标签：general poll reading 形参 abstract around 大于 body break
package java.util;
import java.io.Serializable;
import java.util.function.Consumer;
import java.util.function.Predicate;
import jdk.internal.misc.SharedSecrets;
/**
* 1）Deque 接口的循环可变数组实现，ArrayDeque 没有容量限制并会按需增长。
* 2）ArrayDeque 不是线程安全的，ArrayDeque 不允许使用 null 元素。
* 3）ArrayDeque 用作栈时，性能优于 Stack，用作队列时，性能优于 LinkedList.
* 4）ArrayDeque 的大多数操作基于平摊常数时间，除了 remove* 和 contains。
* 5）ArrayDeque 返回的 iterator 是快速失败的。
*/
public class ArrayDeque<E> extends AbstractCollection<E>
implements Deque<E>, Cloneable, Serializable
{
/*
* VMs excel at optimizing simple array loops where indices are
* incrementing or decrementing over a valid slice, e.g.
*
* for (int i = start; i < end; i++) ... elements[i]
*
* Because in a circular array, elements are in general stored in
* two disjoint such slices, we help the VM by writing unusual
* nested loops for all traversals over the elements. Having only
* one hot inner loop body instead of two or three eases human
* maintenance and encourages VM loop inlining into the caller.
*/
/**
* 存储元素的底层对象数组，尾部的位置总为 null
*/
transient Object[] elements;
/**
* 头部元素索引
*/
transient int head;
/**
* 尾部元素索引，此处的元素一直为 null
*/
transient int tail;
/**
* 最大的对象数组长度
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 底层数组扩容
*/
private void grow(int needed) {
final int oldCapacity = elements.length;
int newCapacity;
// Double capacity if small; else grow by 50%
/**
* 如果容量小于 64，则执行【双倍+2】扩容，否则执行 1.5 倍向下取整扩容。
*/
int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);
/**
* 扩容增量小于所需的空间或总容量大于 Integer.MAX_VALUE - 8
*/
if (jump < needed
|| (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0)
newCapacity = newCapacity(needed, jump);
// 创建新的数组并拷贝原有元素
final Object[] es = elements = Arrays.copyOf(elements, newCapacity);
// Exceptionally, here tail == head needs to be disambiguated
/**
* 尾部索引小于头部索引，尾部索引等于头部索引并且头部元素不为 null
*/
if (tail < head || (tail == head && es[head] != null)) {
// wrap around; slide first leg forward to end of array
int newSpace = newCapacity - oldCapacity;
/**
* 将头部元素拷贝到数组的尾部，并将那些空间置为 null
*/
System.arraycopy(es, head,
es, head + newSpace,
oldCapacity - head);
for (int i = head, to = (head += newSpace); i < to; i++)
es[i] = null;
}
}
/** Capacity calculation for edge conditions, especially overflow. */
private int newCapacity(int needed, int jump) {
final int oldCapacity = elements.length, minCapacity;
if ((minCapacity = oldCapacity + needed) - MAX_ARRAY_SIZE > 0) {
if (minCapacity < 0)
throw new IllegalStateException("Sorry, deque too big");
return Integer.MAX_VALUE;
}
if (needed > jump)
return minCapacity;
return (oldCapacity + jump - MAX_ARRAY_SIZE < 0)
? oldCapacity + jump
: MAX_ARRAY_SIZE;
}
/**
* 创建容量为 16 的 ArrayDeque
*/
public ArrayDeque() {
elements = new Object[16];
}
/**
* 创建指定初始容量的 ArrayDeque
*/
public ArrayDeque(int numElements) {
elements =
new Object[(numElements < 1) ? 1 :
(numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE :
numElements + 1];
}
/**
* 创建包含集合中所有元素的 ArrayDeque
*/
public ArrayDeque(Collection<? extends E> c) {
this(c.size());
addAll(c);
}
/**
* Circularly increments i, mod modulus.
* Precondition and postcondition: 0 <= i < modulus.
* 基于 modulus 计算循环递增索引
*/
static final int inc(int i, int modulus) {
if (++i >= modulus) i = 0;
return i;
}
/**
* Circularly decrements i, mod modulus.
* Precondition and postcondition: 0 <= i < modulus.
* 基于 modulus 计算循环递减索引
*/
static final int dec(int i, int modulus) {
if (--i < 0) i = modulus - 1;
return i;
}
/**
* Circularly adds the given distance to index i, mod modulus.
* Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
* @return index 0 <= i < modulus
* 基于指定的增量，计算循环索引
*/
static final int inc(int i, int distance, int modulus) {
if ((i += distance) - modulus >= 0) i -= modulus;
return i;
}
/**
* Subtracts j from i, mod modulus.
* Index i must be logically ahead of index j.
* Precondition: 0 <= i < modulus, 0 <= j < modulus.
* @return the "circular distance" from j to i; corner case i == j
* is disambiguated to "empty", returning 0.
* 基于指定的递减值，计算循环索引
*/
static final int sub(int i, int j, int modulus) {
if ((i -= j) < 0) i += modulus;
return i;
}
/**
* 获取指定索引处的元素
*/
@SuppressWarnings("unchecked")
static final <E> E elementAt(Object[] es, int i) {
return (E) es[i];
}
/**
* 获取指定索引处的元素，如果元素为空，则抛出 ConcurrentModificationException
*/
static final <E> E nonNullElementAt(Object[] es, int i) {
@SuppressWarnings("unchecked") E e = (E) es[i];
if (e == null)
throw new ConcurrentModificationException();
return e;
}
// The main insertion and extraction methods are addFirst,
// addLast, pollFirst, pollLast. The other methods are defined in
// terms of these.
/**
* 将元素插入到双端队列的头部
*/
public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
final Object[] es = elements;
es[head = dec(head, es.length)] = e;
if (head == tail)
grow(1);
}
/**
* 将元素插入到双端队列的尾部
*/
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
final Object[] es = elements;
es[tail] = e;
if (head == (tail = inc(tail, es.length)))
grow(1);
}
/**
* 通过 addLast 方法将集合中的所有元素插入到 ArrayDeque 的尾部
*/
public boolean addAll(Collection<? extends E> c) {
final int s, needed;
if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
grow(needed);
c.forEach(this::addLast);
return size() > s;
}
/**
* 将元素插入到 ArrayDeque 的头部
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
/**
* 将元素插入到 ArrayDeque 的尾部
*/
public boolean offerLast(E e) {
addLast(e);
return true;
}
/**
* 移除并返回头部元素，如果 ArrayDeque 为空，则抛出 NoSuchElementException 异常
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeFirst() {
E e = pollFirst();
if (e == null)
throw new NoSuchElementException();
return e;
}
/**
* 移除并返回尾部元素，如果 ArrayDeque 为空，则抛出 NoSuchElementException 异常
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeLast() {
E e = pollLast();
if (e == null)
throw new NoSuchElementException();
return e;
}
/**
* 移除并返回头部元素，如果 ArrayDeque 为空，则返回 null。
*/
public E pollFirst() {
final Object[] es;
final int h;
E e = elementAt(es = elements, h = head);
if (e != null) {
es[h] = null;
head = inc(h, es.length);
}
return e;
}
/**
* 移除并返回尾部元素，如果 ArrayDeque 为空，则返回 null。
*/
public E pollLast() {
final Object[] es;
final int t;
E e = elementAt(es = elements, t = dec(tail, es.length));
if (e != null)
es[tail = t] = null;
return e;
}
/**
* 获取头部元素，如果 ArrayDeque 为空，则抛出 NoSuchElementException 异常。
*/
public E getFirst() {
E e = elementAt(elements, head);
if (e == null)
throw new NoSuchElementException();
return e;
}
/**
* 获取尾部元素，如果 ArrayDeque 为空，则抛出 NoSuchElementException 异常。
*/
public E getLast() {
final Object[] es = elements;
E e = elementAt(es, dec(tail, es.length));
if (e == null)
throw new NoSuchElementException();
return e;
}
/**
* 获取头部元素，如果 ArrayDeque 为空，则返回 null
*/
public E peekFirst() {
return elementAt(elements, head);
}
/**
* 获取尾部元素，如果 ArrayDeque 为空，则返回 null
*/
public E peekLast() {
final Object[] es;
return elementAt(es = elements, dec(tail, es.length));
}
/**
* 移除和形参相等的第一个元素，通过 {@link Object#equals(Object)} 进行相等性判断。
*/
public boolean removeFirstOccurrence(Object o) {
if (o != null) {
final Object[] es = elements;
for (int i = head, end = tail, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
if (o.equals(es[i])) {
delete(i);
return true;
}
if (to == end) break;
}
}
return false;
}
/**
* 移除和形参相等的最后一个元素，通过 {@link Object#equals(Object)} 进行相等性判断。
*/
public boolean removeLastOccurrence(Object o) {
if (o != null) {
final Object[] es = elements;
for (int i = tail, end = head, to = (i >= end) ? end : 0;
; i = es.length, to = end) {
for (i--; i > to - 1; i--)
if (o.equals(es[i])) {
delete(i);
return true;
}
if (to == end) break;
}
}
return false;
}
// *** Queue methods ***
/**
* 将目标元素插入到 ArrayDeque 的尾部
*/
public boolean add(E e) {
addLast(e);
return true;
}
/**
* 将目标元素插入到 ArrayDeque 的尾部
*/
public boolean offer(E e) {
return offerLast(e);
}
public E remove() {
return removeFirst();
}
public E poll() {
return pollFirst();
}
public E element() {
return getFirst();
}
public E peek() {
return peekFirst();
}
// *** Stack methods ***
/**
* 将元素加入到 ArrayDeque 栈顶
*/
public void push(E e) {
addFirst(e);
}
/**
* 将元素从 ArrayDeque 栈顶移除并返回
*/
public E pop() {
return removeFirst();
}
/**
* 删除指定索引处的元素
*/
boolean delete(int i) {
final Object[] es = elements;
final int capacity = es.length;
final int h, t;
// number of elements before to-be-deleted elt
final int front = sub(i, h = head, capacity);
// number of elements after to-be-deleted elt
final int back = sub(t = tail, i, capacity) - 1;
if (front < back) {
// move front elements forwards
if (h <= i) {
System.arraycopy(es, h, es, h + 1, front);
} else { // Wrap around
System.arraycopy(es, 0, es, 1, i);
es[0] = es[capacity - 1];
System.arraycopy(es, h, es, h + 1, front - (i + 1));
}
es[h] = null;
head = inc(h, capacity);
return false;
} else {
// move back elements backwards
tail = dec(t, capacity);
if (i <= tail) {
System.arraycopy(es, i + 1, es, i, back);
} else { // Wrap around
System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
es[capacity - 1] = es[0];
System.arraycopy(es, 1, es, 0, t - 1);
}
es[tail] = null;
return true;
}
}
// *** Collection Methods ***
/**
* 返回 ArrayDeque 的元素个数
*/
public int size() {
return sub(tail, head, elements.length);
}
/**
* ArrayDeque 是否为空
*/
public boolean isEmpty() {
return head == tail;
}
/**
* 返回 ArrayDeque 的顺序迭代器
*/
public Iterator<E> iterator() {
return new DeqIterator();
}
/**
* 返回 ArrayDeque 的降序迭代器
*/
public Iterator<E> descendingIterator() {
return new DescendingIterator();
}
private class DeqIterator implements Iterator<E> {
/** Index of element to be returned by subsequent call to next. */
int cursor;
/** Number of elements yet to be returned. */
int remaining = size();
/**
* Index of element returned by most recent call to next.
* Reset to -1 if element is deleted by a call to remove.
*/
int lastRet = -1;
DeqIterator() { cursor = head; }
public final boolean hasNext() {
return remaining > 0;
}
public E next() {
if (remaining <= 0)
throw new NoSuchElementException();
final Object[] es = elements;
E e = nonNullElementAt(es, cursor);
cursor = inc(lastRet = cursor, es.length);
remaining--;
return e;
}
void postDelete(boolean leftShifted) {
if (leftShifted)
cursor = dec(cursor, elements.length);
}
public final void remove() {
if (lastRet < 0)
throw new IllegalStateException();
postDelete(delete(lastRet));
lastRet = -1;
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
int r;
if ((r = remaining) <= 0)
return;
remaining = 0;
final Object[] es = elements;
if (es[cursor] == null || sub(tail, cursor, es.length) != r)
throw new ConcurrentModificationException();
for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
action.accept(elementAt(es, i));
if (to == end) {
if (end != tail)
throw new ConcurrentModificationException();
lastRet = dec(end, es.length);
break;
}
}
}
}
private class DescendingIterator extends DeqIterator {
DescendingIterator() { cursor = dec(tail, elements.length); }
public final E next() {
if (remaining <= 0)
throw new NoSuchElementException();
final Object[] es = elements;
E e = nonNullElementAt(es, cursor);
cursor = dec(lastRet = cursor, es.length);
remaining--;
return e;
}
void postDelete(boolean leftShifted) {
if (!leftShifted)
cursor = inc(cursor, elements.length);
}
public final void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
int r;
if ((r = remaining) <= 0)
return;
remaining = 0;
final Object[] es = elements;
if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
throw new ConcurrentModificationException();
for (int i = cursor, end = head, to = (i >= end) ? end : 0;
; i = es.length - 1, to = end) {
// hotspot generates faster code than for: i >= to !
for (; i > to - 1; i--)
action.accept(elementAt(es, i));
if (to == end) {
if (end != head)
throw new ConcurrentModificationException();
lastRet = end;
break;
}
}
}
}
/**
* 返回后期绑定和快速失败的分割迭代器
*/
public Spliterator<E> spliterator() {
return new DeqSpliterator();
}
final class DeqSpliterator implements Spliterator<E> {
private int fence; // -1 until first use
private int cursor; // current index, modified on traverse/split
/** Constructs late-binding spliterator over all elements. */
DeqSpliterator() {
this.fence = -1;
}
/** Constructs spliterator over the given range. */
DeqSpliterator(int origin, int fence) {
// assert 0 <= origin && origin < elements.length;
// assert 0 <= fence && fence < elements.length;
this.cursor = origin;
this.fence = fence;
}
/** Ensures late-binding initialization; then returns fence. */
private int getFence() { // force initialization
int t;
if ((t = fence) < 0) {
t = fence = tail;
cursor = head;
}
return t;
}
public DeqSpliterator trySplit() {
final Object[] es = elements;
final int i, n;
return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
? null
: new DeqSpliterator(i, cursor = inc(i, n, es.length));
}
public void forEachRemaining(Consumer<? super E> action) {
if (action == null)
throw new NullPointerException();
final int end = getFence(), cursor = this.cursor;
final Object[] es = elements;
if (cursor != end) {
this.cursor = end;
// null check at both ends of range is sufficient
if (es[cursor] == null || es[dec(end, es.length)] == null)
throw new ConcurrentModificationException();
for (int i = cursor, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
action.accept(elementAt(es, i));
if (to == end) break;
}
}
}
public boolean tryAdvance(Consumer<? super E> action) {
Objects.requireNonNull(action);
final Object[] es = elements;
if (fence < 0) { fence = tail; cursor = head; } // late-binding
final int i;
if ((i = cursor) == fence)
return false;
E e = nonNullElementAt(es, i);
cursor = inc(i, es.length);
action.accept(e);
return true;
}
public long estimateSize() {
return sub(getFence(), cursor, elements.length);
}
public int characteristics() {
return Spliterator.NONNULL
| Spliterator.ORDERED
| Spliterator.SIZED
| Spliterator.SUBSIZED;
}
}
/**
* 顺序消费 ArrayDeque 中的每个元素
*/
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
final Object[] es = elements;
for (int i = head, end = tail, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
action.accept(elementAt(es, i));
if (to == end) {
if (end != tail) throw new ConcurrentModificationException();
break;
}
}
}
/**
* 根据函数式断言移除满足条件的所有元素
*/
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
return bulkRemove(filter);
}
/**
* 移除 ArrayDeque 包含在集合 c 中的所有元素，集合差集
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return bulkRemove(e -> c.contains(e));
}
/**
* 保留 ArrayDeque 包含在集合 c 中的所有元素，集合交集
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return bulkRemove(e -> !c.contains(e));
}
private boolean bulkRemove(Predicate<? super E> filter) {
final Object[] es = elements;
// Optimize for initial run of survivors
for (int i = head, end = tail, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
if (filter.test(elementAt(es, i)))
return bulkRemoveModified(filter, i);
if (to == end) {
if (end != tail) throw new ConcurrentModificationException();
break;
}
}
return false;
}
// A tiny bit set implementation
private static long[] nBits(int n) {
return new long[((n - 1) >> 6) + 1];
}
private static void setBit(long[] bits, int i) {
bits[i >> 6] |= 1L << i;
}
private static boolean isClear(long[] bits, int i) {
return (bits[i >> 6] & (1L << i)) == 0;
}
/**
* Helper for bulkRemove, in case of at least one deletion.
* Tolerate predicates that reentrantly access the collection for
* read (but writers still get CME), so traverse once to find
* elements to delete, a second pass to physically expunge.
