| 1 |
/* |
| 2 |
* Written by Doug Lea with assistance from members of JCP JSR-166 |
| 3 |
* Expert Group and released to the public domain. Use, modify, and |
| 4 |
* redistribute this code in any way without acknowledgement. |
| 5 |
*/ |
| 6 |
|
| 7 |
package java.util; |
| 8 |
|
| 9 |
/** |
| 10 |
* This class provides skeletal implementations of some {@link Queue} |
| 11 |
* operations. The implementations in this class are appropriate when |
| 12 |
* the base implementation does <em>not</em> allow <tt>null</tt> |
| 13 |
* elements. Methods {@link #add add}, {@link #remove remove}, and |
| 14 |
* {@link #element element} are based on {@link #offer offer}, {@link |
| 15 |
* #poll poll}, and {@link #peek peek}, respectively but throw |
| 16 |
* exceptions instead of indicating failure via <tt>false</tt> or |
| 17 |
* <tt>null</tt> returns. |
| 18 |
* |
| 19 |
* <p> A <tt>Queue</tt> implementation that extends this class must |
| 20 |
* minimally define a method {@link Queue#offer} which does not permit |
| 21 |
* insertion of <tt>null</tt> elements, along with methods {@link |
| 22 |
* Queue#peek}, {@link Queue#poll}, {@link Collection#size}, {@link |
| 23 |
* Collection#remove}, and a {@link Collection#iterator} supporting |
| 24 |
* {@link Iterator#remove}. If these requirements cannot be met, |
| 25 |
* consider instead subclassing {@link AbstractCollection}. |
| 26 |
* |
| 27 |
* @since 1.5 |
| 28 |
* @author Doug Lea |
| 29 |
*/ |
| 30 |
public abstract class AbstractQueue<E> |
| 31 |
extends AbstractCollection<E> |
| 32 |
implements Queue<E> { |
| 33 |
|
| 34 |
/** |
| 35 |
* Constructor for use by subclasses. |
| 36 |
*/ |
| 37 |
protected AbstractQueue() { |
| 38 |
} |
| 39 |
|
| 40 |
/** |
| 41 |
* Inserts the specified element to this queue, if possible. |
| 42 |
* |
| 43 |
* @param o the element to add. |
| 44 |
* @return <tt>true</tt> if it was possible to add the element to |
| 45 |
* this queue, else <tt>false</tt> |
| 46 |
* @throws NullPointerException if the specified element is <tt>null</tt> |
| 47 |
*/ |
| 48 |
public boolean offer(E o) { return false; } |
| 49 |
// FIXME: Replace above no-op with following abstract version |
| 50 |
// when javac allows it. |
| 51 |
// public abstract boolean offer(E o); |
| 52 |
|
| 53 |
/** |
| 54 |
* Adds the specified element to this queue. This implementation |
| 55 |
* returns <tt>true</tt> if <tt>offer</tt> succeeds, else |
| 56 |
* throws an IllegalStateException. |
| 57 |
* th |
| 58 |
* @param o the element |
| 59 |
* @return <tt>true</tt> (as per the general contract of |
| 60 |
* <tt>Collection.add</tt>). |
| 61 |
* |
| 62 |
* @throws NullPointerException if the specified element is <tt>null</tt> |
| 63 |
* @throws IllegalStateException if element cannot be added |
| 64 |
*/ |
| 65 |
public boolean add(E o) { |
| 66 |
if (offer(o)) |
| 67 |
return true; |
| 68 |
else |
| 69 |
throw new IllegalStateException("Queue full"); |
| 70 |
} |
| 71 |
|
| 72 |
/** |
| 73 |
* Retrieves and removes the head of this queue. |
| 74 |
* This implementation returns the result of <tt>poll</tt> |
| 75 |
* unless the queue is empty. |
| 76 |
* |
| 77 |
* @return the head of this queue. |
| 78 |
* @throws NoSuchElementException if this queue is empty. |
| 79 |
*/ |
| 80 |
public E remove() { |
| 81 |
E x = poll(); |
| 82 |
if (x != null) |
| 83 |
return x; |
| 84 |
else |
| 85 |
throw new NoSuchElementException(); |
| 86 |
} |
| 87 |
|
| 88 |
|
| 89 |
/** |
| 90 |
* Retrieves, but does not remove, the head of this queue. |
| 91 |
* This implementation returns the result of <tt>peek</tt> |
| 92 |
* unless the queue is empty. |
| 93 |
* |
| 94 |
* @return the head of this queue. |
| 95 |
* @throws NoSuchElementException if this queue is empty. |
| 96 |
*/ |
| 97 |
public E element() { |
| 98 |
E x = peek(); |
| 99 |
if (x != null) |
| 100 |
return x; |
| 101 |
else |
| 102 |
throw new NoSuchElementException(); |
| 103 |
} |
| 104 |
|
| 105 |
/** |
| 106 |
