ArrayList實現原理
概述
ArrayList能夠簡單的看做是動態數組，相對於普通的數組它能夠動態的增長容量或者減小容量。要注意的是ArrayList並非線程安全的，所以通常建議在單線程中使用ArrayList。java
實現原理
繼承關係
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList繼承AbstractList實現List, RandomAccess, Cloneable, java.io.Serializable接口數組
關鍵屬性
/**
* 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.
*/
// 數據的數組
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
// 實際數據的數量
private int size;
底層使用數組保存全部元素
transient 若是用transient聲明一個實例變量，當對象存儲時，它的值不須要維持。換句話來講就是，用transient關鍵字標記的成員變量不參與序列化過程安全
構造方法
/**
* 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 = {};
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
默認狀況下初始化空數組（長度爲0的數組）app
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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);
}
}
指定數組的初始容量
當指定的初始容量大於0，初始化指定大小的數組
當指定的初始容量等於0，初始化空數組
當指定的初始容量小於0，拋出IllegalArgumentException異常dom
/**
* 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) {
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;
}
}
初始化指定集合的數組
當指定集合不爲空即長度不爲0，則複製該集合，不然初始化一個空數組ide
E get(int index) 獲取index位置的元素
// Positional Access Operations
// 返回index下標的元素且強制轉化爲E（List<E>中的E）類型
@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) {
// 檢查index是否越界
rangeCheck(index);
// 返回index下標的元素
return elementData(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.
*/
private void rangeCheck(int index) {
// 檢查index是否大於等於size（數組的元素數量），由於數組下標從0開始計算，因此也不能等於元素數量
// 這裏沒有檢查index < 0的狀況，由於index < 0時數組會自動拋出異常，因此並未檢查index<0的狀況
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* Constructs an IndexOutOfBoundsException detail message.
* Of the many possible refactorings of the error handling code,
* this "outlining" performs best with both server and client VMs.
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
首先判斷index是否越界，這裏並無判斷是否小於0，由於下標小於0時數組會拋出異常。越界則拋出IndexOutOfBoundsException異常，反之返回數組對應index位置的元素ui
E set(int index, E element) 設置（覆蓋）index位置的元素
/**
* 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;
}
和get同樣先判斷index（下標）是否越界，不越界則先獲取原來index位置上的元素，接着設置（覆蓋）index位置上的元素，而後返回原來的元素，反之拋出IndexOutOfBoundsException異常this
boolean add(E e) 添加一個元素到列表尾
/**
* 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) {
// 檢查當前容量是否還能夠容納一個元素，不夠則擴容
ensureCapacityInternal(size + 1); // Increments modCount!!
// 添加到數組末尾
// 這個語句能夠分解爲
// elementData[size] = e;
// size += 1;
elementData[size++] = e;
return true;
}
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10; // 默認容量爲10
// 若是數據等於默認數據，返回默認容量和minCapacity（所需容量最小值）的最大值，反之返回所需容量最小值
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++; // 操做數+1
// overflow-conscious code
// 若是所需容量最小值大於實際數組的長度就擴大實際數組容量
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* 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
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; // 數組最大容量爲Integer最大值-8
/**
* 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
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
// 新的容量爲舊的容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
// 若是擴充容量後仍是不夠，則新的容量等於所需容量最小值（通常就是數組實際元素個數）
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
// 若是新的容量大於數組最大容量，再調用hugeCapacity計算新的容量
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
// 複製原來的數據到新的數組，數組容量爲新的容量
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
// 大於數組最大容量返回Integer最大值，反之返回數組最大容量
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
添加一個元素到列表尾，當列表容量不足時自動擴容（一般是擴容至原來的1.5倍），添加成功返回truespa
void add(int index, E element) 在index處放置元素
/**
* 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) {
// 檢查下標是否越界
rangeCheckForAdd(index);
// 檢查當前容量是否還能夠在容納一個元素，不夠則擴容
ensureCapacityInternal(size + 1); // Increments modCount!!
// 將elementData從index開始後面的元素日後移一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
/**
* A version of rangeCheck used by add and addAll.
