arraylist源码面试题：ArrayList源码分析面试你需要他
arraylist源码面试题：ArrayList源码分析面试你需要他/** * 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 * 指定容量的构造函数， */ public ArrayList(int initialCapacity) { if (initialCapacity > 0) { this.elementData = new Object[init
ArrayList源码如下这里我们分析下源码，首先要说的，我们面试讲源码主要还是将下源码的重点。下面我们来分析下重点。
[目标]：从类的基本信息、属性、构造函数、重要方法分析Arraylsit.
[详解]：顶部的注释给我们透露了几点重要信息：ArrayList是List接口的大小可变数组的实现；ArrayList允许null元素；ArrayList的容量可以自动增长；ArrayList不是同步的；ArrayList的iterator和listIterator方法返回的迭代器是fail-fast的.
首先：看下类的基本信息：
继承了AbstractList，实现了List、RandomAccess Cloneable Serializable接口
- 实现了List接口：ArrayList的父类AbstractList也实现了List接口，那为什么子类ArrayList还是去实现一遍呢？collection 的作者Josh说他写这代码的时候觉得这个会有用处，但是其实并没什么用，但因为没什么影响，就一直留到了现在。
- 实现了RandomAccess接口：表明ArrayList支持快速（通常是固定时间）随机访问。在ArrayList中，我们可以通过元素的序号快速获取元素对象，这就是快速随机访问。
- 实现了Cloneable接口：实现了该接口，就可以使用Object.Clone()方法了。 implements java.io.Serializable：表明该类具有序列化功能，该类可以被序列化，什么是序列化？简单的说，就是能够从类变成字节流传输，然后还能从字节流变成原来的类。
public class ArrayList<E> extends AbstractList<E>
implements List<E> RandomAccess Cloneable java.io.Serializable
{}
第二点是属性：
/**
* Default initial capacity.
* 初始化的容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* 指定该ArrayList容量为0时，返回该空数组。
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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.
* 调用无参构造方法时，默认返回改空数组；
* 这个和上面数组的区别就是：上面的是用户指定容量为0的时候返回的，当前的数组是默认构造函数返回的
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 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.
* 保存添加到ArrayList的元素
* 当ArrayList等于无参构造函数得到的数组时，当第一个元素添加时，容量将变成DEFAULT_CAPACITY
* ps:transient修饰此变量，不会序列化到
* 从这里我们可以看到，底层都是通过数组来存放元素的
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
* ArrayList的size
* @serial
*/
private int size;
/**
* 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
* 数组的最大容量，一些vm需要一些头部信息，所以会减8。超过MAX_ARRAY_SIZE会内存溢出
*
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//备注：MAX.VALUE为0x7fffffff，转换成十进制就是2147483647，也就是数组的最大长度是2147483639；
第三：构造函数
（1）ArrayList(int initialCapacity)：构造一个指定容量为capacity的空ArrayList。这是一个带初始容量大小的有参构造函数。 （2）无参构造函数：返回空数组DEFAULTCAPACITY_EMPTY_ELEMENTDATA （3）ArrayList(Collection<? extends E> c):入参是集合，入参长度为0，返回空数组EMPTY_ELEMENTDATA；入参长度不是0，
/**
* 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
* 指定容量的构造函数，
*/
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);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
* 无参构造函数，默认返回空数组
*/
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
* 根据一个集合，构造包含集合元素的ArrayList。他们将按照集合中迭代器的顺序返回
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// defend against c.toArray (incorrectly) not returning Object[]
// (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData size Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
第四:重点方法
/**
* Appends the specified element to the end of this list.
* 添加指定的元素到list的末尾
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size 1); // Increments modCount!!
elementData[size ] = e;
return true;
}
//
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
modCount ;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* 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;
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;
}