* Removes all of the elements from this collection. |
| 107 |
* The collection will be empty after this call returns. |
| 108 |
* <p>This implementation repeatedly invokes {@link #poll poll} until it |
| 109 |
* returns <tt>null</tt>. |
| 110 |
*/ |
| 111 |
public void clear() { |
| 112 |
while (poll() != null) |
| 113 |
; |
| 114 |
} |
| 115 |
|
| 116 |
// Declarations that mostly just remove optionality clauses. |
| 117 |
|
| 118 |
/** |
| 119 |
* Removes a single instance of the specified element from this |
| 120 |
* queue, if one is present. More formally, removes an element |
| 121 |
* <tt>e</tt> such that <tt>(o==null ? e==null : |
| 122 |
* o.equals(e))</tt>, if the queue contains such an element. |
| 123 |
* Returns <tt>true</tt> if the queue contained the specified |
| 124 |
* element (or equivalently, if the queue changed as a result of |
| 125 |
* the call). |
| 126 |
* |
| 127 |
* @param o the element to remove |
| 128 |
* @return true if the element was removed |
| 129 |
*/ |
| 130 |
public abstract boolean remove(Object o); |
| 131 |
|
| 132 |
/** |
| 133 |
* Removes from this queue all of its elements that are contained in |
| 134 |
* the specified collection. <p> |
| 135 |
* |
| 136 |
* This implementation iterates over this queue, checking each |
| 137 |
* element returned by the iterator in turn to see if it's contained |
| 138 |
* in the specified collection. If it's so contained, it's removed from |
| 139 |
* this queue with the iterator's <tt>remove</tt> method.<p> |
| 140 |
* |
| 141 |
* @param c elements to be removed from this collection. |
| 142 |
* @return <tt>true</tt> if this queue changed as a result of the |
| 143 |
* call. |
| 144 |
* @throws NullPointerException if the specified collection is null. |
| 145 |
*/ |
| 146 |
public boolean removeAll(Collection<?> c) { |
| 147 |
return super.removeAll(c); |
| 148 |
} |
| 149 |
|
| 150 |
/** |
| 151 |
* Retains only the elements in this queue that are contained in the |
| 152 |
* specified collection. In other words, removes |
| 153 |
* from this queue all of its elements that are not contained in the |
| 154 |
* specified collection. <p> |
| 155 |
* |
| 156 |
* This implementation iterates over this queue, checking each |
| 157 |
* element returned by the iterator in turn to see if it's contained |
| 158 |
* in the specified collection. If it's not so contained, it's removed |
| 159 |
* from this queue with the iterator's <tt>remove</tt> method.<p> |
| 160 |
* |
| 161 |
* @param c elements to be retained in this collection. |
| 162 |
* @return <tt>true</tt> if this queue changed as a result of the |
| 163 |
* call. |
| 164 |
* @throws NullPointerException if the specified collection is null. |
| 165 |
* |
| 166 |
*/ |
| 167 |
public boolean retainAll(Collection<?> c) { |
| 168 |
return super.retainAll(c); |
| 169 |
} |
| 170 |
|
| 171 |
/** |
| 172 |
* Adds all of the elements in the specified collection to this |
| 173 |
* queue. The behavior of this operation is undefined if the |
| 174 |
* specified collection is modified while the operation is in |
| 175 |
* progress. (This implies that the behavior of this call is |
| 176 |
* undefined if the specified collection is this queue, and this |
| 177 |
* queue is nonempty.) |
| 178 |
* |
| 179 |
* <p> This implementation iterates over the specified collection, |
| 180 |
* and uses the <tt>add</tt> method to insert each object returned by |
| 181 |
* the iterator to this queue, in turn. |
| 182 |
* |
| 183 |
* @param c collection whose elements are to be added to this queue |
| 184 |
* @return <tt>true</tt> if this queue changed as a result of the |
| 185 |
* call. |
| 186 |
* @throws NullPointerException if <tt>c</tt> or any element in <tt>c</tt> |
| 187 |
* is <tt>null</tt> |
| 188 |
* @throws IllegalStateException if any element cannot be added. |
| 189 |
* |
| 190 |
*/ |
| 191 |
public boolean addAll(Collection<? extends E> c) { |
| 192 |
return super.addAll(c); |
| 193 |
} |
| 194 |
|
| 195 |
} |
