自己的电脑建网站,WordPress做头部的插件,百度推广网站建设,上饶做网站建设在Java 8及更高版本中#xff0c;Stream API为集合处理带来了革命性的改变。本文将深入解析如何运用Stream对List进行高效的操作#xff0c;包括筛选#xff08;Filter#xff09;、排序#xff08;Sort#xff09;、分组#xff08;GroupBy#xff09;、求平均值…在Java 8及更高版本中Stream API为集合处理带来了革命性的改变。本文将深入解析如何运用Stream对List进行高效的操作包括筛选Filter、排序Sort、分组GroupBy、求平均值Average和求和Sum。通过实例代码演示以及功能差异对比我们将揭示这些操作在不同应用场景下的最佳实践。
1. Filter操作
filter()方法用于根据给定的条件过滤列表中的元素仅保留满足条件的项。
ListInteger numbers Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
ListInteger evenNumbers numbers.stream().filter(n - n % 2 0).collect(Collectors.toList());上述代码展示了如何使用filter()方法从numbers列表中筛选出所有的偶数。
2. Sort操作
sorted()方法可以对流中的元素进行排序可以使用自然顺序或自定义Comparator。
ListString names Arrays.asList(Bob, Alice, Charlie, David);
ListString sortedNames names.stream().sorted().collect(Collectors.toList()); // 自然排序
ListString reverseSortedNames names.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList()); // 倒序排序3. GroupBy操作
groupBy()方法用于将流中的元素按照指定的属性进行分组返回的是Map类型结果。
ListEmployee employees ... // 假设Employee类有department属性
MapString, ListEmployee groupedEmployees employees.stream().collect(Collectors.groupingBy(Employee::getDepartment));这段代码展示了如何按员工所在的部门进行分组。
4. Average与Sum操作
对于数值型流可以计算平均值average和总和sum。
ListDouble salaries Arrays.asList(50000.0, 70000.0, 60000.0, 80000.0);
double averageSalary salaries.stream().mapToDouble(Double::doubleValue).average().orElse(0.0);double totalSalary salaries.stream().mapToDouble(Double::doubleValue).sum();5. 实战示例及代码详解
当然让我们进一步深入到实战示例中为上述的Filter、Sort、GroupBy、Average和Sum操作提供更详尽的代码详解。
1. Filter过滤
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class StreamFilterExample {public static void main(String[] args) {ListInteger numbers Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);// 过滤出所有偶数ListInteger evenNumbers numbers.stream().filter(n - n % 2 0).collect(Collectors.toList());System.out.println(Even Numbers: evenNumbers); // 输出[2, 4, 6, 8]}
}// 解释
// .stream() 将列表转换成流。
// .filter() 方法接受一个谓词Predicate该谓词是一个返回布尔值的函数用于决定哪些元素应该被保留。
// .collect(Collectors.toList()) 是终端操作它将过滤后的流转换回一个新的List对象。2. Sort排序
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;public class StreamSortExample {public static class Employee {String name;int age;// 构造方法、getters setters 省略Overridepublic String toString() {return Employee{ name name \ , age age };}}public static void main(String[] args) {ListEmployee employees Arrays.asList(new Employee(Alice, 25),new Employee(Bob, 30),new Employee(Charlie, 28),new Employee(David, 32));// 自然排序默认按age升序ListEmployee sortedByAge employees.stream().sorted(Comparator.comparing(Employee::getAge)).collect(Collectors.toList());// 按名字降序排序ListEmployee sortedByNameDesc employees.stream().sorted(Comparator.comparing(Employee::getName).reversed()).collect(Collectors.toList());System.out.println(Sorted by Age: sortedByAge);System.out.println(Sorted by Name Descending: sortedByNameDesc);}
}// 解释
// Comparator.comparing(...) 方法用来创建一个Comparator根据指定的方法提取排序键。
// .reversed() 方法反转Comparator的顺序使其成为降序排列。3. GroupBy分组
import java.util.*;
import java.util.stream.Collectors;public class StreamGroupByExample {public static class Product {String category;String name;double price;// 构造方法、getters setters 省略Overridepublic String toString() {return Product{ category category \ , name name \ , price price };}}public static void main(String[] args) {ListProduct products Arrays.asList(new Product(Electronics, Laptop, 1000.0),new Product(Electronics, Smartphone, 500.0),new Product(Books, Programming Book, 30.0),new Product(Books, Novel, 20.0));MapString, ListProduct groupedProducts products.stream().collect(Collectors.groupingBy(Product::getCategory));for (Map.EntryString, ListProduct entry : groupedProducts.entrySet()) {System.out.println(Category: entry.getKey());System.out.println(Products: entry.getValue());}}
}// 解释
// Collectors.groupingBy(...) 提供了一个收集器可以将流中的元素按照给定的函数进行分类并放入Map中。4. Average平均值与Sum求和
import java.util.Arrays;
import java.util.List;
import java.util.stream.DoubleStream;
import java.util.stream.StreamSupport;public class StreamAverageAndSumExample {public static void main(String[] args) {ListDouble sales Arrays.asList(1000.0, 2000.0, 3000.0, 4000.0, 5000.0);// 计算销售额总和double totalSales sales.stream().mapToDouble(Double::doubleValue).sum();System.out.println(Total Sales: totalSales);// 计算销售额平均值OptionalDouble averageSales sales.stream().mapToDouble(Double::doubleValue).average();System.out.println(Average Sales: averageSales.orElse(0.0));}// 解释// .mapToDouble(...) 将流中的元素映射为double值便于进行数学运算。// .sum() 和 .average() 分别用于计算数值流的总和和平均值.average() 返回的是OptionalDouble类型需使用orElse(...)来处理可能为空的情况。6. 区别总结
filter()用于筛选数据是中间操作。适用于清洗数据、筛选符合条件的数据项。sorted()用于排序数据可作为中间操作也可作为终端操作。在展示数据时需要排序如用户列表、商品列表等。groupingBy()用于分组数据是一个特殊的收集器用于将流元素映射到Map中。数据汇总分析如按地区统计销售额、按部门统计员工人数等。average()和sum()用于数值类型的统计分析是终端操作。常用于数据分析、报表生成等场景例如计算平均薪资、总销售额等。 What is Java technology and why do I need it? Java is a programming language and computing platform first released by Sun Microsystems in 1995. It has evolved from humble beginnings to power a large share of today’s digital world, by providing the reliable platform upon which many services and applications are built. New, innovative products and digital services designed for the future continue to rely on Java, as well.
While most modern Java applications combine the Java runtime and application together, there are still many applications and even some websites that will not function unless you have a desktop Java installed. Java.com, this website, is intended for consumers who may still require Java for their desktop applications – specifically applications targeting Java 8. Developers as well as users that would like to learn Java programming should visit the dev.java website instead and business users should visit oracle.com/java for more information.
Is Java free to download? Yes, Java is free to download for personal use. Java is also free for development: developers can find all the development kits and other useful tools at https://www.oracle.com/javadownload/.
Why should I upgrade to the latest Java patch each quarter when prompted? The latest Java patches contain important enhancements to improve performance, stability and security of the Java applications that run on your machine. Installing these updates will ensure that your Java applications continue to run with the most up-to-date version.
