//定义一个默认的长度10
private static final int DEFAULT_CAPACITY = 10;
//定义空的数组
private static final Object[] EMPTY_ELEMENTDATA = {};
//定义数组用来存储放入ArrayList的元素
private transient Object[] elementData;
//定义记录ArrayList中元素的个数
private int size;
//构造方法--创建指定长度的数组的ArrayList
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
//创建ArrayList数组长度为0的ArrayList,当向其中添加第一个元素的时候，扩展数组的长度为10--默认数组长度
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
//创建含有指定集合元素的ArrayList，通过集合的toArray()方法返回一个数组，让ArrayList的数组指向该数组
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
//当ArrayList的元素个数小于数组长度时候，将数组的长度减小为ArrayList中元素的个数
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
// 向ArrayList中添加一个元素，添加前先检查数组的容量，是否需要扩容，再添加元素进去
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
//注意：minCapavity为添加元素时ArrayList需要最小的容量--确定需要的最小容量
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
//判断数组是否要进行扩容量，需要的容量大于数组长度就需要扩容
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
}
//控制内存溢出使用，对最大容量的限制，最大的数组长度为整型最大值减8
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//扩展容量：先根据原来的容量扩展为原来的1.5倍，再和最小需要的容量比较，假如还是比需要的小，则将最小需要的赋值给数组新容量
假如新容量比最大的容量大，则再次扩容，扩容完成后将数组原来的内容，复制到新的数组中，其实本质实现还是利用的本地方法
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);
}
//再次进行扩容
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
1 //按照下标进行添加，涉及到下标的先检查下标是否越界，检测容量，移动元素，添加元素
2 public void add(int index, E element) {
3 rangeCheckForAdd(index);
4
5 ensureCapacityInternal(size + 1); // Increments modCount!!
6 System.arraycopy(elementData, index, elementData, index + 1,
7 size - index);
8 elementData[index] = element;
9 size++;
10 }
11 //判断下标是否越界
12 private void rangeCheckForAdd(int index) {
13 if (index > size || index < 0)
14 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
15 }
16 //添加集合中所有元素进入ArrayList,先检查容量，在复制，修改元素个数
17 public boolean addAll(Collection<? extends E> c) {
18 Object[] a = c.toArray();
19 int numNew = a.length;
20 ensureCapacityInternal(size + numNew); // Increments modCount
21 System.arraycopy(a, 0, elementData, size, numNew);
22 size += numNew;
23 return numNew != 0;
24 }
25 //从特定下标开始，将集合中的元素加入到ArrayList里面
26 public boolean addAll(int index, Collection<? extends E> c) {
27 rangeCheckForAdd(index);
28
29 Object[] a = c.toArray();
30 int numNew = a.length;
31 ensureCapacityInternal(size + numNew); // Increments modCount
32
33 int numMoved = size - index;
34 if (numMoved > 0)
35 System.arraycopy(elementData, index, elementData, index + numNew,
36 numMoved);
37
38 System.arraycopy(a, 0, elementData, index, numNew);
39 size += numNew;
40 return numNew != 0;
41 }
42 //删除指定下标的元素
43 public E remove(int index) {
44 rangeCheck(index);
45
46 modCount++;
47 E oldValue = elementData(index);
48
49 int numMoved = size - index - 1;
50 if (numMoved > 0)
51 System.arraycopy(elementData, index+1, elementData, index,
52 numMoved);
53 elementData[--size] = null; // clear to let GC do its work
54
55 return oldValue;
56 }
57 //按照对象的内容删除，空和非空两种情况
58 public boolean remove(Object o) {
59 if (o == null) {
60 for (int index = 0; index < size; index++)
61 if (elementData[index] == null) {
62 fastRemove(index);
63 return true;
64 }
65 } else {
66 for (int index = 0; index < size; index++)
67 if (o.equals(elementData[index])) {
68 fastRemove(index);
69 return true;
70 }
71 }
72 return false;
73 }
74 //将特定的下标的元素设置为指定的元素
75 public E set(int index, E element) {
76 rangeCheck(index);
77
78 E oldValue = elementData(index);
79 elementData[index] = element;
80 return oldValue;
81 }
82 //是否包含特定的元素
83 public boolean contains(Object o) {
84 return indexOf(o) >= 0;
85 }
86 //返回特定对象第一次出现的下标，空或非空
87 public int indexOf(Object o) {
88 if (o == null) {
89 for (int i = 0; i < size; i++)
90 if (elementData[i]==null)
91 return i;
92 } else {
93 for (int i = 0; i < size; i++)
94 if (o.equals(elementData[i]))
95 return i;
96 }
97 return -1;
98 }
99 //特定对象最后一次出现的下标
100 public int lastIndexOf(Object o) {
101 if (o == null) {
102 for (int i = size-1; i >= 0; i--)
103 if (elementData[i]==null)
104 return i;
105 } else {
106 for (int i = size-1; i >= 0; i--)
107 if (o.equals(elementData[i]))
108 return i;
109 }
110 return -1;
111 }
112 //返回包含ArrayList元素的数组
113 public Object[] toArray() {
114 return Arrays.copyOf(elementData, size);
115 }
116 //返回一条特定的子ArrayList，SubList在其中是以内部类的形式存在
117 public List<E> subList(int fromIndex, int toIndex) {
118 subListRangeCheck(fromIndex, toIndex, size);
119 return new SubList(this, 0, fromIndex, toIndex);
120 }
121
