ArrayList源码分析
ArrayList
@基于JDK8
ArrayList继承体系结构如下:
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList类主要是继承AbstractList类并实现了List接口，实现Cloneable和Serializable接口使得ArrayList具有克隆和序列化的功能。实现了RandomAccess这个接口的集合是支持 快速随机访问 策略的,如果是实现了这个接口的 List，那么使用for循环的方式获取数据的效率会优于用迭代器获取数据。
主要属性
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.
共享空常量数组
*/
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.
也是共享空常量数组 和上面的区别就是当第一个元素被加入进来的时候它知道如何扩张；在源码中函数add(E e)中的第一行代码中的所在的函数就是这句话的实现。
*/
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.
elementData是个数组，它就是集合存储元素的缓冲区，因此证明ArrayList的底层是一个数组。
当我们创建一个ArrayList不存储任何元素的时候，集合的默认长度是0，不是10，而是在我们第一次往ArrayList里面添加数据的时候，长度会由0拓展为10
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
* size代表的是ArrayList里面存储的元素的实际个数，不是它的长度。
* @serial
*/
private int size;
构造方法
ArrayList有三个构造方法
1.无参构造函数
/**
* Constructs an empty list with an initial capacity of ten.
创建一个空的list，初始容量为10
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
这里会有疑问，DEFAULTCAPACITY_EMPTY_ELEMENTDATA是一个空数组，然后赋值给了elementData，为何注释说其容量就变成10了呢？其实这里的解释要放在add方法里面，后面会做解释。
2.指定容量作为参数的构造函数
/**
* Constructs an empty list with the specified initial capacity.
创建一个空的数组，其容量为我们指定的长度
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
//如果指定的长度大于0，则创建一个新的指定长度的数组，并把该数组赋值给elementData
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
//如果指定的长度等于0，则把element赋值为空数组
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
从源码可以看到，就是根据参数的大小作为容量来实例化底层的数组对象，其中对参数的3种情况进行了处理。当参数小于0时，抛异常。当参数等于0时，用空的常量数组对象EMPTY_ELEMENTDATA来初始化底层数组elementData。
3.Collection作为参数的构造函数
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*创建一个包含指定元素的数组，参数为Collection类型，最终ArrayList里面的元素迭代顺序与Collection中的元素顺序保持一致
*
* @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) {
/*将传递进来带有元素的集合转换为数组，然后赋值给elementData，作为自己的元素
如果参数c是null，则会抛出NullPointerException的异常*/
elementData = c.toArray();
//把elementData的长度赋值给size并判断是否为0，如果不为0，就进行数组拷贝
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 {
// 如果为0，那说明传递进来的集合是个空集合，那就把elementData赋值为空数组
this.elementData = EMPTY_ELEMENTDATA;
}
}
从源码可以看到，将容器Collection转化为数组赋给数组elementData，还对Collection转化是否转化为了Object[]进行了检查判断。如果Collection为空，则就将空的常量数组对象EMPTY_ELEMENTDATA赋给了elementData；
Add方法
public boolean add(E e)
/**
* Appends the specified element to the end of this list.
在列表尾部添加指定元素
*
* @param e element to be appended to this list
* @return true (as specified by {@link Collection#add})
*/
public boolean add(E e) {
//进入新的函数，开始进行容量确认工作
//size此时为0
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
ensureCapacityInternal函数：
private void ensureCapacityInternal(int minCapacity) {
//int minCapacity 就是我们传递进来的 size+1=1
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
calculateCapacity函数：
//这个函数用来计算当前的集合的容量
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//如果是空集合，就返回默认容量10
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
//如果不是空数组，就返回实际大小
return minCapacity;
}
ensureExplicitCapacity函数：
private void ensureExplicitCapacity(int minCapacity) {
//代表实际修改集合的次数
modCount++;
// 如果预期的容量（实际容量+1）大于实际的容量，进行扩容
if (minCapacity - elementData.length > 0)
//扩容
grow(minCapacity);
}
grow函数：
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
* 增加集合的容量，为了确保能容纳指定minCapacity参数指定的参数数量
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// 实际容量
int oldCapacity = elementData.length;
//扩容1.5倍后的新容量
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果新容量小于实际容量 不采取扩容，把预期容量赋值给新容量即可
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果新容量大于最大容量（Integer.MAX_VALUE - 8），则分配超大容量
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
//数组拷贝并赋值
elementData = Arrays.copyOf(elementData, newCapacity);
}
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).
*
* @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) {
//位置有效性检查
rangeCheckForAdd(index);
//添加修改次数以及判断是否需要扩张数组长度
ensureCapacityInternal(size + 1); // Increments modCount!!
//数组拷贝
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
//
elementData[index] = element;
size++;
}
rangeCheckForAdd函数
/**
* A version of rangeCheck used by add and addAll.
*/
private void rangeCheckForAdd(int index) {
//添加的索引如果大于size或者小于0 抛出异常 注意size比最大索引大1
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
public boolean addAll(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.)
* 把指定集合的元素按照给定的顺序添加到当前集合中 遍历的顺序跟指定集合中存储的顺序一致
*
* @param c collection containing elements to be added to this list
* @return true 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
//将a中的所有元素拷贝到数组elementData最后一个元素的后面
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
public boolean addAll(int index, 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.
*
* @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 true 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) {
//位置有效性检查
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
//添加修改次数以及判断是否需要扩张数组长度
ensureCapacityInternal(size + numNew); // Increments modCount
//计算需要移动的元素数量
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
//size增加numNew个
size += numNew;
return numNew != 0;
}
get方法
public E get(int index)
由于ArrayList底层是借助于数组来实现，因此，从ArrayList中获取元素就相当简单了。直接利用了数组随机访问能力强的特点。
/**
* Returns the element at the specified position in this list.
