Java中的Stream mapToDouble（）及其示例-yiteyi-C++库

流映射ToDouble（ToDouble函数映射器） 返回一个DoubleStream，其中包含将给定函数应用于该流元素的结果。

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流映射ToDouble（ToDouble函数映射器）是一个 中间操作 .这些操作总是懒惰的。在流实例上调用中间操作，完成处理后，它们将流实例作为输出。

语法：

DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)

Where, A sequence of primitive double-valued
elements 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.

例1： mapToDouble（），用于选择满足给定函数的元素。

                     // Java code for Stream mapToDouble                   
                     // (ToDoubleFunction mapper) to get a                   
                     // DoubleStream 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(                               "10"                               ,                               "6.548"                               ,                               "9.12"                               ,                   
                                         "11"                               ,                               "15"                               );                   
                             
                                         // Using Stream mapToDouble(ToDoubleFunction mapper)                   
                                         // and displaying the corresponding DoubleStream                   
                                         list.stream().mapToDouble(num -> Double.parseDouble(num))                   
                                         .filter(num -> (num * num) *                               2                               ==                               450                               )                   
                                         .forEach(System.out::println);                   
                                         }                   
                     }                   

输出：

15.0

例2： mapToDouble（），用于返回字符串长度平方的流。

                     // Java code for Stream mapToDouble                   
                     // (ToDoubleFunction mapper) to get a                   
                     // DoubleStream 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(                               "CSE"                               ,                               "JAVA"                               ,                               "gfg"                               ,                   
                                         "C++"                               ,                               "C"                               );                   
                             
                                         // Using Stream mapToDouble(ToDoubleFunction mapper)                   
                                         // and displaying the corresponding DoubleStream                   
                                         // which contains square of length of each element in                   
                                         // given Stream                   
                                         list.stream().mapToDouble(str -> str.length() * str.length())                   
                                         .forEach(System.out::println);                   
                                         }                   
                     }                   

输出：

9.0
16.0
9.0
9.0
1.0

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