Queue
Queue 是 Collection 接口的子接口,和List,Set 出於同一層級。
除了基本Collection操作外,隊列還提供其他插入,提取和檢查操作。這些方法中的每一種都以兩種形式存在:一種在操作失敗時引發異常,另一種返回一個特殊值(取決於操作,爲null或false)。插入操作的後一種形式是專爲與容量受限的Queue 實現一起使用而設計的;在大多數實現中,插入操作不會失敗。
操作 | 引發異常 | 返回特殊值 |
---|---|---|
插入 | add(e) | offer(e) |
移除 | remove() | poll() |
檢查 | element() | peek() |
隊列通常但不一定以FIFO(先進先出)的方式對元素進行排序。例外情況包括優先級隊列(根據提供的比較器 comparator 對元素進行排序或元素的自然排序)和LIFO隊列(或堆棧),對LIFO進行排序(後進先出)。無論使用哪種順序,可以通過調用remove()或poll() 刪除該隊列的開頭的元素(head of the queue)。在FIFO隊列中,所有新元素都插入隊列的尾部(the tail of queue)。其他種類的隊列可能使用不同的放置規則。每個Queue實現必須指定其排序屬性。
add(E e) 和 offer(E e)
可以看看AbstractQueue的源代碼:
public boolean add(E var1) {
if (this.offer(var1)) {
return true;
} else {
throw new IllegalStateException("Queue full");
}
}
可以看到,抽象類裏,add()方法在執行成功後會返回true,執行失敗會拋出IllegalStateException(“Queue full”);
而offer(E e) 方法沒有實現。會在具體的子類中實現,比如ArrayBlockingQueue.java
/**
* Inserts the specified element at the tail of this queue if it is
* possible to do so immediately without exceeding the queue's capacity,
* returning {@code true} upon success and {@code false} if this queue
* is full. This method is generally preferable to method {@link #add},
* which can fail to insert an element only by throwing an exception.
*
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {
Objects.requireNonNull(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (count == items.length)
return false;
else {
enqueue(e);
return true;
}
} finally {
lock.unlock();
}
}
/**
* Inserts element at current put position, advances, and signals.
* Call only when holding lock.
*/
private void enqueue(E e) {
// assert lock.isHeldByCurrentThread();
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = e;
if (++putIndex == items.length) putIndex = 0;
count++;
notEmpty.signal();
}
如果e爲null, 就會拋 NullPointerException().
/**
* Checks that the specified object reference is not {@code null}. This
* method is designed primarily for doing parameter validation in methods
* and constructors, as demonstrated below:
* <blockquote><pre>
* public Foo(Bar bar) {
* this.bar = Objects.requireNonNull(bar);
* }
* </pre></blockquote>
*
* @param obj the object reference to check for nullity
* @param <T> the type of the reference
* @return {@code obj} if not {@code null}
* @throws NullPointerException if {@code obj} is {@code null}
*/
public static <T> T requireNonNull(T obj) {
if (obj == null)
throw new NullPointerException();
return obj;
}
這裏的NullPointerException 是運行時異常,繼承 RuntimeException. 在java.lang包下。
還有一個NullPointerException.c在java.base/share/native/libjava/下。是C文件,需要翻翻虛擬機代碼了:
https://github.com/kaffe/kaffe
扯遠了。可以看到, ArrayBlockingQueue 在添加前會加鎖。這也應正了它的名字。
可以參考 https://docs.oracle.com/javase/7/docs/technotes/guides/collections/index.html 瞭解更多。
LinkedList
先看看LinkedList.java的實現:
可以看到它有關鍵的first Node 和 last Node.
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0;
/**
* Pointer to first node.
*/
transient Node<E> first;
/**
* Pointer to last node.
*/
transient Node<E> last;
//...................
}
而一個Node元素會有一個prev,指向它的前一個Node,有一個next,指向它的後一個Node.還有一個泛型屬性E,用於保存Node節點的值。
private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;
Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}
對於LinkedList.它的 index 查詢是怎麼實現的呢?
/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
先把size擴大兩倍,然後用index和它比較。其實就是看index是在中間以前,還是在中間以後,如果在中間以前,就從前半部分遍歷,直到找到該Node,如果是在後半部分,就從末尾開始遍歷。
再看看它的各種操作:
/**
* Links e as first element.
*/
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
/**
* Links e as last element.
*/
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
/**
* Inserts element e before non-null Node succ.
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev;
final Node<E> newNode = new Node<>(pred, e, succ);
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
/**
* Unlinks non-null first node f.
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element;
}
/**
* Unlinks non-null last node l.
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}
/**
* Unlinks non-null node x.
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
/**
* Links e as first element.
*/
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
/**
* Links e as last element.
*/
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
/**
* Inserts element e before non-null Node succ.
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev;
final Node<E> newNode = new Node<>(pred, e, succ);
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
/**
* Unlinks non-null first node f.
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element;
}
/**
* Unlinks non-null last node l.
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}
/**
* Unlinks non-null node x.
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
/**
* Links e as first element.
* 添加頭部元素
*/
private void linkFirst(E e) {
// 新建一個Node保存原來老的first
final Node<E> f = first;
// 創建一個新節點 newNode, 新加的 e 作爲保存之, 老的 first 節點作爲這個新節點的next
final Node<E> newNode = new Node<>(null, e, f);
// 把創建的新節點 newNode 作爲 first.
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
/**
* Links e as last element.
*/
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
/**
* Inserts element e before non-null Node succ.
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev;
final Node<E> newNode = new Node<>(pred, e, succ);
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
/**
* Unlinks non-null first node f.
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element;
}
/**
* Unlinks non-null last node l.
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}
/**
* Unlinks non-null node x.
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
PriorityQueue
PriorityQueue 繼承了 AbstractQueue,看看源碼:
public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable {
private static final long serialVersionUID = -7720805057305804111L;
private static final int DEFAULT_INITIAL_CAPACITY = 11;
transient Object[] queue;
private int size;
private final Comparator<? super E> comparator;
transient int modCount;
private static final int MAX_ARRAY_SIZE = 2147483639;
// ......省略
}
可以看到他是以 Object[] 數組來保存元素的。這和LinkedList不一樣。
它還有一個 final Comparator 屬性。