我有兩個測試類：

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();}}

先構造一個集合Collections：

private static List<User> users = Arrays.asList( new User(1, "張三", 1), new User(2, "李四", 2), new User(3,"王五", 3), new User(4, "趙六", 4));

// 獲得一個并行流：

Stream userStream = users.parallelStream();

// 獲得一個映射函數，該函數配合stream使用，能把所有User實例的Age字段映射成IntStream

ToIntFunction mapper = User::getAgeWrapper;

IntStream intstream = userStream.mapToInt(mapper);

// 得到輸出：10

System.out.println("Total: " + intstream.sum());

如果重復調用sum：

System.out.println("Total2: " + intstream.sum());

會遇到錯誤消息：

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自帶的sum方法，也可以手動用reduce計算：

int total = intstream.reduce(0, (x, y) -> x + y);

一樣能得到10的結果。

全部測試代碼：

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, "張三", 1), new User(2, "李四", 2), new User(3,"王五", 3), new User(4, "趙六", 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"));}}

總結

以上是生活随笔為你收集整理的利用Java流进行类的整型字段求和的例子的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。