Java源码分析之ArrayList

先看私有属性

//保存ArrayList中的内容
transient Object[] elementData; // non-private to simplify nested class access
//表示元素的数量
private int size;

构造函数

//没有参数时，构建一个空的
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

//当有一个参数时，就是代表开多大的空间，注意，这里并没有设size的值哦
private static final Object[] EMPTY_ELEMENTDATA = {};
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);
}
}

//用集合构造，转成数组后，返回给elementData，返回若不是Object[]将调用Arrays.copyOf方法将其转为Object[]
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}

基本的方法

get方法

//正常的get方法
public E get(int index) {
rangeCheck(index);

return elementData(index);
}
//对index的检测
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//注意这个是没有边界检测的
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}

set方法

//有index的检测，返回的是旧的值
public E set(int index, E element) {
rangeCheck(index);

E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}

add方法

//ensureCapacityInternal稍后讲，除了为了有足够大的空间，还有其他的作用
public boolean add(E e) {
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}

//在index位置处插入element
public void add(int index, E element) {
rangeCheckForAdd(index);

ensureCapacityInternal(size + 1);  // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
//这里检测是否在那个区间内，个人感觉有点儿问题，比如，初始构造给了参数，接下来在某个位置add一个数，换句话说，有了空间，但该位置没数，此时该不该抛出越界异常
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

addAll

//加入集合中的全部，多开空间
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;
}

//在index位置上加入集合中的全部
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);

Object[] a = c.toArray();
int numNew = a.length;
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;
}

remove方法

//先看下是否越界，再移除，返回的是旧的值
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;
}

//移除第一个与o相等的对象，注意remove(int index)是移除index位置的，如果想要移除的Object本身就是int该怎么办，hiahia
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;
}
//和remove(int index)一样，为什么不直接调用呢？
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
}

//移除fromIndex，toIndex中的元素(这个没有判断越界？
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);

// clear to let GC do its work
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}

size方法

public int size() {
return size;
}

isEmpty()方法

public boolean isEmpty() {
return size == 0;
}

indexOf方法(O(n))

//返回o的第一次出现的下标，注意是从前往后依次遍历的
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;
}

//返回o最后一次出现的位置，是从后往前依次遍历的
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;
}

contains方法

//就是判断第一次出现的位置，也是O(n)，(我还以为会用HashSet之类的呢
public boolean contains(Object o) {
return indexOf(o) >= 0;
}

toArray方法

public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}

clear

public void clear() {
modCount++;

// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;

size = 0;
}

subList

public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
//这里fromIndex > toIndex并不会交换
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > size)
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex +
") > toIndex(" + toIndex + ")");
}

稍高级点儿的方法

add时候的空间检测

private static final int DEFAULT_CAPACITY = 10;
//取当前需要的空间和10的最大值
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}
//看element的已有空间，是否能满足
private void ensureExplicitCapacity(int minCapacity) {
modCount++;

// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//取需要的空间与原来空间1.5倍中最大值
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);
}
//所需空间非常大时候的判断，如果是Integer.MAX_VALUE，需要84M，注意爆栈
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

求elementData与一个集合的交集和差集

//差集
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
//交集
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
//设一个complement变量来判断是留或者去掉，感觉好机智啊
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}

modCount

对于这个变量，大家可能会好奇是什么用，是为了多线程中判断是否被修改时使用，举个例子

//可能很多人会问，明明上面刚刚赋值了expectedModCount ，只做了一个排序的操作，怎么可能会变了呢。在这个线程1中，当然是不会变的，注意到modCount，修改这个值的函数，大概有删除elementData中的值，修改elementData中的值，清除等，也就是说，如果其他线程2中值被修改了，modCount值就会变，那么在线程1中modCount != expectedModCount了。但这还不是线程安全的，为什么呢，因为知识抛出了异常，并没有做出什么操作(比如锁等)，所以并不是线程安全的
public void sort(Comparator<? super E> c) {
final int expectedModCount = modCount;
Arrays.sort((E[]) elementData, 0, size, c);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}

至此，对ArrayList有了基本的了解，其中很重要的Itr没有说明，打算专门开一个说。