java.lang.Object  ↳     java.util.AbstractCollection<E>  ↳     java.util.AbstractList<E>  ↳     java.util.ArrayList<E> //ArrayList 继承关系  public class ArrayList<E> extends AbstractList<E>  implements List<E>, RandomAccess, Cloneable, java.io.Serializable {} //注意看这里 

ArrayList()  ArrayList(int capacity) // capacity 创建默认容量，容量是动态的，不足时会增加一半  ArrayList(Collection<? extends E> collection) // 这里包含了 collection 

// 继承于 Collection boolean add(E object) boolean addAll(Collection<? extends E> collection) void clear() boolean contains(Object object) boolean containsAll(Collection<?> collection) boolean equals(Object object) int hashCode() boolean isEmpty() Iterator<E> iterator() boolean remove(Object object) boolean removeAll(Collection<?> collection) boolean retainAll(Collection<?> collection) int size() <T> T[] toArray(T[] array) Object[] toArray() // 继承于 AbstractCollection void add(int location, E object) boolean addAll(int location, Collection<? extends E> collection) E get(int location) int indexOf(Object object) int lastIndexOf(Object object) ListIterator<E> listIterator(int location) ListIterator<E> listIterator() E remove(int location) E set(int location, E object) List<E> subList(int start, int end) // ArrayList 新增 Object clone() void ensureCapacity(int minimumCapacity) void trimToSize() void removeRange(int fromIndex, int toIndex) 

