Java Stream map()与实例

Stream map(Function mapper) 返回一个由对该流的元素应用给定函数的结果组成的流。

Stream map(Function mapper)是一个 中间操作 这些操作始终是懒惰的。中间操作是在一个流实例上调用的，在它们完成处理后，会给出一个流实例作为输出:

语法:

<R> Stream<R> map(Function<? super T, ? extends R> mapper)

其中，R是新流的元素类型。流是一个接口，T是流元素的类型。流元素的类型。Mapper是一个无状态函数 它被应用于每个元素，该函数 返回新的流。

例1： 流映射()函数，对流中的每个元素进行数字*3的操作。

// Java code for Stream map(Function mapper)
// to get a stream by applying the
// given function to this stream.
import java.util.*;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
  
        System.out.println("The stream after applying "
                           + "the function is : ");
  
        // Creating a list of Integers
        List<Integer> list = Arrays.asList(3, 6, 9, 12, 15);
  
        // Using Stream map(Function mapper) and
        // displaying the corresponding new stream
        list.stream().map(number -> number * 3).forEach(System.out::println);
    }
}

输出:

The stream after applying the function is : 
9
18
27
36
45

例2： Stream map()函数的操作是将小写字母转换为大写字母。

// Java code for Stream map(Function mapper)
// to get a stream by applying the
// given function to this stream.
import java.util.*;
import java.util.stream.Collectors;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
  
        System.out.println("The stream after applying "
                           + "the function is : ");
  
        // Creating a list of Integers
        List<String> list = Arrays.asList("geeks", "gfg", "g",
                                          "e", "e", "k", "s");
  
        // Using Stream map(Function mapper) to
        // convert the Strings in stream to
        // UpperCase form
        List<String> answer = list.stream().map(String::toUpperCase).
        collect(Collectors.toList());
  
        // displaying the new stream of UpperCase Strings
        System.out.println(answer);
    }
}

输出:

The stream after applying the function is : 
[GEEKS, GFG, G, E, E, K, S]

例3： Stream map()函数的操作是将字符串的长度映射为字符串。

// Java code for Stream map(Function mapper)
// to get a stream by applying the
// given function to this stream.
import java.util.*;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
  
        System.out.println("The stream after applying "
                           + "the function is : ");
  
        // Creating a list of Strings
        List<String> list = Arrays.asList("Geeks", "FOR", "GEEKSQUIZ",
                                          "Computer", "Science", "gfg");
  
        // Using Stream map(Function mapper) and
        // displaying the length of each String
        list.stream().map(str -> str.length()).forEach(System.out::println);
    }
}

输出:

The stream after applying the function is : 
5
3
9
8
7
3