*
* @param beg valid index of first element to be deleted
*/
private boolean bulkRemoveModified(
Predicate<? super E> filter, final int beg) {
final Object[] es = elements;
final int capacity = es.length;
final int end = tail;
final long[] deathRow = nBits(sub(end, beg, capacity));
deathRow[0] = 1L; // set bit 0
for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
; i = 0, to = end, k -= capacity) {
for (; i < to; i++)
if (filter.test(elementAt(es, i)))
setBit(deathRow, i - k);
if (to == end) break;
}
// a two-finger traversal, with hare i reading, tortoise w writing
int w = beg;
for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
; w = 0) { // w rejoins i on second leg
// In this loop, i and w are on the same leg, with i > w
for (; i < to; i++)
if (isClear(deathRow, i - k))
es[w++] = es[i];
if (to == end) break;
// In this loop, w is on the first leg, i on the second
for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
if (isClear(deathRow, i - k))
es[w++] = es[i];
if (i >= to) {
if (w == capacity) w = 0; // "corner" case
break;
}
}
if (end != tail) throw new ConcurrentModificationException();
circularClear(es, tail = w, end);
return true;
}
/**
* ArrayDeque 是否包含目标元素
*/
public boolean contains(Object o) {
if (o != null) {
final Object[] es = elements;
for (int i = head, end = tail, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
if (o.equals(es[i]))
return true;
if (to == end) break;
}
}
return false;
}
/**
* 移除在 ArrayDeque 中第一次匹配形参的目标元素
*/
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}
/**
* 清空 ArrayDeque，head 和 tail 指针都置为 0
*/
public void clear() {
circularClear(elements, head, tail);
head = tail = 0;
}
/**
* Nulls out slots starting at array index i, upto index end.
* Condition i == end means "empty" - nothing to do.
*/
private static void circularClear(Object[] es, int i, int end) {
// assert 0 <= i && i < es.length;
// assert 0 <= end && end < es.length;
for (int to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++) es[i] = null;
if (to == end) break;
}
}
/**
* 返回包含 ArrayDeque 所有元素的对象数组
*/
public Object[] toArray() {
return toArray(Object[].class);
}
private <T> T[] toArray(Class<T[]> klazz) {
final Object[] es = elements;
final T[] a;
final int head = this.head, tail = this.tail, end;
if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
// Uses null extension feature of copyOfRange
a = Arrays.copyOfRange(es, head, end, klazz);
} else {
// integer overflow!
a = Arrays.copyOfRange(es, 0, end - head, klazz);
System.arraycopy(es, head, a, 0, es.length - head);
}
if (end != tail)
System.arraycopy(es, 0, a, es.length - head, tail);
return a;
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
final int size;
if ((size = size()) > a.length)
return toArray((Class<T[]>) a.getClass());
final Object[] es = elements;
for (int i = head, j = 0, len = Math.min(size, es.length - i);
; i = 0, len = tail) {
System.arraycopy(es, i, a, j, len);
if ((j += len) == size) break;
}
if (size < a.length)
a[size] = null;
return a;
}
// *** Object methods ***
/**
* Returns a copy of this deque.
*/
public ArrayDeque<E> clone() {
try {
@SuppressWarnings("unchecked")
ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
result.elements = Arrays.copyOf(elements, elements.length);
return result;
} catch (CloneNotSupportedException e) {
throw new AssertionError();
}
}
private static final long serialVersionUID = 2340985798034038923L;
/**
* Saves this deque to a stream (that is, serializes it).
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
// Write out size
s.writeInt(size());
// Write out elements in order.
final Object[] es = elements;
for (int i = head, end = tail, to = (i <= end) ? end : es.length;
; i = 0, to = end) {
for (; i < to; i++)
s.writeObject(es[i]);
if (to == end) break;
}
}
/**
* Reconstitutes this deque from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
// Read in size and allocate array
int size = s.readInt();
SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size + 1);
elements = new Object[size + 1];
this.tail = size;
// Read in all elements in the proper order.
for (int i = 0; i < size; i++)
elements[i] = s.readObject();
}
/** debugging */
void checkInvariants() {
// Use head and tail fields with empty slot at tail strategy.
// head == tail disambiguates to "empty".
try {
int capacity = elements.length;
// assert 0 <= head && head < capacity;
// assert 0 <= tail && tail < capacity;
// assert capacity > 0;
// assert size() < capacity;
// assert head == tail || elements[head] != null;
// assert elements[tail] == null;
// assert head == tail || elements[dec(tail, capacity)] != null;
} catch (Throwable t) {
System.err.printf("head=%d tail=%d capacity=%d%n",
head, tail, elements.length);
System.err.printf("elements=%s%n",
Arrays.toString(elements));
throw t;
}
}
}
标签：general poll reading 形参 abstract around 大于 body break
原文地址：https://www.cnblogs.com/zhuxudong/p/9312012.html