package java.util;  public class ArrayList<E> extends AbstractList<E>  implements List<E>, RandomAccess, Cloneable, java.io.Serializable {  private static final long serialVersionUID = 8683452581122892189L;   // 刚才提到的 elementData，用于存储数值的数组  private transient Object[] elementData;   // 目前的实际数组大小  private int size;   // 默认构造函数，带有初始容量(initial)  public ArrayList(int initialCapacity) {  super();  if (initialCapacity < 0)  throw new IllegalArgumentException("Illegal Capacity: "+  initialCapacity);  // 新建数组  this.elementData = new Object[initialCapacity];  }   // 未声明初始容量的时候，默认为 10  public ArrayList() {  this(10);  }   // 创建一个包含 collection 的 ArrayList  public ArrayList(Collection<? extends E> c) {  elementData = c.toArray();  size = elementData.length;  // c.toArray might (incorrectly) not return Object[] (see 6260652)  if (elementData.getClass() != Object[].class)  elementData = Arrays.copyOf(elementData, size, Object[].class);  }    // 动态调整容量，将容量设为实际元素个数  public void trimToSize() {  modCount++;  int oldCapacity = elementData.length;  if (size < oldCapacity) {  elementData = Arrays.copyOf(elementData, size);  }  }    // 如果容量不足，扩展数组：容量=(容量x3)/2 + 1  public void ensureCapacity(int minCapacity) {  modCount++;  int oldCapacity = elementData.length;  // 如果容量不足，扩展数组：容量=(容量x3)/2 + 1  if (minCapacity > oldCapacity) {  Object oldData[] = elementData;  int newCapacity = (oldCapacity * 3)/2 + 1;  if (newCapacity < minCapacity)  newCapacity = minCapacity;  elementData = Arrays.copyOf(elementData, newCapacity);  }  }   // 添加元素e  public boolean add(E e) {  ensureCapacity(size + 1); // Increments modCount!!  // 添加e到ArrayList中  elementData[size++] = e;  return true;  }   // 返回实际大小  public int size() {  return size;  }   public boolean contains(Object o) {  return indexOf(o) >= 0;  }   // 检查 ArrayList 是否为空  public boolean isEmpty() {  return size == 0;  }   // 正向查找元素的索引值  // 这里跟 String Methods 的各种函数都差不多  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;  }   // 反向查找元素的索引值  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;  }   // 反向查找元素(o)的索引值  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;  }    public Object[] toArray() {  return Arrays.copyOf(elementData, size);  }   public <T> T[] toArray(T[] a) {  // 新建一个T[]数组，数组大小是目前元素数，克隆数组元素  if (a.length < size)  return (T[]) Arrays.copyOf(elementData, size, a.getClass());   System.arraycopy(elementData, 0, a, 0, size);  if (a.length > size)  a[size] = null;  return a;  }   // 获取 index 位置的元素值  // 类似 charAt  public E get(int index) {  RangeCheck(index);   return (E) elementData[index];  }   public E set(int index, E element) {  RangeCheck(index);   E oldValue = (E) elementData[index];  elementData[index] = element;  return oldValue;  }   public boolean add(E e) {  ensureCapacity(size + 1); // Increments modCount!!  elementData[size++] = e;  return true;  }   // 将 e 添加到ArrayList的指定位置  public void add(int index, E element) {  if (index > size || index < 0)  throw new IndexOutOfBoundsException(  "Index: "+index+", Size: "+size);   ensureCapacity(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 = (E) elementData[index];   int numMoved = size - index - 1;  if (numMoved > 0)  System.arraycopy(elementData, index+1, elementData, index,  numMoved);  elementData[--size] = null; // Let gc do its work   return oldValue;  }   // 删除指定位置的元素  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;  }    // 快速删除第 index 个元素  private void fastRemove(int index) {  modCount++;  int numMoved = size - index - 1;  // 从 index+1 开始，用后面的元素替换前面的元素  //位移  if (numMoved > 0)  System.arraycopy(elementData, index+1, elementData, index,  numMoved);  // 将最后一个元素设为 null  elementData[--size] = null; // Let gc do its work  }   // 删除元素  // 区别在于这里需要查找元素  public boolean remove(Object o) {  if (o == null) {  for (int index = 0; index < size; index++)  if (elementData[index] == null) {  fastRemove(index);  return true;  }  } else {  // 便利 ArrayList，找到 元素o，删除  for (int index = 0; index < size; index++)  if (o.equals(elementData[index])) {  fastRemove(index);  return true;  }  }  return false;  }   // 清空ArrayList  public void clear() {  modCount++;   for (int i = 0; i < size; i++)  elementData[i] = null;   size = 0;  }   // 将集合 c 加到 ArrayList 中  public boolean addAll(Collection<? extends E> c) {  Object[] a = c.toArray();  int numNew = a.length;  ensureCapacity(size + numNew); // Increments modCount  System.arraycopy(a, 0, elementData, size, numNew);  size += numNew;  return numNew != 0;  }   // 从 index 位置开始，将集合 c 添加到 ArrayList  public boolean addAll(int index, Collection<? extends E> c) {  if (index > size || index < 0)  throw new IndexOutOfBoundsException(  "Index: " + index + ", Size: " + size);   Object[] a = c.toArray();  int numNew = a.length;  ensureCapacity(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fromIndex 到toIndex 之间的全部元素  protected void removeRange(int fromIndex, int toIndex) {  modCount++;  int numMoved = size - toIndex;  System.arraycopy(elementData, toIndex, elementData, fromIndex,  numMoved);   // Let gc do its work  int newSize = size - (toIndex-fromIndex);  while (size != newSize)  elementData[--size] = null;  }   private void RangeCheck(int index) {  if (index >= size)  throw new IndexOutOfBoundsException(  "Index: "+index+", Size: "+size);  }    // 克隆函数  public Object clone() {  try {  ArrayList<E> v = (ArrayList<E>) super.clone();  // 将当前 ArrayList 的全部元素克隆到 v 中  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();  }  }    // java.io.Serializabl e的写入函数  // 将ArrayList的 容量、元素值 都写入到输出流中  private void writeObject(java.io.ObjectOutputStream s)  throws java.io.IOException{  // Write out element count, and any hidden stuff  int expectedModCount = modCount;  s.defaultWriteObject();   // 写入数组的容量  s.writeInt(elementData.length);   // 写入元素  for (int i=0; i<size; i++)  s.writeObject(elementData[i]);   if (modCount != expectedModCount) {  throw new ConcurrentModificationException();  }   }    // java.io.Serializable 的读取函数，基本一致  private void readObject(java.io.ObjectInputStream s)  throws java.io.IOException, ClassNotFoundException {  s.defaultReadObject();   int arrayLength = s.readInt();  Object[] a = elementData = new Object[arrayLength];   for (int i=0; i<size; i++)  a[i] = s.readObject();  } } 

import java.util.ArrayList; // 调用库  public class Example {  public static void main(String []args)  {  ArrayList list = new ArrayList();  // ...  } }