ArrayList源码浅析（jdk1.8）
ArrayList的实质就是动态数组。所以可以通过下标准确的找到目标元素，因此查找的效率高。但是添加或删除元素会涉及到大量元素的位置移动，所以效率低。
一、构造方法
ArrayList提供了3个构造方法
1、无参的，就是把表示集合的数组赋值为空。
public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }
2、带初始化大小的构造方法，就是new一个指定大小的数组赋值给表示集合的数组。
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); } }
3、带集合的构造方法，就是把传过来的集合转成数组在赋值给表示集合的数组。
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; } }
二、添加元素
1、add(E e)方法，添加元素前会先判断数组是否为空，如果为空，则创建一个长度为10的空数组，
然后在把所添加的元素加入到数组中。如果不为空，则判断集合大小是否超出数组的长度，
如果超出则进行数组扩容，不然直接进行赋值。数组扩容的规则是原先数组长度加上其一半。即扩容1.5倍。
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(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); }
2、add(int index, E element)方法，添加元素到指定位置。添加前先判断所传参数是否超出集合大小或小于0，
如果是则抛出数组越界异常，符合条件则在进行和add(E e)方法一样的数组大小的检查，看看是否扩容。
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 rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(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); }
3、addAll(Collection<? extends E> c)方法，添加集合到集合中。把集合转成数组，然后检查数组是否进行扩容，
如果不须就直接进行添加。
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; }
三、删除元素
1、remove(int index)根据下标删除元素，先检查下标是否越界，然后获取本下标的值用于返回，最后进行数组的变动。
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); elementData[--size] = null; // clear to let GC do its work return oldValue; } private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } E elementData(int index) { return (E) elementData[index]; }
2、remove(Object o)根据元素删除值，for循环整个集合来检查是否有和参数相同的元素，有就根据索引进行删除。
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; } 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 }
3、removeRange(int fromIndex, int toIndex)根据下标范围进行删除元素。
protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // clear to let GC do its work int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; }
4、removeAll(Collection<?> c)只要所传集合中的元素在本身集合中就删除，取差集的意思。
public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); } private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; 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; }
四、其他方法
1、retainAll(Collection<?> c)，只要所传集合中的元素不在本身集合中就删除，取并集的意思。
public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; 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; }
2、set(int index, E element)，取代指定位置的元素。
public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }
3、trimToSize()，裁剪空间。
public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } }
五、其他部分源码
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { private static final long serialVersionUID = 8683452581122892189L; /** * 默认集合初始容量为10 */ private static final int DEFAULT_CAPACITY = 10; private static final Object[] EMPTY_ELEMENTDATA = {}; private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * 存储集合元素的数组 */ transient Object[] elementData; // non-private to simplify nested class access /** * 集合大小（元素个数） * * @serial */ private int size; /** * 构造一个带具体初始容量的空集合 * 就是new一个指定大小的object数组 * @param initialCapacity 集合的初始容量 */ 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); } } /** * 构造一个初始为10的空集合 */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } /** * 参数为集合的构造函数，集合元素被复制到数组中 * * @param c 集合参数*/ public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { // 防止c.toArray()返回的不是数组 if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // 空数组 this.elementData = EMPTY_ELEMENTDATA; } } /** * 确保容量 */ public void ensureCapacity(int minCapacity) { int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) // any size if not default element table ? 0 // larger than default for default empty table. It's already // supposed to be at default size. : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } // ensureExplicitCapacity(minCapacity); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * 最大容量 */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; /** * 容量扩展 * @param minCapacity the desired minimum capacity */ 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); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } /** * 返回集合大小 * * @return the number of elements in this list */ public int size() { return size; } /** * 集合为空返回true * * @return <tt>true</tt> if this list contains no elements */ public boolean isEmpty() { return size == 0; } /** * 判断集合是否包含一个元素 */ public boolean contains(Object o) { return indexOf(o) >= 0; } /** * 判断集合是否包含一个元素的具体方法 * 就是for循环整个数组进行比较，存在就返回下标，否则返回-1 */ public int indexOf(Object o) { 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; } return -1; } /** * 从集合的尾部开始判断是否含有某个元素 * 规则同indexOf（）方法 */ 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; } /** * 克隆 */ 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); } } /** * 集合转数组 */ public Object[] toArray() { return Arrays.copyOf(elementData, size); } // 返回数组指定下标的元素 @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } /** * 根据指定位置获取元素 */ public E get(int index) { // 越界检查 rangeCheck(index); return elementData(index); } /** * 把某元素塞到集合指定的位置(替换)，然后返回旧的值 */ public E set(int index, E element) { // 越界检查 rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } /** * 添加元素 */ public boolean add(E e) { // 集合容量检查 ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } /** * 把元素添加到指定位置 */ 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++; } /** * 删除指定位置的元素 */ 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); elementData[--size] = null; // clear to let GC do its work return oldValue; } /** * 删除指定元素 */ 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; } /* * 删除指定位置的元素 */ 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 } /** * 清空集合 */ public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; } /** * 添加集合到集合中 */ 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; } /** * 从指定位置添加集合 */ 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; } /** * 删除指定范围的集合元素 */ protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // clear to let GC do its work int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; } /** * 集合越界检查 */ private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * 添加元素时进行越界检查 */ private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * 越界信息提示 */ private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; } /** * 根据所传集合的元素进行删除指定的元素（差集） */ public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); } /** * 根据所传集合和本身集合取交集（交集） */ public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; 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; } }
