blocks|key|678559|text|Collection+c;

Iterator+iter+=+c.iterator();

Object+first+=+iter.next();|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|678560|(这是最接近拥有Set的“第一”元素的地方。您应该意识到，它对于大多数Set实现来说完全没有意义。这可能对LinkedHashSet和TreeSet有意义，但对HashSet没有意义。)|unstyled|offset|length|style|CODE|678561|entityMap^0|0|8|3|Z|3|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|N|8|@$G|O|H|P|I|J]|$G|Q|H|R|I|J]]|9|@]|A|$]]|$1|K|3|-4|5|F|7|S|8|@]|9|@]|A|$]]]|L|$]]

<pre><code>Collection c;

Iterator iter = c.iterator();

Object first = iter.next();
</code></pre>

(This is the closest you'll get to having the "first" element of a <code>Set</code>. You should realize that it has absolutely no meaning for most implementations of <code>Set</code>. This may have meaning for LinkedHashSet and TreeSet, but not for HashSet.)

blocks|key|678645|text|请参阅javadoc|type|unstyled|depth|inlineStyleRanges|entityRanges|data|678646|List的|offset|length|678647|list.get(0);|code-block|syntax|javascript|678648|或Set|678649|set.iterator().next();|678650|并在使用上述方法之前通过调用isEmpty()检查大小|style|CODE|678651|!list_or_set.isEmpty()|678652|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/1.5.0/docs/api/java/util/List.html|1|http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Set.html^0|0|0|4|0|0|0|1|3|1|0|0|E|9|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|15|8|@]|9|@$D|16|E|17|1|18]]|A|$]]|$1|F|3|G|5|H|7|19|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|9|@$D|1B|E|1C|1|1D]]|A|$]]|$1|M|3|N|5|H|7|1E|8|@]|9|@]|A|$I|J]]|$1|O|3|P|5|6|7|1F|8|@$D|1G|E|1H|Q|R]]|9|@]|A|$]]|$1|S|3|T|5|H|7|1I|8|@]|9|@]|A|$I|J]]|$1|U|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]|12|$5|X|Y|Z|A|$10|13]]]]

See the javadoc 

of <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/List.html" rel="noreferrer">List</a>

<pre><code>list.get(0);
</code></pre>

or <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Set.html" rel="noreferrer">Set</a>

<pre><code>set.iterator().next();
</code></pre>

and check the size before using the above methods by invoking <code>isEmpty()</code>

<pre><code>!list_or_set.isEmpty()
</code></pre>

blocks|key|463821|text|在Java中，您还可以使用流>=8：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|463822|Optional<String>+first+=+set.stream().findFirst();|code-block|syntax|javascript|463823|(如果集合/列表可能为空，则非常有用。)|463824|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|$]]

In Java >=8 you could also use the Streaming API:

<pre><code>Optional&lt;String&gt; first = set.stream().findFirst();
</code></pre>

(Useful if the Set/List may be empty.)

blocks|key|678972|text|令我惊讶的是，还没有人建议芭乐解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|678973|com.google.common.collect.Iterables.get(collection,+0)
//+or
com.google.common.collect.Iterables.get(collection,+0,+defaultValue)
//+or
com.google.common.collect.Iterables.getFirst(collection,+defaultValue)|code-block|syntax|javascript|678974|或者，如果您希望使用单个元素：|678975|com.google.common.collect.Iterables.getOnlyElement(collection,+defaultValue)
//+or
com.google.common.collect.Iterables.getOnlyElement(collection)|678976|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I'm surprised that nobody suggested guava solution yet:

<pre><code>com.google.common.collect.Iterables.get(collection, 0)
// or
com.google.common.collect.Iterables.get(collection, 0, defaultValue)
// or
com.google.common.collect.Iterables.getFirst(collection, defaultValue)
</code></pre>

or if you expect single element:

