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基本介绍
ArrayList底层由数组实现，默认初始化数组为空数组，可以实现数组的动态扩容，第一次添加数据时组数默认大小为10，后续每次扩容，默认为当前数组大小的1.5倍。默认线程不安全
底层数据结构
/**
* 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.
*/
transient Object[] elementData; // non-private to simplify nested class access
构造函数
初始化构造函数是否是无参构造，如果为无参构造方法则默认初始换为空数组，如果为有参构造，构造参数为int类型时，会先判断传入的大小是否为0，如果为0，则默认为空数组，如果大于0，则new 一个新的并且容量大小为当前传入值的object数组；如果传入参数为集合，会按照集合的迭代器返回的顺序构造一个包含指定集合元素的列表。
/**
* Constructs an empty list with the specified initial capacity.
*
* @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.
*/
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.
*
* @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) {
Object[] a = c.toArray();
if ((size = a.length) != 0) {
if (c.getClass() == ArrayList.class) {
elementData = a;
} else {
elementData = Arrays.copyOf(a, size, Object[].class);
}
} else {
// replace with empty array.
elementData = EMPTY_ELEMENTDATA;
}
}
添加元素与自动扩容
add()方法
添加一个元素进入list的过程
1、调用ensureCapacityInternal方法，确保数组容量足够
2、调用calculateCapacity方法，计算数组容量，如果当前数组是初始化空数组，则返回默认数组容量大小的值与传入minCapacity（该值为当前数组元素个数+1）值中最大的一个；如果当前数组不为初始化数组，则返回minCapacity。
3、继续调用ensureExplicitCapacity方法，首先将modCount参数加1，判断需要的最小容量是否大于当前数组的容量，如果大于，则需要进行数组扩容，反之，执行elementData[size++] = e;方法，元素进入，添加完成
4、如果当前容量不满足元素进入，调用grow方法，记录oldCapacity为当前数组容量，记录newCapacity为oldCapacity的1.5倍，判断新的数组容量是否满足当前元素进入后的最小容量，如果小于，则将minCapacity赋值给newCapacity，继续判断newCapacity是否大于（Integer.MAX_VALUE - 8）的值，如果大于判断 (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;并重新设置newCapacity的值，最后创建一个大小为newCapacity的新数组并将原数组的数据拷贝过来
/**
* 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).
*
* @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++;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
addAll()方法
1、将传入的对象先进行toArray操作
2、再记录toArray后数组的长度
3、同add操作一样，判断当前数组容量，扩容机制同add方法一样
4、调用System.arraycopy方法，将toArray操作后的数组拷贝至当前数组
/**
* 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.)
*
* @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. 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.
*
* @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
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
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;
}
set()方法
用指定元素替换数组中指定位置的元素
1、调用rangeCheck，检查给定的索引是否在范围内
2、记录原位置的元素值
3、将当前指定位置的元素替换为新的元素
4、返回旧的元素值
/**
* Replaces the element at the specified position in this list with
* the specified element.
*
* @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;
}
get()方法
返回此列表中指定位置的元素
1、检查给定的索引是否在范围内
2、返回此列表中指定位置的元素
/**
* 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);
}
删除方法
指定位置删除 remove(int index)方法
移除此列表中指定位置的元素。 将任何后续元素向左移动（从它们的索引中减去一个）。
1、检查给定的索引是否在范围内
2、操作数量统计加一
3、记录旧的元素值
4、记录需要移动元素的个数
5、将剩余元素进行向左移动，并将最后位置的元素置为null，size减一
6、返回旧的元素值
/**
* 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);
//原数组 elementData = {"1","2","3","4"}
//将2删除后，元素移动
//当前数组elementData = {"1","3","4","4"}
//将最后一位元素设置为null elementData = {"1","3","4",null}
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
指定元素的删除 remove(Object o)方法
从此列表中删除第一次出现的指定元素（如果存在）。 如果列表不包含该元素，则它保持不变。 更正式地，删除具有最低索引i的元素，使得(o==null ? get(i)==null : o.equals(get(i))) （如果存在这样的元素）。 如果此列表包含指定的元素（或等效地，如果此列表因调用而更改），则返回true 。
/**
* 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
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @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;
}
调整数组实际大小 trimToSize()方法
将此ArrayList实例的容量修剪为列表的当前大小。 应用程序可以使用此操作来最小化ArrayList实例的存储空间
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
总结
因为ArrayList底层是基于数组实现的，所以数组的优缺点也是ArrayList的优缺点，典型的数组优点在于随机指定所以存储和查询方便，按照索引遍历数组时方便，而在进行数组扩容和删除元素的操作的时候，调用System.arrayCopy方法时会导致大量的元素移动，性能较差。
