我有两个测试类：
import java.util.Arrays; import java.util.List; import java.util.function.ToIntFunction; import java.util.stream.IntStream; import java.util.stream.Stream; class Age{ private int age; public Age(int age){ this.age = age; } public int getAgeValue(){ return this.age; } } @SuppressWarnings("unused") class User{ private int id; private String name; private Age age; public User(int id, String name, int age){ this.id = id; this.name = name; this.age = new Age(age); } public Age getAge(){ return this.age; } public int getAgeWrapper(){ return this.age.getAgeValue(); } }
先构造一个集合Collections：
private static List<User> users = Arrays.asList( new User(1, "张三", 1), new User(2, "李四", 2), new User(3,"王五", 3), new User(4, "赵六", 4));
// 获得一个并行流：
Stream userStream = users.parallelStream();
// 获得一个映射函数，该函数配合stream使用，能把所有User实例的Age字段映射成IntStream
ToIntFunction mapper = User::getAgeWrapper;
IntStream intstream = userStream.mapToInt(mapper);
// 得到输出：10
System.out.println("Total: " + intstream.sum());
如果重复调用sum：
System.out.println("Total2: " + intstream.sum());
会遇到错误消息：
Exception in thread “main” java.lang.IllegalStateException: stream has already been operated upon or closed
at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:229)
at java.base/java.util.stream.IntPipeline.reduce(IntPipeline.java:491)
at java.base/java.util.stream.IntPipeline.sum(IntPipeline.java:449)
at java8.stream.SumTest.main(SumTest.java:68)
如果不想用stream自带的sum方法，也可以手动用reduce计算：
int total = intstream.reduce(0, (x, y) -> x + y);
一样能得到10的结果。
全部测试代码：
package java8.stream; import java.util.Arrays; import java.util.List; import java.util.function.ToIntFunction; import java.util.stream.IntStream; import java.util.stream.Stream; class Age{ private int age; public Age(int age){ this.age = age; } public int getAgeValue(){ return this.age; } } @SuppressWarnings("unused") class User{ private int id; private String name; private Age age; public User(int id, String name, int age){ this.id = id; this.name = name; this.age = new Age(age); } public Age getAge(){ return this.age; } public int getAgeWrapper(){ return this.age.getAgeValue(); } } public class SumTest { private static List<User> users = Arrays.asList( new User(1, "张三", 1), new User(2, "李四", 2), new User(3,"王五", 3), new User(4, "赵六", 4)); public static String covert2SnakeCase(final String camelCase) { final String regex = "([a-z])([A-Z])"; final String replacement = "$1_$2"; return camelCase.replaceAll(regex, replacement).toLowerCase(); } public static void main(String[] args) { //double sum = users.parallelStream().mapToInt(User::getAge).getAgeValue().reduce(0, (x, y) -> x + y); double sum2 = users.parallelStream().mapToInt(User::getAgeWrapper).sum(); double sum = 0; Stream<User> userStream = users.parallelStream(); ToIntFunction<User> mapper = User::getAgeWrapper; IntStream intstream = userStream.mapToInt(mapper); int total = intstream.reduce(0, (x, y) -> x + y); System.out.println("sum: " + total); // System.out.println("Total: " + intstream.sum()); //System.out.println("Total2: " + intstream.sum()); System.out.println("sum1: " + sum); //System.out.println(" sum2: " + sum2); System.out.println(covert2SnakeCase("helloJerry")); System.out.println(covert2SnakeCase("CongratulationsJerry")); } }
