blocks|key|1053364|text|来源：http://docs.oracle.com/javase/6/docs/api/java/util/Queue.html|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1053365|如果可能，offer方法插入一个元素，否则返回false。这与Collection.add方法不同，后者只能通过抛出未检查的异常而无法添加元素。offer方法设计用于故障是正常的，而不是异常发生的情况，例如，在固定容量(或“有界”)队列中。|1053366|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/6/docs/api/java/util/Queue.html^0|3|1P|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

Source: <a href="http://docs.oracle.com/javase/6/docs/api/java/util/Queue.html" rel="nofollow">http://docs.oracle.com/javase/6/docs/api/java/util/Queue.html</a>

The offer method inserts an element if possible, otherwise returning false. This differs from the Collection.add method, which can fail to add an element only by throwing an unchecked exception. The offer method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.

blocks|key|1268104|text|来自jdk+7的源代码，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1268105|public+boolean+add(E+e)+{
++++if+(offer(e))
++++++++return+true;
++++else
++++++++throw+new+IllegalStateException("Queue+full");
}|code-block|syntax|javascript|1268106|我们可以很容易地知道，当成功地将新元素添加到队列中时，add函数将返回true，但在失败时会抛出异常。|1268107|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

from the source code in jdk 7 as follow:

<pre><code>public boolean add(E e) {
 if (offer(e))
 return true;
 else
 throw new IllegalStateException("Queue full");
}
</code></pre>

we can easily know that the add function will return true when successfully add a new element into the queue, but throw a exception when failed .

blocks|key|1268164|text|Queue接口指定，如果当前没有可用的空间，add()将抛出IllegalStateException+(否则返回true)，而如果由于容量限制而无法插入元素，则offer()将返回false。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1268165|它们在PriorityQueue中相同的原因是该队列被指定为无界的，即没有容量限制。在没有容量限制的情况下，add()和offer()的合约表现出相同的行为。|1268166|entityMap^0|0|5|M|5|U|L|1L|4|29|7|2J|5|0|3|D|1I|5|1O|7|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|I|$]]

The <code>Queue</code> interface specifies that <code>add()</code> will throw an <code>IllegalStateException</code> if no space is currently available (and otherwise return <code>true</code>) while <code>offer()</code> will return <code>false</code> if the element couldn't be inserted due to capacity restrictions.

The reason they are the same in a <code>PriorityQueue</code> is that this queue is specified to be unbounded, i.e. there are no capacity restrictions. In the case of no capacity restrictions, the contracts of <code>add()</code> and <code>offer()</code> display the same behaviour.

blocks|key|1053524|text|我将为offer方法和add方法编写java契约示例代码，以显示它们之间的区别。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1053525|BlockingQueue<String>+queue+=+new+ArrayBlockingQueue<>(2);
++++++++queue.add("TestQuue1");+++++
++++++++queue.add("TestQuue2");+
++++++++queue.add("TestQuue3");++//+will+throw+"java.lang.IllegalStateException:+Queue+full

BlockingQueue<String>+queue+=+new+ArrayBlockingQueue<>(2);
++++++++queue.offer("TestQuue1");+++++++
++++++++queue.offer("TestQuue2");+++
++++++++queue.offer("TestQuue3");+//+will+not+throw+any+exception|code-block|syntax|javascript|1053526|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I will write the java contract example code for offer method and add method showing how they differ.

<pre><code>BlockingQueue&lt;String&gt; queue = new ArrayBlockingQueue&lt;&gt;(2);
 queue.add("TestQuue1"); 
 queue.add("TestQuue2"); 
 queue.add("TestQuue3"); // will throw "java.lang.IllegalStateException: Queue full

BlockingQueue&lt;String&gt; queue = new ArrayBlockingQueue&lt;&gt;(2);
 queue.offer("TestQuue1"); 
 queue.offer("TestQuue2"); 
 queue.offer("TestQuue3"); // will not throw any exception
</code></pre>

Take the <code>PriorityQueue</code> for example <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/PriorityQueue.html#offer(E)" rel="noreferrer">http://java.sun.com/j2se/1.5.0/docs/api/java/util/PriorityQueue.html#offer(E)</a>

Can anyone give me an example of a <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Queue.html" rel="noreferrer"><code>Queue</code></a> where the <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#add-E-" rel="noreferrer"><code>add</code></a> and <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#offer-E-" rel="noreferrer"><code>offer</code></a> methods are different?

According to the <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Collection.html" rel="noreferrer"><code>Collection</code></a> doc, the <code>add</code> method will often seek to ensure that an element exists within the <code>Collection</code> rather than adding duplicates. So my question is, what is the difference between the <code>add</code> and <code>offer</code> methods?

Is it that the <code>offer</code> method will add duplicates regardless? (I doubt that it is because if a <code>Collection</code> should only have distinct elements this would circumvent that).

EDIT:
In a <code>PriorityQueue</code> the <code>add</code> and <code>offer</code> methods are the same method (see my answer below). Can anyone give me an example of a class where the <code>add</code> and <code>offer</code> methods are different?

What is the difference between the add and offer methods in a Queue in Java?

以为例，使用PriorityQueue谁能给我举一个和方法不同的的例子？根据文档，add方法通常会确保元素存在于Collection中，而不是添加重复的元素。所以我的问题是，add和offer方法之间的区别是什么？offer方法是否无论如何都会添加重复项？(我怀疑这是因为如果Collection应该只有不同的元素，这将...

问在Java中，队列中的add和offer方法有什么不同？
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