一.继承结构图
二.arrayList介绍
arrayList是基于数组实现的,(是一个动态数组,容量自动增长),从上面继承结构图可以看出她实现了cloneable接口,所以可以调用object的clone()方法,实现了serializable接口,因此它支持序列化。实现了RandomAccess接口,支持快速随机访问(其实就是通过下标号进行快速访问)
三.总结
(为了方便阅读,我把总结放到上面来,建议还是先阅读源码再来看总结)
1.arrayList扩容原理?
当向arrayList中添加元素时,首先会调用ensureCapacityInternal(size+1)方法,该方法首先会判断是否要进行扩容,如果需要就调用grow()方法,该方法会创建一个原数组1.5倍的新数组,并调用Arrays.copyof()方法进行数组拷贝,从而实现扩容2.ArrayList频繁增加或删除对象为什么效率低?
arraylist每次删除或增加的操作都会涉及到通过system.arraycopy()进行数组的拷贝,十分影响效率3.arraylist遍历的方式有哪些?
(1)for循环遍历,(2)foreach遍历,(3)list.iteratro获取迭代器去遍历4.迭代器再进行遍历的时候应该注意什么?
迭代器在进行初始化的时候会将当前的modCount值赋值给exceptedmodCount变量,在每次迭代时都会校验modcount和exceptedCount是否相等,如果不相等就会抛出java.util.ConcurrentModificationException异常,所以在迭代过程中要进行元素的remove和add方法时应该使用迭代器自身的方法而不是去用集合的方法,因为集合的remove和add方法都会改变modcount值,但是迭代器的方法不会
四.源码阅读
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { //序列版本号 private static final long serialVersionUID = 8683452581122892189L; //默认容量为10 private static final int DEFAULT_CAPACITY = 10; //空数组实例 private static final Object[] EMPTY_ELEMENTDATA = {}; private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; //arrayList基于该数组实现,用来保存数据 transient Object[] elementData; // non-private to simplify nested class access //arraylist中实际数据的数量 private int size; //ArrayList带容量大小的构造函数 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); } } //Arraylist空参构造函数 public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } //包含一个collection的Arraylist 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; } } /将当前容量设置为实际元素个数 public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } } //扩容函数,根据期望容量minCapacity来扩大arraylist容量 public void ensureCapacity(int minCapacity) { //最小可扩展容量,当前集合为空,可扩充容量为10,否则为0 int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0: DEFAULT_CAPACITY; //如果期望扩充容量>最小可扩展容量,进行扩容操作 if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } //计算扩容容量,如果是未初始化的集合,就设置其扩容容量为默认容量(10),否则返回扩容容量 private static int calculateCapacity(Object[] elementData, int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; } //相当于是计算扩容容量+进行扩容操作 private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } //线程安全的扩容函数 private void ensureExplicitCapacity(int minCapacity) { //线程安全的相关变量(fail-fast机制) modCount++; // 调用grow函数进行扩容 if (minCapacity - elementData.length > 0) grow(minCapacity); } //集合的最大大小(和jvm的性能有关,那8字节是保存下标的) private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; private void grow(int minCapacity) { // 容量变为原来的1.5倍 int oldCapacity = elementData.length; int newCapacity = oldCapacity + (oldCapacity >> 1); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); // 完成扩容后拷贝新数组 elementData = Arrays.copyOf(elementData, newCapacity); } //获取Arraylist的最大容量值 private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } //当前Arraylist大小 public int size() { return size; } //当前Arraylist是否为空 public boolean isEmpty() { return size == 0; } //判断集合是否包含某个元素 public boolean contains(Object o) { return indexOf(o) >= 0; } //某个元素首次出现的位置 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; } //某个元素最后一次出现的位置 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; } //arraylist重写的clone仍然是调用的object的clone()方法,只是为新的集合定义了一个用于线程 //安全的modCount对象 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); } } //集合转数组的第一种重载,转为Object类型的数组 public Object[] toArray() { return Arrays.copyOf(elementData, size); } //集合转数组的第二种重载,转换为特定类型的数组 public <T> T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } //获得某个下标代表的元素(不带范围检验) E elementData(int index) { return (E) elementData[index]; } //获得某个下标代表的元素(带范围检验) public E get(int index) { rangeCheck(index); return elementData(index); } //设置某个下标对应的元素 public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } //添加元素前先检查是否需要扩容,如果需要的话进行扩容操作 public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } //在指定位置进行添加 public void add(int index, E element) { rangeCheckForAdd(index); ensureCapacityInternal(size + 1); // Increments modCount!! //原数组 原数组起始 目标数组 目标数组起始 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) //检查范围 修改modCount 将删除元素之后的元素向前移动一个位置 list末尾元素置空 System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work return oldValue; } //删除arraylist中首次出现的元素(如果存在的话) 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])) { //调用fastremove增加效率 fastRemove(index); return true; } } return false; } //省去了边界检查,私有的方法,用于被remove(object o)调用 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 } //将元素全部置空,等待被垃圾回收器回收 public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; } //将集合 c的全部元素都拷贝到该数组中 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; } //从某个位置开始进行拷贝 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; }