blocks|key|4038779|text|可以看看Google的Guava库：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4038780|http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Maps.html|offset|length|4038781|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|2J|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@$D|O|E|P|1|Q]]|A|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|C]]]]

Could take a look at Google's Guava library:

<a href="http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Maps.html" rel="nofollow">http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Maps.html</a>

blocks|key|374592|text|您可以尝试芭乐，但您可能会发现，如果您使用匿名类，它将比仅使用循环更复杂和更长。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|374593|List<OldType>+list1+=
List<NewType>+list2+=+new+ArrayList<NewType>(list1.size());
for(OldType+element:+list1);
+++list2.add(transform(element));|code-block|syntax|javascript|374594|值得记住的是，Java不是一种函数式语言，使用循环通常更好。也许当Java有闭包的时候...叹一口气。|374595|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|$]]

You can try guava, but you are likely to find that if you use an anonymous class its will be more complicated and longer than just using a loop.

<pre><code>List&lt;OldType&gt; list1 =
List&lt;NewType&gt; list2 = new ArrayList&lt;NewType&gt;(list1.size());
for(OldType element: list1);
 list2.add(transform(element));
</code></pre>

Its worth remembering that Java is not a functional language and its often better to just use a loop. Perhaps when Java has closures .... sigh.

blocks|key|4038841|text|有几个提到here的函数库，其中大部分可能涵盖了map：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4038842|4038843|+http://www.cs.chalmers.se/~bringert/hoj/程序员(包括Java5.0泛型支持)。很少的文档。|blockquote|4038844|4038845|http://jakarta.apache.org/commons/sandbox/functor/看起来不像是被维护的，不支持泛型。很少的文档。|4038846|http://devnet.developerpipeline.com/documents/s=9851/q=1/ddj0511i/0511i.html库。|4038847|http://functionalj.sourceforge.net|4038848|http://www.functologic.com/orbital/|4038849|java的http://jga.sourceforge.net/编程(包括泛型)。期待更多的文档，也许能更好地组织API.|4038850|entityMap|0|LINK|mutability|MUTABLE|url|http://lambda-the-ultimate.org/node/1223|1|http://www.cs.chalmers.se/~bringert/hoj/|2|http://jakarta.apache.org/commons/sandbox/functor/|3|http://devnet.developerpipeline.com/documents/s=9851/q=1/ddj0511i/0511i.html|4|http://functionalj.sourceforge.net/|5|6|http://jga.sourceforge.net/^0|5|4|0|0|0|1|14|1|0|0|0|1E|2|0|0|24|3|0|0|Y|4|0|0|Z|5|0|5|R|6|0^^$0|@$1|2|3|4|5|6|7|1B|8|@]|9|@$A|1C|B|1D|1|1E]]|C|$]]|$1|D|3|-4|5|6|7|1F|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1G|8|@]|9|@$A|1H|B|1I|1|1J]]|C|$]]|$1|H|3|-4|5|6|7|1K|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|1L|8|@]|9|@$A|1M|B|1N|1|1O]]|C|$]]|$1|K|3|L|5|6|7|1P|8|@]|9|@$A|1Q|B|1R|1|1S]]|C|$]]|$1|M|3|N|5|6|7|1T|8|@]|9|@$A|1U|B|1V|1|1W]]|C|$]]|$1|O|3|P|5|6|7|1X|8|@]|9|@$A|1Y|B|1Z|1|20]]|C|$]]|$1|Q|3|R|5|6|7|21|8|@]|9|@$A|22|B|23|1|24]]|C|$]]|$1|S|3|-4|5|6|7|25|8|@]|9|@]|C|$]]]|T|$U|$5|V|W|X|C|$Y|Z]]|10|$5|V|W|X|C|$Y|11]]|12|$5|V|W|X|C|$Y|13]]|14|$5|V|W|X|C|$Y|15]]|16|$5|V|W|X|C|$Y|17]]|18|$5|V|W|X|C|$Y|P]]|19|$5|V|W|X|C|$Y|1A]]]]

There are several functional libraries mentioned <a href="http://lambda-the-ultimate.org/node/1223" rel="nofollow">here</a>, most of which probably cover map:

<blockquote>
 <a href="http://www.cs.chalmers.se/~bringert/hoj/" rel="nofollow">http://www.cs.chalmers.se/~bringert/hoj/</a>
 programmer (includes Java 5.0 generics
 support). Little documentation.
 
 <a href="http://jakarta.apache.org/commons/sandbox/functor/" rel="nofollow">http://jakarta.apache.org/commons/sandbox/functor/</a>
 doesn't look like it is maintained,
 doesn't support generics. Little
 documentation.
 
