记录一下自己看的源码，就当复习了
引用
成员变量
//默认容量
private static final int DEFAULT_CAPACITY = 10;
//空数组，用于无参构造函数
private static final Object[] EMPTY_ELEMENTDATA = {};
//默认空数组，用于有参构造函数
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//实际存储元素的数组
transient Object[] elementData;
//集合元素数量
private int size;
//集合最大容量
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//父类变量，表示List容量修改次数
protected transient int modCount = 0;
构造函数
1.无参构造函数
public ArrayList() {
//无参构造函数，elementData 指向DEFAULTCAPACITY_EMPTY_ELEMENTDATA 空数组
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
2.有参构造函数
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
//初始化为一个容量为initialCapacity的数组
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
//elementData 指向EMPTY_ELEMENTDATA空数组
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
3.参数为集合的构造函数
public ArrayList(Collection<? extends E> c) {
//参数集合转为Array赋给存储元素的数组
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[]
if (elementData.getClass() != Object[].class)
//元素为空时，复制数组，copyOf方法中有类型检查
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// 若参数集合为空，则返回空数组EMPTY_ELEMENTDATA.
this.elementData = EMPTY_ELEMENTDATA;
}
}
添加
1.添加单个元素
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//用无参构造函数，添加单个元素，大小为默认容量10
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
//容量变化次数增加1
modCount++;
//判断需要的实际元素容量是否大于数组容量，是则扩容
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
//容量增加为1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
//newCapacity 可能为0，此时采用传入的minCapacity作为扩容后的容量
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0)
throw new OutOfMemoryError();
//容量不能超过Integer.MAX_VALUE
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
扩容机制如下：
1、先默认将列表大小newCapacity增加原来一半，即如果原来是10，则新的大小为15；
2、如果新的大小newCapacity依旧不能满足add进来的元素总个数minCapacity,则将列表大小改为和minCapacity一样大；即如果扩大一半后newCapacity为15，但add进来的总元素个数minCapacity为20，则15明显不能存储20个元素，那么此时就将newCapacity大小扩大到20，刚刚好存储20个元素；
3、如果扩容后的列表大小大于2147483639,也就是说大于Integer.MAX_VALUE - 8,此时就要做额外处理了，因为实际总元素大小有可能比Integer.MAX_VALUE还要大，当实际总元素大小minCapacity的值大于Integer.MAX_VALUE，即大于2147483647时，此时minCapacity的值将变为负数，因为int是有符号的，当超过最大值时就变为负数
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
// 在创建新数组对象之前会先对传入的数据类型进行判定
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
//参数为原array，原起始位置，目标array，目标起始位置，需要复制的数组长度
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
2.指定位置添加元素
public void add(int index, E element) {
//位置检查
rangeCheckForAdd(index);
//扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
//复制数组，将原index及其以后的元素复制到index+1
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
//index位置赋值为添加的元素
elementData[index] = element;
size++;
}
3.添加集合中的元素
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
//扩容
ensureCapacityInternal(size + numNew); // Increments modCount
//将集合中的元素复制到扩容后的数组后
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
4.指定位置添加集合中的元素
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;
//判断是否在数组中间插入元素
if (numMoved > 0)
//将原index到index+numMoved-1的元素复制到index+numNew及以后
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
//将集合元素复制到index到index+numNew-1
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
删除
1.删除指定位置元素
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
//将index+1及以后的元素复制到Index，相当于全部往前挪一位
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//最后一个元素设置为null
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
2.删除元素
public boolean remove(Object o) {
//循环判断，找到元素的位置，然后调用删除指定位置元素的方法
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 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
}
3.删除指定范围的元素
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
//将toIndex之后的元素挪动到fromIndex
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
int newSize = size - (toIndex-fromIndex);
//数组末尾元素全部置null
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
4.删除集合中的所有元素
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
5.保留集合中的元素，删除其他元素（取交集）
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
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++)
//根据complete条件判断是否删除该元素
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;
}
获取集合对象
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
E elementData(int index) {
return (E) elementData[index];
}
其他方法
1.指定位置设值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
2.判断元素是否存在
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
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;
}
序列化机制
1.writeObject
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
s.writeInt(size);
// Write out all elements in the proper order.
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
从上面的代码中我们可以看出ArrayList其实是有对elementData进行序列化的，只不过这样做的原因是因为elementData中可能会有很多的null元素，为了不把null元素也序列化出去，所以自定义了writeObject和readObject方法。
2.readObject
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in capacity
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}
迭代器
public Iterator<E> iterator() {
return new Itr();
}
新建了一个叫Itr的对象，Itr类其实是ArrayList的一个内部类
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
