Java Stream mapToLong()示例

Stream mapToLong(ToLongFunction mapper) 返回一个LongStream，它由对这个流的元素应用给定函数的结果组成。

Stream mapToLong(ToLongFunction 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Stream实例上调用的，在它们完成处理后，会给出一个Stream实例作为输出:

语法:

LongStream mapToLong(ToLongFunction < ? super T > mapper)

其中，LongStream是一串原始的长值元素的序列，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： mapToLong()函数的操作是返回满足指定函数的流。

// Java code for Stream mapToLong
// (ToLongFunction mapper) to get a
// LongStream by applying the given function
// to the elements of 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("25", "225", "1000",
                                                  "20", "15");
  
        // Using Stream mapToLong(ToLongFunction mapper)
        // and displaying the corresponding LongStream
        list.stream().mapToLong(num -> Long.parseLong(num))
            .filter(num -> Math.sqrt(num) / 5 == 3 )
            .forEach(System.out::println);
    }
}

输出:

The stream after applying the function is : 
225

例2： mapToLong()函数，返回字符串长度中的设定位数。

// Java code for Stream mapToLong
// (ToLongFunction mapper) to get a
// LongStream by applying the given function
// to the elements of this stream.
import java.util.*;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating a list of Strings
        List<String> list = Arrays.asList("Data Structures", "JAVA", "OOPS",
                                             "GeeksforGeeks", "Algorithms");
  
        // Using Stream mapToLong(ToLongFunction mapper)
        // and displaying the corresponding LongStream
        // which contains the number of one-bits in 
        // binary representation of String length
        list.stream().mapToLong(str -> Long.bitCount(str.length()))
            .forEach(System.out::println);
    }
}

输出: