java8 函数式编程

    /***
     * 测试统计API <br>
     * 小结:<br>
     * 1.逻辑代码应用{} 包起来  不写{} 默认  + return <br>
     * 2.重载方法参数必须加类型声明<br>
     * 3.书写风格一个表达式只做一次抽象转换 <br>
     * 4.最后断链了 stream 内部会关闭流,再执行流之类操作会抛异常<br>
     * 5.表达式可以象管道一样绑定,接口与另一实例方法绑定<br>
     * */
    public static class Test4 {
        
        
        public static void main(String[] args) {
            List<Person> list1 = Person.ofList();
             
            //sum 操作
            long count = list1.stream().mapToInt(e -> e.getAge()).sum();
            //reduce 操作
            long count2 = list1.stream().mapToInt(e -> e.getAge()).reduce(0, (a,b)->a+b);
                        
            //max 操作
            OptionalInt max = list1.stream().mapToInt(e -> e.getAge()).max();
            //avg 操作
            OptionalDouble count1 = list1.stream().mapToInt(e -> e.getAge()).average();
            
            //返回统计对象
            IntSummaryStatistics statistics=list1.stream().mapToInt(e -> e.getAge()).summaryStatistics();
        
            //groupBy 操作
            Map<Integer, List<Person>> map=list1.stream().collect(Collectors.groupingBy(Person::getAge));
            
            //collect 集合结束操作
            Collectors.toList();
            Collectors.toSet();
            
            //每次it 回调peek 
            list1.stream().filter(e->e.getAge()>2).mapToInt(e -> e.getAge()).peek( e->System.out.println("peek : "+e)).count();
            
            //接口与实例方法绑定
            list1.stream().forEach(System.out::println);
            
            //并行流
            list1.parallelStream().count();
            
            System.out.println("count : " + count);
            System.out.println("count2 : " + count2);
            
            System.out.println("aver : " + count1.getAsDouble());
            System.out.println("max : " + max.getAsInt());
            System.out.println("statistics count : " + statistics.getCount());
            System.out.println("groupBy size : " + map.size());
            

            Map<String,String> m = new HashMap<>();
            m.getOrDefault("a", "5");
            m.merge("a", "addValue", (a,b)-> a+b);
            System.out.println("map merge : " + m.get("a"));
    
             
        }
    }

import java.util.ArrayList;
import java.util.List;

public class Person {
    private String name;
    private int age;

    public static Person of(String name, int age) {
        Person result = new Person();
        result.name = name;
        result.age = age;
        return result;
    }

    public Person() {
    }

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public static List<Person> ofList() {
        List<Person> result = new ArrayList<>();
        for (int i = 0; i < 50; i++) {
            result.add(of("a" + i, i % 5));
        }
        return result;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

}