 <a href="http://devnet.developerpipeline.com/documents/s=9851/q=1/ddj0511i/0511i.html" rel="nofollow">http://devnet.developerpipeline.com/documents/s=9851/q=1/ddj0511i/0511i.html</a>
 library.
 
 <a href="http://functionalj.sourceforge.net" rel="nofollow">http://functionalj.sourceforge.net</a> 
 
 <a href="http://www.functologic.com/orbital/" rel="nofollow">http://www.functologic.com/orbital/</a> 
 
 <a href="http://jga.sourceforge.net/" rel="nofollow">http://jga.sourceforge.net/</a>
 programming in java (includes
 generics). Looking forward to more
 documentation, perhaps better
 organization of the API.
</blockquote>

blocks|key|2442522|text|看看lambdaj和它的convert+method|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2442523|entityMap|0|LINK|mutability|MUTABLE|url|http://code.google.com/p/lambdaj|1|http://code.google.com/p/lambdaj/source/browse/trunk/src/main/java/ch/lambdaj/Lambda.java?r=1452#896^0|2|7|0|C|E|1|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]|$A|R|B|S|1|T]]|C|$]]|$1|D|3|-4|5|6|7|U|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]]]

Take a look at <a href="http://code.google.com/p/lambdaj" rel="nofollow">lambdaj</a> and its <a href="http://code.google.com/p/lambdaj/source/browse/trunk/src/main/java/ch/lambdaj/Lambda.java?r=1452#896" rel="nofollow">convert method</a>

blocks|key|1538897|text|仍然不存在|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1538898|将在Java+8+-+Project+Lambda中添加函数式编程功能|offset|length|style|BOLD|1538899|我认为Google+Guava现在最适合您的需求|1538900|entityMap|0|LINK|mutability|MUTABLE|url|http://openjdk.java.net/projects/lambda/|1|http://code.google.com/p/guava-libraries/^0|0|S|5|2|N|0|0|3|C|1|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@$D|V|E|W|F|G]]|9|@$D|X|E|Y|1|Z]]|A|$]]|$1|H|3|I|5|6|7|10|8|@]|9|@$D|11|E|12|1|13]]|A|$]]|$1|J|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q]]|R|$5|M|N|O|A|$P|S]]]]

still not exist

functional programming features will be added in <a href="http://openjdk.java.net/projects/lambda/" rel="noreferrer">Java 8 - Project Lambda</a>

I think <a href="http://code.google.com/p/guava-libraries/" rel="noreferrer">Google Guava</a> is best for your needs now

blocks|key|1536585|text|这是我的解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1536586|public+abstract+class+MapF<T,S>
++++{
++++++++public+abstract+T+f(S+s)+throws+Exception;

++++++++public+List<T>+map(List<S>+input)+throws+Exception
++++++++{
++++++++++++LinkedList<T>+output+=+new+LinkedList<T>();
++++++++++++for+(S+s:+input)
++++++++++++++++output.add(f(s));
++++++++++++return+output;
++++++++}
++++}|code-block|syntax|javascript|1536587|简单地说，扩展这个定义函数f的类并调用列表上的方法map。|1536588|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 my solution:

<pre><code>public abstract class MapF&lt;T,S&gt;
 {
 public abstract T f(S s) throws Exception;

 public List&lt;T&gt; map(List&lt;S&gt; input) throws Exception
 {
 LinkedList&lt;T&gt; output = new LinkedList&lt;T&gt;();
 for (S s: input)
 output.add(f(s));
 return output;
 }
 }
</code></pre>

Simply, extends this class defining your function f and call the method map on the list.

