概述
jdk17
ArrayList实现了list接口，是顺序容器,(元素按照存放的顺序排列)，允许null，底层通过数组实现，ArrayList并不是线程安全的。
ArrayList底层实现
结构
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
@java.io.Serial
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
* 默认初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* 初始化时的空数组
*/
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.
* 用于默认大小的空实例与EMPTY_ELEMENTDATA区分开来，以了解添加第一个元素时要膨胀多少
*/
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.
* 存放具体数据
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
* 多少个数据
*/
private int size;
/**
* 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
* 构造函数
* 根据传入的initialCapacity来创建底层数组的大小
*/
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);
}
}
自动扩容
每次添加数据时，都会去检查添加后元素的个数是否会超出当前数组的长度，如果超出了，数组就会进行扩容，以满足添加数据的要求。
ArrayList中扩容通过ensureCapacity(int minCapacity)方法实现，如过需要添加大量的元素，也可以调用这个方法来手动增加ArrayList的容量，以减少递增式在分配的数量。
数组每次扩容会在原容量的基础上增加1.5倍，每次的扩容都会新建一个数组，然后将老数组中的数据拷贝过去
这样就会导致扩容的代价是高昂的，如果频繁的扩容，肯定导致性能问题，所以在实际使用时，应该尽量避免数组容量的扩张，在能够预知要保存数据有多少时，尽可能在创建的时候指定好容量，以避免扩容的发生。
/**
* Increases the capacity of this {@code ArrayList} instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
if (minCapacity > elementData.length
&& !(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
&& minCapacity <= DEFAULT_CAPACITY)) {
modCount++;
grow(minCapacity);
}
}
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
* @throws OutOfMemoryError if minCapacity is less than zero
*/
private Object[] grow(int minCapacity) {
int oldCapacity = elementData.length;
if (oldCapacity > 0 || elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
int newCapacity = ArraysSupport.newLength(oldCapacity,
minCapacity - oldCapacity, /* minimum growth */
oldCapacity >> 1 /* preferred growth */);
return elementData = Arrays.copyOf(elementData, newCapacity);
} else {
return elementData = new Object[Math.max(DEFAULT_CAPACITY, minCapacity)];
}
}
private Object[] grow() {
return grow(size + 1);
}
方法
add()
/**
* This helper method split out from add(E) to keep method
* bytecode size under 35 (the -XX:MaxInlineSize default value),
* which helps when add(E) is called in a C1-compiled loop.
*/
private void add(E e, Object[] elementData, int s) {
if (s == elementData.length)
elementData = grow();
elementData[s] = e;
size = s + 1;
}
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
modCount++;
add(e, elementData, size);
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);
modCount++;
final int s;
Object[] elementData;
if ((s = size) == (elementData = this.elementData).length)
elementData = grow();
System.arraycopy(elementData, index,
elementData, index + 1,
s - index);
elementData[index] = element;
size = s + 1;
}
新增 public boolean add(E e) ：
调用add方法后，底层去调用了add(e, elementData, size) 方法，如果size = 数组的容量，就会扩容，
然后在数组后面添加数据，添加完size+1
在索引index上新增 public void add(int index, E element)
add(int index, E e)需要先对元素进行移动，然后完成插入操作，也就意味着该方法有着线性的时间复杂度。
addAll()
addAll()方法能够一次添加多个元素，根据位置不同也有两个版本，一个是在末尾添加的addAll(Collection<? extends E> c)方法，一个是从指定位置开始插入的addAll(int index, Collection<? extends E> c)方法。跟add()方法类似，在插入之前也需要进行空间检查，如果需要则自动扩容；如果从指定位置插入，也会存在移动元素的情况。 addAll()的时间复杂度不仅跟插入元素的多少有关，也跟插入的位置相关。
set()
set()方法就是简单的直接对数组的指定位置赋值
/**
* 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) {
// 下标越界检查
Objects.checkIndex(index, size);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
get()
跟普通的获取数组元素一样 arr[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) {
Objects.checkIndex(index, size);
return elementData(index);
}
remove()
remove()方法也有两个版本，一个是remove(int index)删除指定位置的元素，另一个是remove(Object o)删除第一个满足o.equals(elementData[index])的元素。删除操作是add()操作的逆过程，需要将删除点之后的元素向前移动一个位置。需要注意的是为了让GC起作用，必须显式的为最后一个位置赋null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) {
Objects.checkIndex(index, size);
final Object[] es = elementData;
@SuppressWarnings("unchecked") E oldValue = (E) es[index];
fastRemove(es, index);
return oldValue;
}
trimToSize()
ArrayList还给我们提供了将底层数组的容量调整为当前列表保存的实际元素的大小的功能。它可以通过trimToSize方法来实现
/**
* Trims the capacity of this {@code ArrayList} instance to be the
* list's current size. An application can use this operation to minimize
* the storage of an {@code ArrayList} instance.
