https://www.cnblogs.com/ryjJava/p/14314573.html
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