JDK1.8 AbstractCollection
1、AbstractCollection抽象类描叙
AbstractCollection这个抽象类是开发者基于Collection接口的基础上实现了很多功能。有时候具体的实现类并不想实现接口的所有方法，而如果继承AbstractCollection这个抽象类，只需要实现其抽象方法就行，这也是集合元素所必须的，其他的方法就可以选择性的进行重写。
2、主机构图
抽象类AbstractCollection实现了Collection接口
3、源码分析
public abstract class AbstractCollection<E> implements Collection<E> {
/**
* 用于子类构造函数调用，通常是隐式的。
*
*/
protected AbstractCollection() {
}
// Query Operations
/**
* 返回此集合中包含的元素的迭代器。
*
*/
public abstract Iterator<E> iterator();
//获取大小
public abstract int size();
//判空
public boolean isEmpty() {
return size() == 0;
}
/**
*
* 判断集合是否包含指定元素 包含返回true否则返回false
* @throws ClassCastException {@inheritDoc 类型转换异常
* @throws NullPointerException {@inheritDoc} 空指针异常
*/
public boolean contains(Object o) {
Iterator<E> it = iterator();
if (o==null) {
while (it.hasNext())
if (it.next()==null)
return true;
} else {
while (it.hasNext())
if (o.equals(it.next()))
return true;
}
return false;
}
/**
* {@inheritDoc}
*
* 如果迭代器返回的元素数量太大而不适合指定的数组，
* 那么元素将在新分配的数组中返回，其长度等于迭代器返回的元素数，
* 即使此集合的大小发生更改 在迭代期间
*
* <p>This method is equivalent to:
*
* <pre> {@code
* List<E> list = new ArrayList<E>(size());
* for (E e : this)
* list.add(e);
* return list.toArray();
* }</pre>
*/
public Object[] toArray() {
// Estimate size of array; be prepared to see more or fewer elements
Object[] r = new Object[size()];
Iterator<E> it = iterator();
for (int i = 0; i < r.length; i++) {
if (! it.hasNext()) // fewer elements than expected
return Arrays.copyOf(r, i);
r[i] = it.next();
}
return it.hasNext() ? finishToArray(r, it) : r;
}
/**
* {@inheritDoc}
*
* 如果迭代器返回的元素数量太大而不适合指定的数组，
* 那么元素将在新分配的数组中返回，其长度等于迭代器返回的元素数，
* 即使此集合的大小发生更改 在迭代期间
*
* <p>This method is equivalent to:
*
* <pre> {@code
* List<E> list = new ArrayList<E>(size());
* for (E e : this)
* list.add(e);
* return list.toArray(a);
* }</pre>
*
* @throws ArrayStoreException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
// Estimate size of array; be prepared to see more or fewer elements
int size = size();
T[] r = a.length >= size ? a :
(T[])java.lang.reflect.Array
.newInstance(a.getClass().getComponentType(), size);
Iterator<E> it = iterator();
for (int i = 0; i < r.length; i++) {
if (! it.hasNext()) { // fewer elements than expected
if (a == r) {
r[i] = null; // null-terminate
} else if (a.length < i) {
return Arrays.copyOf(r, i);
} else {
System.arraycopy(r, 0, a, 0, i);
if (a.length > i) {
a[i] = null;
}
}
return a;
}
r[i] = (T)it.next();
}
// more elements than expected
return it.hasNext() ? finishToArray(r, it) : r;
}
/**
* 准许数组的最大值
* 由于vm原因导致内存问题
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 在迭代器时重新分配toArray中使用的数组返回比预期更多的元素，并完成填充
* 迭代器。
*
* @param r 以前存储的元素的数组
* @param it 这个集合的正在进行的迭代器
* @return 包含给定数组中的元素，以及迭代器返回的任何其他元素的数组，按大小进行裁剪
*
*/
@SuppressWarnings("unchecked")
private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
int i = r.length;
while (it.hasNext()) {
int cap = r.length;
if (i == cap) {
int newCap = cap + (cap >> 1) + 1;
// overflow-conscious code
if (newCap - MAX_ARRAY_SIZE > 0)
newCap = hugeCapacity(cap + 1);
r = Arrays.copyOf(r, newCap);
}
r[i++] = (T)it.next();
}
// trim if overallocated
return (i == r.length) ? r : Arrays.copyOf(r, i);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError
("Required array size too large");
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
// Modification Operations
/**
* {@inheritDoc}
*
* 该方法当前类没有具体实现，需自行实现。
* <tt>UnsupportedOperationException</tt>.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IllegalStateException {@inheritDoc}
*/
public boolean add(E e) {
throw new UnsupportedOperationException();
}
/**
* {@inheritDoc}
*
* 移除集合中的元素
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public boolean remove(Object o) {
Iterator<E> it = iterator();
if (o==null) {
while (it.hasNext()) {
if (it.next()==null) {
it.remove();
return true;
}
}
} else {
while (it.hasNext()) {
if (o.equals(it.next())) {
it.remove();
return true;
}
}
}
return false;
}
// Bulk Operations
/**
* {@inheritDoc}
*
* 指定集合是否包含在当前集合中 包含返回true 否则返回false
*
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @see #contains(Object)
*/
public boolean containsAll(Collection<?> c) {
for (Object e : c)
if (!contains(e))
return false;
return true;
}
/**
* {@inheritDoc}
*
* 将指定集合添加到当前集合中
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IllegalStateException {@inheritDoc}
*
* @see #add(Object)
*/
public boolean addAll(Collection<? extends E> c) {
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}
/**
* {@inheritDoc}
*
* 移除指定集合中的元素
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
boolean modified = false;
Iterator<?> it = iterator();
while (it.hasNext()) {
if (c.contains(it.next())) {
it.remove();
modified = true;
}
}
return modified;
}
/**
* {@inheritDoc}
*
* 取两集合的交集
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
boolean modified = false;
Iterator<E> it = iterator();
while (it.hasNext()) {
if (!c.contains(it.next())) {
it.remove();
modified = true;
}
}
return modified;
}
/**
* {@inheritDoc}
*
* 清除集合
*
* @throws UnsupportedOperationException {@inheritDoc}
*/
public void clear() {
Iterator<E> it = iterator();
while (it.hasNext()) {
it.next();
it.remove();
}
}
// String conversion
/**
*
* 将集合字符化
*
* @return 字符串
*/
public String toString() {
Iterator<E> it = iterator();
if (! it.hasNext())
return "[]";
StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = it.next();
sb.append(e == this ? "(this Collection)" : e);
if (! it.hasNext())
return sb.append(']').toString();
sb.append(',').append(' ');
}
}
}
4、方法结构图