* 将这个ArrayList实例的容量修剪为列表的当前大小。应用程序可以使用该操作最小化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);
}
}
indexOf(), lastIndexOf()
indexOf()获取元素的第一次出现的index:
lastIndexOf()获取元素的最后一次出现的index:
/**
* 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 {@code i} such that
* {@code Objects.equals(o, get(i))},
* or -1 if there is no such index.
* 返回指定元素在此列表中最后一次出现的索引，如果该列表不包含该元素，则返回-1。
* 更正式地，返回最高索引i，使Objects。等于(o, get(i))，如果没有这样的索引，则为-1。
*/
public int indexOf(Object o) {
return indexOfRange(o, 0, size);
}
int indexOfRange(Object o, int start, int end) {
Object[] es = elementData;
if (o == null) {
for (int i = start; i < end; i++) {
if (es[i] == null) {
return i;
}
}
} else {
for (int i = start; i < end; i++) {
if (o.equals(es[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 {@code i} such that
* {@code Objects.equals(o, get(i))},
* or -1 if there is no such index.
* 返回指定元素在此列表中最后一次出现的索引，如果该列表不包含该元素，则返回-1。
* 更正式地，返回最高索引i，使Objects。等于(o, get(i))，如果没有这样的索引，则为-1。
*/
public int lastIndexOf(Object o) {
return lastIndexOfRange(o, 0, size);
}
int lastIndexOfRange(Object o, int start, int end) {
Object[] es = elementData;
if (o == null) {
for (int i = end - 1; i >= start; i--) {
if (es[i] == null) {
return i;
}
}
} else {
for (int i = end - 1; i >= start; i--) {
if (o.equals(es[i])) {
return i;
}
}
}
return -1;
}
Fail-Fast机制:
ArrayList也采用了快速失败的机制，通过记录modCount参数来实现。在面对并发的修改时，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险。
/**
* The number of times this list has been <i>structurally modified</i>.
* Structural modifications are those that change the size of the
* list, or otherwise perturb it in such a fashion that iterations in
* progress may yield incorrect results.
*
* <p>This field is used by the iterator and list iterator implementation
* returned by the {@code iterator} and {@code listIterator} methods.
* If the value of this field changes unexpectedly, the iterator (or list
* iterator) will throw a {@code ConcurrentModificationException} in
* response to the {@code next}, {@code remove}, {@code previous},
* {@code set} or {@code add} operations. This provides
* <i>fail-fast</i> behavior, rather than non-deterministic behavior in
* the face of concurrent modification during iteration.
*
* <p><b>Use of this field by subclasses is optional.</b> If a subclass
* wishes to provide fail-fast iterators (and list iterators), then it
* merely has to increment this field in its {@code add(int, E)} and
* {@code remove(int)} methods (and any other methods that it overrides
* that result in structural modifications to the list). A single call to
* {@code add(int, E)} or {@code remove(int)} must add no more than
* one to this field, or the iterators (and list iterators) will throw
* bogus {@code ConcurrentModificationExceptions}. If an implementation
* does not wish to provide fail-fast iterators, this field may be
* ignored.
* 此列表被结构修改的次数。结构修改是指改变列表的大小，或者以某种方式扰乱列表，
* 从而导致正在进行的迭代可能产生不正确的结果。
* 该字段由迭代器和listIterator方法返回的迭代器和列表迭代器实现使用。
* 如果该字段的值意外更改，迭代器(或列表迭代器)
* 将抛出ConcurrentModificationException异常，
* 以响应next、remove、previous、set或add操作。这提供了快速故障行为，
* 而不是在迭代期间面对并发修改时的非确定性行为。子类是否使用此字段是可选的。
* 如果一个子类希望提供快速失败迭代器(和列表迭代器)，
* 那么它只需要在它的add(int, E)和remove(int)方法
* (以及它覆盖的导致列表结构修改的任何其他方法)
* 中增加这个字段。单个调用add(int, E)或remove(int)必须向该字段添加不超过一个，
* 否则迭代器(和列表迭代器)将抛出虚假的concurrentmodificationexception异常。
* 如果实现不希望提供快速失败迭代器，则可以忽略此字段。
*/
protected transient int modCount = 0;