Java中的 Stream map() 示例

流映射（函数映射器）返回一个流，该流由将给定函数应用于该流的元素的结果组成。

Stream map(函数 mapper)是一个中间操作。这些操作总是懒惰的。在 Stream 实例上调用中间操作，在它们完成处理后，它们给出一个 Stream 实例作为输出。

句法 ：

 Stream map(Function mapper)

where, R is the element type of the new stream.
Stream is an interface and T is the type 
of stream elements. mapper is a stateless function 
which is applied to each element and the function
returns the new stream.

示例 1： Stream map()函数，对流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 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

示例 2： Stream map()函数，具有将小写转换为大写的操作。

// Java code for Stream map(Function mapper)
// to get a stream by applying the
// given function to this stream.
import java.util.*;
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 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 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]

示例 3： Stream map()函数，使用映射字符串长度代替字符串的操作。

// 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 Strings
        List 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