Java 中的 Stream mapToDouble()，示例
原文:https://www . geesforgeks . org/stream-maptodouble-Java-examples/
Stream mapto double(to doublefunction mapper)返回一个由给定函数应用于该流元素的结果组成的双流。
流映射器到双功能映射器(ToDoubleFunction mapper)是一个 中间操作 。这些操作总是偷懒。中间操作在流实例上被调用，在它们完成处理后，它们给出一个流实例作为输出。
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.
示例 1 : mapToDouble()具有选择满足给定函数的元素的操作。
// 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
示例 2 : mapToDouble()具有返回字符串长度平方的流的操作。
// 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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