java容器
ArrayList
概述
基于动态数组实现，支持随机访问，实现了List接口，顺序容器（元素存放的数据与放心去的顺序相同，允许放入null元素，底层通过数组实现）
每个ArrayList都有一个容量(capacity)，表示底层数组的实际大小，容器内存储元素的个数不能多余当前容量。添加元素时，若容量不足时，自动扩容。这里的数组是一个Object数组，以便能够容纳任何类型的对象
ps: ArrayList没有实现同步，如果需要多个线程并发访问用户可以手动同步，也可使用Vector替代
底层数据结构
//transient 不需要序列化的属性
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
*/
private int size;
构造函数
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final int DEFAULT_CAPACITY = 10;
//构造一个固定容量到小的arrayList
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);
}
}
//默认数组大小为10
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 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) {
//如果数组缓冲区的类型不是Object[]的，要用Arrays.copyOf()方法转化为Object[]，例如new ArrayList 和 Arrays.asList()在toArray()后返回的类型不同，后者返回java.util.Arrays&ArrayList,toArray()后返回类java.lang.String
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// 否则返回空链表
this.elementData = EMPTY_ELEMENTDATA;
}
}
添加元素 add(), addAll()
-
Add(e) 和add(index ,e)这两个方法都是向容器中添加新元素
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } //先对元素进行移动，然后完成插入操作 public void add(int index, E element) { //检验插入位置 rangeCheckForAdd(index); ensureCapacityInternal(size + 1); // Increments modCount!! //移动元素 System.arraycopy(elementData, index, elementData, index + 1, size - index); //插入元素 elementData[index] = element; size++; } private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } -
addAll()
//在末尾添加 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; } //指定位置添加 public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); Object[] a = c.toArray(); int numNew = a.length; //手动扩容 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; } //时间复杂度不仅跟插入元素的多少有关，也跟插入的位置有关
修改元素 Set()
//直接对数组指定的位置赋值
public E set(int index, E element) {
//校验数组下标是否越界
rangeCheck(index);
E oldValue = elementData(index);
//赋值到指定的位置，复制的仅仅是引用
elementData[index] = element;
return oldValue;
}
E elementData(int index) {
return (E) elementData[index];
}
获取元素Get()
public E get(int index) {
rangeCheck(index);
//注意类型转换
return elementData(index);
}
E elementData(int index) {
return (E) elementData[index];
}
//获取元素第一次出现的index
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;
}
//获取元素最后一次出现的index
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;
}
删除元素 remove()
//删除指定位置的元素，将需要删除点之后的元素向前移动一个位置
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
//需移动的数组长度
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//清除该位置的引用，让GC起作用
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
//ps:对象能否被GC的依据是是否还有引用指向它，否则原来的对象就一直不会回收
自动扩容
- 添加元素时可能会导致容量不足，在添加元素之前，都要检查添加元素后是否会超出数组长度，如果需要则自动扩容，最终通过grow()方法完成
//数组扩容，在实际添加大量元素前，也可使用此方法来手动增加ArrayList实例的容量，以减少递增式再分配的数量
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) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
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;
//数组扩容时，将老数组中的元素拷贝一份到新数组中，每次数组容量的增长大约是原容量的1.5倍
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;
}
调整容量
//将底层数组的容量调整为当前列表保存的实际元素的大小
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
Fail-Fast机制
采用了快速失败的机制，通过记录modCount参数来实现。在面对并发的修改时，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险。
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