java8 函數式編程

/***
     * 測試統計API <br>
     * 小結:<br>
     * 1.邏輯代碼應用{} 包起來  不寫{} 默認  + return <br>
     * 2.重載方法參數必須加類型聲明<br>
     * 3.書寫風格一個表達式只作一次抽象轉換 <br>
     * 4.最後斷鏈了 stream 內部會關閉流,再執行流之類操做會拋異常<br>
     * 5.表達式能夠象管道同樣綁定,接口與另外一實例方法綁定<br>
     * */
    public static class Test4 {
        
        
        public static void main(String[] args) {
            List<Person> list1 = Person.ofList();
             
            //sum 操做
            long count = list1.stream().mapToInt(e -> e.getAge()).sum();
            //reduce 操做
            long count2 = list1.stream().mapToInt(e -> e.getAge()).reduce(0, (a,b)->a+b);
                        
            //max 操做
            OptionalInt max = list1.stream().mapToInt(e -> e.getAge()).max();
            //avg 操做
            OptionalDouble count1 = list1.stream().mapToInt(e -> e.getAge()).average();
            
            //返回統計對象
            IntSummaryStatistics statistics=list1.stream().mapToInt(e -> e.getAge()).summaryStatistics();
        
            //groupBy 操做
            Map<Integer, List<Person>> map=list1.stream().collect(Collectors.groupingBy(Person::getAge));
            
            //collect 集合結束操做
            Collectors.toList();
            Collectors.toSet();
            
            //每次it 回調peek 
            list1.stream().filter(e->e.getAge()>2).mapToInt(e -> e.getAge()).peek( e->System.out.println("peek : "+e)).count();
            
            //接口與實例方法綁定
            list1.stream().forEach(System.out::println);
            
            //並行流
            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;
    }

}