*/
private void rangeCheckForAdd(int index) {
// 當index等於size時至關於添加元素到列表尾
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
將elementData數組從index開始後面的元素日後移一位，接着在index處放置元素
模擬添加數據(lierabbit)到index=4過程以下
線程
boolean addAll(Collection<? extends E> c) 添加一個集合裏的全部元素到列表尾
/**
* 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;
// 檢查當前容量是否還能夠在容納a數組的元素，不夠則擴容
ensureCapacityInternal(size + numNew); // Increments modCount
// 將a數組裏的元素添加到elementData末尾
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
// a數組不爲空（長度不爲0）時返回true，反之false
return numNew != 0;
}
將要添加的集合變爲數組，而後將其複製到elementData數組末尾
boolean addAll(int index, Collection<? extends E> c) 添加一個集合裏的全部元素到index位置
/**
* 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) {
// 檢查下標是否越界
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
// 檢查當前容量是否還能夠在容納a數組的元素，不夠則擴容
ensureCapacityInternal(size + numNew); // Increments modCount
// 須要日後移動幾個位置
int numMoved = size - index;
if (numMoved > 0)
// 從index開始，日後的元素向後移動numMoved個位置
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
// 將a數組裏的全部元素複製到elementData從index到index + numNew -1的位置上
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
// a數組不爲空（長度不爲0）時返回true，反之false
return numNew != 0;
}
將要添加的集合變爲數組，而後把elementData數組從index開始，日後的元素向後移動numMoved個位置，接着將要添加的數組裏的全部元素複製到elementData從index到index + numNew -1的位置上
void trimToSize() 改變列表內部數組容量至列表實際元素數量
/**
* Trims the capacity of this <tt>ArrayList</tt> instance to be the
* list's current size. An application can use this operation to minimize
* the storage of an <tt>ArrayList</tt> instance.
*/
public void trimToSize() {
modCount++; // 操做數+1
// 若是數組實際元素數量小於數組長度
if (size < elementData.length) {
// 若是數組實際元素數量等於0則數組被賦值爲空數組，反之建立一個新的元素數量等於數組長度的數組
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
當數據穩定了以後可使用這個方法來減小內存的使用
int indexOf(Object o) 查找o元素在列表第一次出現的位置
/**
* 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) {
//元素能夠爲null，若是爲null返回null的下標
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;
}
// 沒有找到對應的元素返回-1
return -1;
}
ArrayList中能夠存放null元素，indexof是返回elementData數組中值相同的首個元素的下標，indexof中比較方法是equals而equals是比較元素的值，所以必須對null單獨查找。若是未找到該元素則返回-1
E remove(int index) 刪除index位置上的元素
/**
* 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) {
// 檢查下標是否越界
rangeCheck(index);
modCount++; // 操做數+1
E oldValue = elementData(index); // 獲取index位置上的元素
int numMoved = size - index - 1; // 須要往前移動幾個位置
if (numMoved > 0)
// 從index + 1開始，日後的元素向前移動1個位置
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
// 將數組末尾元素置空
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
模擬刪除index=4（值爲lierabbit）過程以下
boolean remove(Object o) 刪除o元素
/**
* 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) {
// 元素能夠爲null，分開搜索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;
}
}
// 沒有找到返回false
return false;
}
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
// 因爲已經找到元素，則元素一定存在，則index一定合理，因此不須要在檢查index是否越界
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
}
經過尋找o元素，能夠得到其下標，再根據下標刪除o元素
forEach(Consumer<? super E> action)遍歷列表
/**
* 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.
*/
protected transient int modCount = 0;//操做數
@Override
public void forEach(Consumer<? super E> action) {
// 確保不爲空
Objects.requireNonNull(action);
final int expectedModCount = modCount;
@SuppressWarnings("unchecked")
final E[] elementData = (E[]) this.elementData;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
action.accept(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
/**
* 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:
* <blockquote><pre>
* public Foo(Bar bar) {
* this.bar = Objects.requireNonNull(bar);
* }
* </pre></blockquote>
*
* @param obj the object reference to check for nullity
* @param <T> 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;
}
這裏能夠看到modCount的用處，當modCount發生改變後，馬上拋出ConcurrentModificationException異常。經過以前的分析能夠知道當列表內容被修改時modCount會增長。也就是說若是在遍歷ArrayList的過程當中有其餘線程修改了ArrayList，那麼將拋出ConcurrentModificationException異常
ArrayList小結
- ArrayList是List接口的一個可變大小的數組的實現
- ArrayList的內部是使用一個Object對象數組來存儲元素的
- 初始化ArrayList的時候，能夠指定初始化容量的大小，若是不指定，就會使用默認大小，爲10
- 當添加一個新元素的時候，首先會檢查容量是否足夠添加這個元素，若是夠就直接添加，若是不夠就進行擴容，擴容爲原數組容量的1.5倍
- 當在index處放置一個元素的時候，會將數組index處右邊的元素所有右移
- 當在index處刪除一個元素的時候，會將數組index處右邊的元素所有左移
本文首發： https://lierabbit.cn/2018/01/...
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