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。
01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。
02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.
03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 work return 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 ? get(i)==null : 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。