<pre><code>com.google.common.collect.Iterables.getOnlyElement(collection, defaultValue)
// or
com.google.common.collect.Iterables.getOnlyElement(collection)
</code></pre>

blocks|key|679011|text|java8和更多|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|679012|Set<String>+set+=+new+TreeSet<>();
++++set.add("2");
++++set.add("1");
++++set.add("3");
++++String+first+=+set.stream().findFirst().get();|code-block|syntax|javascript|679013|这将帮助您检索列表或集合的第一个元素。假设集合或列表不为空(get()+on+empty+optional将抛出java.util.NoSuchElementException)|style|CODE|679014|orElse()可以用作：(这只是一种变通方法-不推荐)|679015|String+first+=+set.stream().findFirst().orElse("");
set.removeIf(String::isEmpty);|679016|下面是适当的方法：|679017|Optional<String>+firstString+=+set.stream().findFirst();
if(firstString.isPresent()){
++++String+first+=+firstString.get();
}|679018|类似地，可以检索列表的第一个元素。|679019|希望这能有所帮助。|679020|entityMap|0|LINK|mutability|MUTABLE|url|/questions/tagged/java8^0|0|5|0|0|0|U|5|1K|W|0|0|8|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@$A|17|B|18|1|19]]|C|$]]|$1|D|3|E|5|F|7|1A|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|1B|8|@$A|1C|B|1D|K|L]|$A|1E|B|1F|K|L]]|9|@]|C|$]]|$1|M|3|N|5|6|7|1G|8|@$A|1H|B|1I|K|L]]|9|@]|C|$]]|$1|O|3|P|5|F|7|1J|8|@]|9|@]|C|$G|H]]|$1|Q|3|R|5|6|7|1K|8|@]|9|@]|C|$]]|$1|S|3|T|5|F|7|1L|8|@]|9|@]|C|$G|H]]|$1|U|3|V|5|6|7|1M|8|@]|9|@]|C|$]]|$1|W|3|X|5|6|7|1N|8|@]|9|@]|C|$]]|$1|Y|3|-4|5|6|7|1O|8|@]|9|@]|C|$]]]|Z|$10|$5|11|12|13|C|$14|15]]]]

<a href="/questions/tagged/java8" class="post-tag" title="show questions tagged &#39;java8&#39;" rel="tag">java8</a> and further

<pre><code>Set&lt;String&gt; set = new TreeSet&lt;&gt;();
 set.add("2");
 set.add("1");
 set.add("3");
 String first = set.stream().findFirst().get();
</code></pre>

This will help you retrieve the first element of the list or set.
Given that the set or list is not empty (<code>get()</code> on empty optional will throw <code>java.util.NoSuchElementException</code>)

<code>orElse()</code> can be used as: (this is just a work around - not recommended) 

<pre><code>String first = set.stream().findFirst().orElse("");
set.removeIf(String::isEmpty);
</code></pre>

Below is the appropriate approach : 

<pre><code>Optional&lt;String&gt; firstString = set.stream().findFirst();
if(firstString.isPresent()){
 String first = firstString.get();
}
</code></pre>

Similarly first element of the list can be retrieved. 

Hope this helps.

blocks|key|463832|text|设置|type|unstyled|depth|inlineStyleRanges|entityRanges|data|463833|set.toArray()[0];|code-block|syntax|javascript|463834|列表|463835|list.get(0);|463836|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Set

<pre><code>set.toArray()[0];
</code></pre>

List

<pre><code>list.get(0);
</code></pre>

blocks|key|678873|text|这不是这个问题的确切答案，但如果对象需要排序，SortedSet有一个first()方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|678874|SortedSet<String>+sortedSet+=+new+TreeSet<String>();
sortedSet.add("2");
sortedSet.add("1");
sortedSet.add("3");
String+first+=+sortedSet.first();+//first="1"|code-block|syntax|javascript|678875|排序后的对象必须实现可比较接口(就像String一样)|678876|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|$]]

This is not an exact answer to this question, but in case the objects should be sorted SortedSet has a first() method:

<pre><code>SortedSet&lt;String&gt; sortedSet = new TreeSet&lt;String&gt;();
sortedSet.add("2");
sortedSet.add("1");
sortedSet.add("3");
String first = sortedSet.first(); //first="1"
</code></pre>

The sorted objects must implement the Comparable interface (like String does)

blocks|key|678608|text|可以使用get(index)方法访问列表中的元素。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|678609|根据定义，集合只包含元素，没有特定的顺序。因此，没有可以获取的“第一”元素，但是可以使用迭代器(使用for+each循环)遍历它，或者使用toArray()方法将它转换为数组。|678610|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

You can use the get(index) method to access an element from a List. 

Sets, by definition, simply contain elements and have no particular order. Therefore, there is no "first" element you can get, but it is possible to iterate through it using iterator (using the for each loop) or convert it to an array using the toArray() method.

I'd like to know if I can get the first element of a list or set. Which method to use?

How to get the first element of the List or Set?

Java

我想知道我是否可以得到列表或集合的第一个元素。使用哪种方法？

问如何获取列表或集合的第一个元素？
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问如何获取列表或集合的第一个元素？EN

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