Java中List是一个必须要掌握的基础知识，List是一个接口，实现List接口的基础类有很多，其中最具有代表性的两个：ArrayList和LinkedList。

1、变量

ArrayList是一个底层基于数组实现动态大小扩容的数据结构，快速访问、可复制、序列化的。继承自 AbstractList，实现了 List 接口。写代码时，我们经常用到这个数据结构来做大数据量的访问和（程序）缓存存储等。

     /*** Default initial capacity.* 默认初始容量10*/private static final int DEFAULT_CAPACITY = 10;/*** Shared empty array instance used for empty instances.* 共享空数组实例值*/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.* 跟第一个空数组实例区分，当第一个元素添加进来时，数组会扩展多大*/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.* 太难翻译了，transient用来表示一个域不是该对象序行化的一部分*/transient Object[] elementData; // non-private to simplify nested class access/*** The size of the ArrayList (the number of elements it contains).* 数组的大小* @serial*/private int size;

以上是arrayList的基本变量组成，private int size; 和 transient Object[] elementData; 则是ArrayList的成员变量。

2、构造函数

2.1 无参构造函数

     /*** Constructs an empty list with an initial capacity of ten.*/public ArrayList() {this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}

注释说初始给数组赋值空间大小时为10的list集合，当第一次添加元素时即给数组容量扩大了10倍。

2.2 带初始容量大小的构造函数

    /*** 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*/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);}}

当initialCapacity = 0时，则把EMPTY_ELEMENTDATA赋值给 elementData；当initialCapacity大于0时，初始化为一个大小为initialCapacity的Object数组，并赋值给elementData。否则，抛出异常“非法的大小”；即初始容量大小大于等于0.

3、主要操作方法

 3.1 add（）添加方法

往数组中添加一个元素e，数组容量大小增加一。

    /*** Appends the specified element to the end of this list.** @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 static int calculateCapacity(Object[] elementData, int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {return Math.max(DEFAULT_CAPACITY, minCapacity);}return minCapacity;}

每次添加元素到集合中时都会先确认下集合容量大小，随后再增加1。

3.2 remove()移除方法

分2个，一个是移除元素在数组中的位置的方法。一个是移除元素的方法。

/*** Removes the element at the specified position in this list.* Shifts any subsequent elements to the left (subtracts one from their* indices).** @param index the index of the element to be removed* @return the element that was removed from the list* @throws IndexOutOfBoundsException {@inheritDoc}*/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 workreturn oldValue;}/*** Removes the first occurrence of the specified element from this list,* if it is present.  If the list does not contain the element, it is* unchanged.  More formally, removes the element with the lowest index* <tt>i</tt> such that* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>* (if such an element exists).  Returns <tt>true</tt> if this list* contained the specified element (or equivalently, if this list* changed as a result of the call).** @param o element to be removed from this list, if present* @return <tt>true</tt> if this list contained the specified element*/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 remove method that skips bounds checking and does not* return the value removed.*/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}

可以很快速的理解：移除元素在数组中位置的方法是，把该元素在数组的中的下标位置对应的元素移除。先检查对应入参的下标位置是否超出数组的大小限制（数组溢出），然后把该元素移除，再把数组大小减一。

移除元素，当元素为空的时候，匹配数组中是否有空的元素，有则移除，数组大小减一。元素不为空的时候，循环数组元素，匹配元素后，移除数组，数组大小减一。

3.3 isEmpty()和size()等常用函数方法

    /*** Returns the number of elements in this list.** @return the number of elements in this list*/public int size() {return size;}/*** Returns <tt>true</tt> if this list contains no elements.** @return <tt>true</tt> if this list contains no elements*/public boolean isEmpty() {return size == 0;}

在进行数组的大小比较和判断操作时用的一些基本方法。

ArrayList 底层基于数组实现容量大小动态可变。 扩容机制为首先扩容为原始容量的 1.5 倍。如果1.5倍太小的话，则将我们所需的容量大小赋值给 newCapacity，如果1.5倍太大或者我们需要的容量太大，那就直接拿 newCapacity = (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE 来扩容。