for (String word : words) { collector.collect(word); }
时间: 2024-02-15 19:51:01 浏览: 20
这段代码看起来像是在使用 Hadoop MapReduce 中的 Mapper 类的代码,其中 words 是一个字符串数组,这里的 for 循环将数组中的每一个字符串取出来,然后通过 collector 的 collect 方法将它们收集起来。这个 collect 方法将会把这些字符串作为 Mapper 类的输出发送给 Reduce 阶段。在 MapReduce 中,Mapper 类负责将输入数据解析成键值对,并通过 collect 方法将中间结果发送给 Reduce 阶段;而 Reduce 类则负责将 Mapper 输出的结果进行汇总和计算。
相关问题
collector.map
collector.map 是一种数据结构,它是 Java 8 中的 java.util.stream.Collectors 类的一个静态工厂方法。它用于将流中的元素映射到一个 Map 实例中,其中键是元素,值是对应的元素数量。
举个例子:如果有一个单词列表,可以使用 collector.map 来统计每个单词出现的次数:
```
List<String> words = Arrays.asList("apple", "banana", "apple", "cherry", "banana");
Map<String, Long> wordCounts = words.stream()
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
```
这样,wordCounts 变量就包含了每个单词的出现次数:
```
{apple=2, banana=2, cherry=1}
```
Collectors.collectingAndThen
`Collectors.collectingAndThen` is a static method in the `java.util.stream.Collectors` class that allows you to perform a final transformation on the result of a collector. It takes two arguments: a collector and a `Function` that performs the final transformation.
The `collectingAndThen` method returns a collector that applies the specified collector to the elements of a stream, and then applies the specified function to the result of the collector.
Here's an example of how to use `collectingAndThen` to collect a list of strings, convert it to uppercase, and then return the result as an unmodifiable list:
```
List<String> words = Arrays.asList("hello", "world", "java");
List<String> upperCaseWords = words.stream()
.collect(Collectors.collectingAndThen(
Collectors.toList(),
list -> Collections.unmodifiableList(list.stream()
.map(String::toUpperCase)
.collect(Collectors.toList()))));
System.out.println(upperCaseWords);
```
Output:
```
[HELLO, WORLD, JAVA]
```
In this example, the `collectingAndThen` method first applies the `toList` collector to the stream of strings, which collect them into a mutable `List<String>`. Then, it applies a lambda expression that transforms the list into an unmodifiable list of uppercase strings using the `map` and `collect` methods. Finally, it returns the resulting unmodifiable list.