blocks|key|1536599|text|从Java8开始，这要归功于Stream+API使用一个适当的映射器+Function，我们将使用它将A类的实例转换为B类的实例。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|CODE|entityRanges|data|1536600|伪代码将是：|1536601|List<A>+input+=+//+a+given+list+of+instances+of+class+A
Function<A,+B>+function+=+//+a+given+function+that+converts+an+instance+
++++++++++++++++++++++++++//+of+A+to+an+instance+of+B
//+Call+the+mapper+function+for+each+element+of+the+list+input
//+and+collect+the+final+result+as+a+list
List+output+=+input.stream().map(function).collect(Collectors.toList());|code-block|syntax|javascript|1536602|下面是一个具体的示例，它将使用Integer.valueOf(String)作为映射器函数将String的List转换为Integer的List：|1536603|List<String>+input+=+Arrays.asList("1",+"2",+"3");
List<Integer>+output+=+input.stream().map(Integer::valueOf).collect(Collectors.toList());
System.out.println(output);|1536604|输出：|1536605|[1,+2,+3]|1536606|1536607|对于以前版本的Java+，您仍然可以使用Google+Guava中的FluentIterable来替换Stream，并使用com.google.common.base.Function代替java.util.function.Function作为<代码>e129映射器函数。|1536608|上一个示例将被重写为：|1536609|List<Integer>+output+=+FluentIterable.from(input)
++++.transform(
++++++++new+Function<String,+Integer>()+{
++++++++++++public+Integer+apply(final+String+value)+{
++++++++++++++++return+Integer.valueOf(value);
++++++++++++}
++++++++}
++++).toList();|1536610|1536611|[1,+2,+3]|1536612|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html|1|https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html|2|https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#valueOf-java.lang.String-|3|https://google.github.io/guava/releases/snapshot/api/docs/com/google/common/collect/FluentIterable.html|4|https://google.github.io/guava/releases/snapshot/api/docs/com/google/common/base/Function.html^0|1|5|V|3|E|A|Z|8|1F|1|1N|1|E|A|0|Z|8|1|0|0|6|0|0|F|N|1A|6|1H|4|1O|7|1W|4|F|N|2|0|0|0|3|0|0|0|7|4|Y|E|1F|6|1P|V|2M|R|K|C|Y|E|3|1P|V|4|0|A|1|0|0|0|3|0|0^^$0|@$1|2|3|4|5|6|7|1L|8|@$9|1M|A|1N|B|C]|$9|1O|A|1P|B|C]|$9|1Q|A|1R|B|D]|$9|1S|A|1T|B|D]|$9|1U|A|1V|B|D]|$9|1W|A|1X|B|D]]|E|@$9|1Y|A|1Z|1|20]|$9|21|A|22|1|23]]|F|$]]|$1|G|3|H|5|6|7|24|8|@$9|25|A|26|B|C]]|E|@]|F|$]]|$1|I|3|J|5|K|7|27|8|@]|E|@]|F|$L|M]]|$1|N|3|O|5|6|7|28|8|@$9|29|A|2A|B|D]|$9|2B|A|2C|B|D]|$9|2D|A|2E|B|D]|$9|2F|A|2G|B|D]|$9|2H|A|2I|B|D]]|E|@$9|2J|A|2K|1|2L]]|F|$]]|$1|P|3|Q|5|K|7|2M|8|@]|E|@]|F|$L|M]]|$1|R|3|S|5|6|7|2N|8|@$9|2O|A|2P|B|C]]|E|@]|F|$]]|$1|T|3|U|5|K|7|2Q|8|@]|E|@]|F|$L|M]]|$1|V|3|-4|5|6|7|2R|8|@]|E|@]|F|$]]|$1|W|3|X|5|6|7|2S|8|@$9|2T|A|2U|B|D]|$9|2V|A|2W|B|D]|$9|2X|A|2Y|B|D]|$9|2Z|A|30|B|D]|$9|31|A|32|B|D]|$9|33|A|34|B|C]]|E|@$9|35|A|36|1|37]|$9|38|A|39|1|3A]]|F|$]]|$1|Y|3|Z|5|6|7|3B|8|@$9|3C|A|3D|B|C]]|E|@]|F|$]]|$1|10|3|11|5|K|7|3E|8|@]|E|@]|F|$L|M]]|$1|12|3|S|5|6|7|3F|8|@$9|3G|A|3H|B|C]]|E|@]|F|$]]|$1|13|3|14|5|K|7|3I|8|@]|E|@]|F|$L|M]]|$1|15|3|-4|5|6|7|3J|8|@]|E|@]|F|$]]]|16|$17|$5|18|19|1A|F|$1B|1C]]|1D|$5|18|19|1A|F|$1B|1E]]|1F|$5|18|19|1A|F|$1B|1G]]|1H|$5|18|19|1A|F|$1B|1I]]|1J|$5|18|19|1A|F|$1B|1K]]]]

Starting from Java 8, it can be done thanks to the <a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html" rel="nofollow"><code>Stream API</code></a> using an appopriate mapper <a href="https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html" rel="nofollow"><code>Function</code></a> that we will use to convert our instances of class <code>A</code> to instances of class <code>B</code>.