*返回指定位置的元素
*
* @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) {
//位置有效性检查
rangeCheck(index);
return elementData(index);
}
rangeCheck函数：
/**
* 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.
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
set方法
public E set(int index, E element)
/**
* Replaces the element at the specified position in this list with
* the specified element.
* 使用指定的元素替换指定位置的元素 并返回旧值
* @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) {
//位置有效性检查
rangeCheck(index);
//记录旧值
E oldValue = elementData(index);
//插入新值
elementData[index] = element;
//返回旧值
return oldValue;
}
remove方法
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).
* 移除指定位置的元素，把指定位置后面的所有元素往左移动1位，它们的索引全部减1
* 并返回旧值
* @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) {
//位置有效性检查
rangeCheck(index);
//实际修改次数加1
modCount++;
//记录旧值
E oldValue = elementData(index);
//需要移动的元素个数
int numMoved = size - index - 1;
if (numMoved > 0)
//执行数组拷贝
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//此时size还是原始size 先减去1 然后将最后一位索引处的元素置为null，让GC来回收
elementData[--size] = null; // clear to let GC do its work
//返回旧值
return oldValue;
}
rangeCheck函数：
/**
* 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.
*/
//检查指定的索引是否合法，否则抛出异常
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
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
* 删除第一次出现的指定元素 如果指定的元素在list中不存在，那么不会对集合修改。更确切的说就是
删除最小索引的并且与指定元素相同的元素。
* 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).
*
* @param o element to be removed from this list, if present
* @return true 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;
}
从源码中可以看到，无论是指定对象o是否为null，都是在ArrayList中找到与此第一个相等的元素的位置，然后调用fastRemove(index)来进行移除；如果没有找到指定对象o的位置，则返回false，表示没有移除成功。
fastRemove函数：
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
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; // clear to let GC do its work
}
fastRemove函数没有对index进行有效性检查，以及没有返回移除的旧值。原因就在于既然调用了fastTemove函数，就已经在 remove(Object o)函数中找到了相对应要删除的元素，那么索引也就确认了，索引就不用再去验证非法性了。
public boolean removeAll(Collection c)
/**
* Removes from this list all of its elements that are contained in the
* specified collection.
* 删除该集合中指定集合的全部存在元素，不存在的不操作
* @param c collection containing elements to be removed from this list
* @return {@code 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)
* @throws 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 Collection#contains(Object)
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
requireNonNull函数
/**
* 检查指定的对象引用是否不为空。该方法主要用于对方法和构造函数进行参数验证
* Checks that the specified object reference is not {@code null}. This
* method is designed primarily for doing parameter validation in methods
* and constructors, as demonstrated below:
*
* public Foo(Bar bar) {
* this.bar = Objects.requireNonNull(bar);
* }
*
*
* @param obj the object reference to check for nullity
* @param the type of the reference
* @return {@code obj} if not {@code null}
* @throws NullPointerException if {@code obj} is {@code null}
*/
public static <T> T requireNonNull(T obj) {
if (obj == null)
throw new NullPointerException();
return obj;
}
batchRemove函数(批量删除功能)
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0; //r作为遍历参数数组的索引 w为原数组的索引
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;
}
removeIf函数
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
// figure out which elements are to be removed
// any exception thrown from the filter predicate at this stage
// will leave the collection unmodified
int removeCount = 0;
final BitSet removeSet = new BitSet(size);
final int expectedModCount = modCount;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
@SuppressWarnings("unchecked")
final E element = (E) elementData[i];
if (filter.test(element)) {
removeSet.set(i);
removeCount++;
}
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
// shift surviving elements left over the spaces left by removed elements
final boolean anyToRemove = removeCount > 0;
if (anyToRemove) {
final int newSize = size - removeCount;
for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
i = removeSet.nextClearBit(i);
elementData[j] = elementData[i];
}
for (int k=newSize; k < size; k++) {
elementData[k] = null; // Let gc do its work
}
this.size = newSize;
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
return anyToRemove;
}
clear方法
/**
* Removes all of the elements from this list. The list will
* be empty after this call returns.
* 删除集合中的所有元素 并把原来的位置对应的元素全部置为null
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
sort方法
//对集合进行指定比较器的排序
@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++;
}
clone方法
/**
* //返回这个ArrayList实例的浅拷贝。(元素本身不会被复制。)
* Returns a shallow copy of this ArrayList instance. (The
* elements themselves are not copied.)
*
* @return a clone of this ArrayList 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);
}
}
indexOf方法
/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
返回在列表中第一次出现的指定元素的索引，如果不存在，返回-1
* 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.
*/
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
//null用==判断
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
isEmpty方法
/**
* Returns true if this list contains no elements.
* 返回这个列表是否没有元素 也就是size是否为0
* @return true if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
}
lastIndexOf方法
/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
返回在列表中最后一次出现的指定元素的索引，如果不存在，返回-1
* 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.
*/
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;
}
retainAll方法
/**
* 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.
* 仅保留此列表中包含在指定集合中的元素。换句话说，从该列表中删除指定集合中不包含的所有元素。
* @param c collection containing elements to be retained in this list
* @return {@code true} if this list changed as a result of the call
* @throws ClassCastException if the class of an element of this list
* is incompatible with the specified collection
* (optional)
* @throws 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 Collection#contains(Object)
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
toArray方法
/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
* 将列表转换为数组
* 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 an array containing all of the elements in this list in
* proper sequence
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}