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
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
/** * 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
/** * 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
// 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
/** * 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
/** * 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
/** * 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
/** * 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
/** * 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
/** * 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
/** * 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
/** * 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
/** * 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
/** * 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
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