entityMap|blocks|key|4upem|text|您不需要链接mapping收集器。默认情况下，单个参数groupingBy将为您提供一个Map<Long,+List<Student>>。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|ft93|Map<Long,+List<Student>>+map+=+
++++list.stream()
++++++++.collect(Collectors.groupingBy(Student::getId));|code-block|syntax|javascript|8fnob^0|6|7|R|A|18|O|0|0^^$0|$]|1|@$2|3|4|5|6|7|8|M|9|@$A|N|B|O|C|D]|$A|P|B|Q|C|D]|$A|R|B|S|C|D]]|E|@]|F|$]]|$2|G|4|H|6|I|8|T|9|@]|E|@]|F|$J|K]]|$2|L|4|-4|6|7|8|U|9|@]|E|@]|F|$]]]]

You don't need to chain the <code>mapping</code> collector. The single argument <code>groupingBy</code> will give you a <code>Map&lt;Long, List&lt;Student&gt;&gt;</code> by default.
<pre><code>Map&lt;Long, List&lt;Student&gt;&gt; map = 
 list.stream()
 .collect(Collectors.groupingBy(Student::getId));
</code></pre>

entityMap|0|type|LINK|mutability|MUTABLE|data|url|https://stackoverflow.com/a/67294187/10819573|1|https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#groupingBy-java.util.function.Function-java.util.function.Supplier-java.util.stream.Collector-|blocks|key|4i2h5|text|answer+by+Eran是非常准确的。除此之外，你还可以使用Supplier，例如|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|cp4ie|Map<Long,+List<Student>>+map+=+
++++list.stream()
++++++++.collect(Collectors.groupingBy(Student::getId,+TreeMap::new,+Collectors.toList()));|code-block|syntax|javascript|89qld^0|W|8|0|E|0|W|8|1|0|0^^$0|$1|$2|3|4|5|6|$7|8]]|9|$2|3|4|5|6|$7|A]]]|B|@$C|D|E|F|2|G|H|U|I|@$J|V|K|W|L|M]]|N|@$J|X|K|Y|C|Z]|$J|10|K|11|C|12]]|6|$]]|$C|O|E|P|2|Q|H|13|I|@]|N|@]|6|$R|S]]|$C|T|E|-4|2|G|H|14|I|@]|N|@]|6|$]]]]

The <a href="https://stackoverflow.com/a/67294187/10819573">answer by Eran</a> is spot-on. Just to add to that, you can also use a <a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#groupingBy-java.util.function.Function-java.util.function.Supplier-java.util.stream.Collector-" rel="noreferrer"><code>Supplier</code></a> e.g.
<pre><code>Map&lt;Long, List&lt;Student&gt;&gt; map = 
 list.stream()
 .collect(Collectors.groupingBy(Student::getId, TreeMap::new, Collectors.toList()));
</code></pre>

entityMap|0|type|LINK|mutability|MUTABLE|data|url|https://www.eclipse.org/collections/|blocks|key|e03h0|text|对于这些简单的用例，我更喜欢使用Eclipse+Collections，而不是依赖于创建Stream的开销。|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|8h8ru|结果是一样的，它提供了一个java.util.Map，我发现它的语法更简洁|1k530|MutableList<Student>+list+=+Lists.mutable.of();
list.add(new+Student("1",+"test+1"));
list.add(new+Student("3",+"test+1"));
list.add(new+Student("3",+"test+3"));

Map<String,+List<Student>>+map+=+list.groupBy(Student::getId).toMap(ArrayList::new);|code-block|syntax|javascript|9m360^0|18|6|G|J|0|0|D|D|0|0^^$0|$1|$2|3|4|5|6|$7|8]]]|9|@$A|B|C|D|2|E|F|U|G|@$H|V|I|W|J|K]]|L|@$H|X|I|Y|A|Z]]|6|$]]|$A|M|C|N|2|E|F|10|G|@$H|11|I|12|J|K]]|L|@]|6|$]]|$A|O|C|P|2|Q|F|13|G|@]|L|@]|6|$R|S]]|$A|T|C|-4|2|E|F|14|G|@]|L|@]|6|$]]]]

For these kind of easy use cases, I prefer to look at <a href="https://www.eclipse.org/collections/" rel="nofollow noreferrer">Eclipse Collections</a> instead of relying on the overhead of creating a <code>Stream</code>.
The result is the same, it gives you a <code>java.util.Map</code> and I find the syntax more concise
<pre class="lang-java prettyprint-override"><code>MutableList&lt;Student&gt; list = Lists.mutable.of();
list.add(new Student(&quot;1&quot;, &quot;test 1&quot;));
list.add(new Student(&quot;3&quot;, &quot;test 1&quot;));
list.add(new Student(&quot;3&quot;, &quot;test 3&quot;));