The pseudo code would be:

<pre><code>List&lt;A&gt; input = // a given list of instances of class A
Function&lt;A, B&gt; function = // a given function that converts an instance 
 // of A to an instance of B
// Call the mapper function for each element of the list input
// and collect the final result as a list
List&lt;B&gt; output = input.stream().map(function).collect(Collectors.toList());
</code></pre>

Here is a concrete example that will convert a <code>List</code> of <code>String</code> to a <code>List</code> of <code>Integer</code> using <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#valueOf-java.lang.String-" rel="nofollow"><code>Integer.valueOf(String)</code></a> as mapper function:

<pre><code>List&lt;String&gt; input = Arrays.asList("1", "2", "3");
List&lt;Integer&gt; output = input.stream().map(Integer::valueOf).collect(Collectors.toList());
System.out.println(output);
</code></pre>

Output:

<pre><code>[1, 2, 3]
</code></pre>

<hr>

For previous versions of <code>Java</code>, you can still use <a href="https://google.github.io/guava/releases/snapshot/api/docs/com/google/common/collect/FluentIterable.html" rel="nofollow"><code>FluentIterable</code></a> from Google Guava to replace the <code>Stream</code> and use <a href="https://google.github.io/guava/releases/snapshot/api/docs/com/google/common/base/Function.html" rel="nofollow"><code>com.google.common.base.Function</code></a> instead of <code>java.util.function.Function</code> as mapper function.

The previous example would then be rewritten as next:

<pre><code>List&lt;Integer&gt; output = FluentIterable.from(input)
 .transform(
 new Function&lt;String, Integer&gt;() {
 public Integer apply(final String value) {
 return Integer.valueOf(value);
 }
 }
 ).toList();
</code></pre>

Output:

<pre><code>[1, 2, 3]
</code></pre>

blocks|key|1536627|text|从Java+8开始，Python的map在Java中的等价物称为...地图！|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1536628|package+stackOverflow;

import+java.util.Arrays;
import+java.util.List;
import+java.util.stream.Collectors;


public+class+ListMapExample
{

++++public+static+void+main(String[]+args)+{
++++++++List<Integer>+integers+=+Arrays.asList(1,+2,+3,+4,+5);
++++++++List<int[]>+arrays+=+integers.stream().map(size+->+new+int[size])
++++++++++++++++.collect(Collectors.toList());
++++++++arrays.forEach(ary+->+System.out.println(Arrays.toString(ary)));
++++}

}|code-block|syntax|javascript|1536629|此示例将整数列表转换为具有相应大小的数组列表：|1536630|[0]
[0,+0]
[0,+0,+0]
[0,+0,+0,+0]
[0,+0,+0,+0,+0]|1536631|您还可以在map中编写多个语句：|1536632|List<String>+strings+=+Arrays.asList("1",+"2",+"3",+"4",+"5");
Stream<int[]>+arraysStream+=+strings.stream().map(string+->+{
++++int+size+=+Integer.valueOf(string);
++++int+size2+=+size+*+2;
++++return+new+int[size2];
});
arraysStream.map(Arrays::toString).forEach(System.out::println);|1536633|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Since Java 8, the Java equivalent for Python's map is called ... map !

<pre><code>package stackOverflow;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;


public class ListMapExample
{

 public static void main(String[] args) {
 List&lt;Integer&gt; integers = Arrays.asList(1, 2, 3, 4, 5);
 List&lt;int[]&gt; arrays = integers.stream().map(size -&gt; new int[size])
 .collect(Collectors.toList());
 arrays.forEach(ary -&gt; System.out.println(Arrays.toString(ary)));
 }

}
</code></pre>

This examples converts a list of integers to a list of arrays with the corresponding size :

<pre><code>[0]
[0, 0]
[0, 0, 0]
[0, 0, 0, 0]
[0, 0, 0, 0, 0]
</code></pre>

You can also write more than one statement inside map :

<pre><code>List&lt;String&gt; strings = Arrays.asList("1", "2", "3", "4", "5");
Stream&lt;int[]&gt; arraysStream = strings.stream().map(string -&gt; {
 int size = Integer.valueOf(string);
 int size2 = size * 2;
 return new int[size2];
});
arraysStream.map(Arrays::toString).forEach(System.out::println);
</code></pre>

I would like to easily transform a collection (list) of objects of class A to a collection of objects of class B, just as Python's map function does it. Is there any "well-known" implementation of this (some kind of library)? I've already searched for it in Apache's commons-lang but with no luck.

Is there a Java equivalent for Python's map function?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我想轻松地将类A的对象集合(列表)转换为类B的对象集合，就像Python的map函数所做的那样。有没有什么“知名”的实现(某种类库)？我已经在Apache的commons-lang中搜索过它，但没有找到。

问有没有与Python的map函数等效的Java函数？
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问有没有与Python的map函数等效的Java函数？EN