Map&lt;String, List&lt;Student&gt;&gt; map = list.groupBy(Student::getId).toMap(ArrayList::new);
</code></pre>

entityMap|blocks|key|22id7|text|作为使用toMap的groupingBy的替代品|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|c9iee|Map<Long,+List<Student>>+result+=+list.stream()
++++++++++++++++++++++++++++++++++++++.collect(Collectors.toMap(Student::getId,
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++s->+{+List<Student>+l+=+new+ArrayList<>();
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++l.add(s);
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++return+l;
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++},
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++(l1,l2)->+{+l1.addAll(l2);+
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++return+l1;}));|code-block|syntax|javascript|ft1l6^0|4|5|A|A|0|0^^$0|$]|1|@$2|3|4|5|6|7|8|M|9|@$A|N|B|O|C|D]|$A|P|B|Q|C|D]]|E|@]|F|$]]|$2|G|4|H|6|I|8|R|9|@]|E|@]|F|$J|K]]|$2|L|4|-4|6|7|8|S|9|@]|E|@]|F|$]]]]

As alternative of <code>groupingBy</code> using <code>toMap</code>:
<pre><code>Map&lt;Long, List&lt;Student&gt;&gt; result = list.stream()
 .collect(Collectors.toMap(Student::getId,
 s-&gt; { List&lt;Student&gt; l = new ArrayList&lt;&gt;();
 l.add(s);
 return l;
 },
 (l1,l2)-&gt; { l1.addAll(l2); 
 return l1;}));
</code></pre>

entityMap|blocks|key|14i0|text|然后，@Eran提供的答案是直接的，但是如果您想要将此上下文中的数据类型String更改为Long，则可以使用lamda+expression将String转换为Long|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|ranc|Map<Long,+List<Student>>+map+=+
++++list.stream()
++++++++.collect(Collectors.groupingBy(s+->Long.parseLong(s.getId())));|code-block|syntax|javascript|3uct6^0|10|6|19|4|1J|G|20|6|29|4|0|0^^$0|$]|1|@$2|3|4|5|6|7|8|M|9|@$A|N|B|O|C|D]|$A|P|B|Q|C|D]|$A|R|B|S|C|D]|$A|T|B|U|C|D]|$A|V|B|W|C|D]]|E|@]|F|$]]|$2|G|4|H|6|I|8|X|9|@]|E|@]|F|$J|K]]|$2|L|4|-4|6|7|8|Y|9|@]|E|@]|F|$]]]]

then answer provided by @Eran is straight forward but if you want to change the data type in this context <code>String</code> to <code>Long</code> then you can use <code>lamda expression</code> to convert <code>String</code> to <code>Long</code>
<pre><code>Map&lt;Long, List&lt;Student&gt;&gt; map = 
 list.stream()
 .collect(Collectors.groupingBy(s -&gt;Long.parseLong(s.getId())));
</code></pre>

I want to convert a list of Student objects to <code>Map&lt;Long, List&lt;&gt;&gt;</code> using <code>streams</code>.
<pre><code>List&lt;Student&gt; list = new ArrayList&lt;Student&gt;();
list.add(new Student(&quot;1&quot;, &quot;test 1&quot;));
list.add(new Student(&quot;3&quot;, &quot;test 1&quot;));
list.add(new Student(&quot;3&quot;, &quot;test 3&quot;));
</code></pre>
I want the final outcome in the following way:
Map
Key: 1
Value List: <code>Student(&quot;1&quot;, &quot;test 1&quot;)</code>
Key: 3
Value List: <code>Student(&quot;3&quot;, &quot;test 1&quot;), Student(&quot;3&quot;, &quot;test 3&quot;)</code>
I tried the following code, but it is reinitializing the <code>Student</code> objects. Can anyone help me fix the below code?
<pre class="lang-java prettyprint-override"><code>Map&lt;Long, List&lt;Student&gt;&gt; map = list.stream()
 .collect(Collectors.groupingBy(
 Student::getId,
 Collectors.mapping(Student::new, Collectors.toList())
 ));
</code></pre>

Convert a list of objects to Map<Long, List<>> using streams

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。

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我想使用streams将学生对象列表转换为Map<Long, List<>>。 List<Student> list = new ArrayList<Student>();list.add(new Student("1", "test 1"));list.add(new Student("3", "test 1"));list.add(new Student("3", "test 3")); 我

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