java8 自定义收集器

java8自定义收集器

案例：接收参数int n，并将前n个自然数分为质数和非质数。

第一种方式自定义收集器第一步：实现收集器接口，重写收集器方法，以及定义业务逻辑方法

package com.java8.CustomCollector;

import java.util.*;

import java.util.function.*;

import java.util.stream.Collector;

public class PrimeNumbersCollector implements Collector<Integer,Map<Boolean,List<Integer>>,Map<Boolean,List<Integer>>>{

/**



 * 作用：创建结果容器



 * @return



 */



@Override



public Supplier<Map<Boolean,List<Integer>>> supplier() {



    return () -> new HashMap<Boolean,List<Integer>>(){{



        put(true,new ArrayList<>());



        put(false,new ArrayList<>());



    }};



}







/**



 * 作用：将元素添加到结果容器



 * @return



 */



@Override



public BiConsumer<Map<Boolean,List<Integer>>,Integer> accumulator() {



    return (Map<Boolean,List<Integer>> acc,Integer candidate) ->{



        acc.get(isPrime(acc.get(true)



                ,candidate)



        ).add(candidate);



    };



}







/**



 * 对结果容器进行转换



 * @return



 */



@Override



public BinaryOperator<Map<Boolean,List<Integer>>> combiner() {



    return (Map<Boolean,List<Integer>> map1,



            Map<Boolean,List<Integer>> map2) ->{







        map1.get(true).addAll(map2.get(true));



        map1.get(false).addAll(map2.get(false));



        return map1;



    };



}







/**



 * 作用:当为并行流时，合并两个或多个结果容器



 * @return



 */



@Override



public Function<Map<Boolean,List<Integer>>,Map<Boolean,List<Integer>>> finisher() {



    return Function.identity();



}







/**



 * 作用：返回一个不可变的 Characteristics 集合



 * @return



 */



@Override



public Set<Characteristics> characteristics() {



    return Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));



}







/**



 * 行为参数化方法，



 * @param list



 * @param p



 * @param <A>



 * @return



 */



public static <A> List<A> takeWhile(List<A> list, Predicate<A> p){



    int  i = 0;



    for(A item : list){



        if(!p.test(item)){



            return list.subList(0,i);



        }



        i++;



    }



    return list;



}







/**



 * 主要判断逻辑（）判断是否是质数



 * @param primes



 * @param candidate



 * @return



 */



public static boolean isPrime(List<Integer> primes,int candidate){



    int candidateRoot = (int)Math.sqrt((double)candidate);



    return takeWhile(primes,i -> i <= candidateRoot).stream().noneMatch(p ->candidate % p == 0);



}

}

第二步：应用方法与java8内置收集归约函数做对比

package com.java8.CustomCollector;

import java.util.List;

import java.util.Map;

import java.util.stream.IntStream;

import static java.util.stream.Collectors.partitioningBy;

public class Test {

public static void main(String[] arg){



    long fastest  = Long.MAX_VALUE;



    for(int i = 0; i < 10 ;i++){



        long start = System.nanoTime();



        partitionPrimes(1000000);



        long duration = (System.nanoTime() - start) / 1000000;



        if (duration < fastest ) fastest = duration;



    }



    System.out.println("Fastest execution done in " + fastest +" msecs");







    for(int i = 0; i < 10 ;i++){



        long start = System.nanoTime();



        partitionPaimesWithCustomCollector(1000000);



        long duration = (System.nanoTime() - start) / 1000000;



        if (duration < fastest ) fastest = duration;



    }



    System.out.println("CustomCollector Fastest execution done in " + fastest +" msecs");



}











/**



 * 自定义收集器调用



 * @param n



 * @return



 */



public static  Map<Boolean,List<Integer>> partitionPaimesWithCustomCollector(int n){



    return IntStream.rangeClosed(2,n).boxed().collect(new PrimeNumbersCollector());



}











/**



 *采用系统分区函数



 * @param n



 * @return



 */



public static Map<Boolean,List<Integer>> partitionPrimes(int n){



    return IntStream.rangeClosed(2,n).



            boxed().



            collect(partitioningBy(candidate -> isPrime(candidate)));



}











/**



 *判断是否为质数



 * @param candidate



 * @return



 */



public static boolean isPrime(int candidate){



    int candidateRoot = (int)Math.sqrt((double)candidate);



    return IntStream.rangeClosed(2,candidateRoot).boxed().noneMatch(i -> candidate % i == 0);



}

}

函数输出（提高了55%）

Fastest execution done in 865 msecs

CustomCollector Fastest execution done in 382 msecs

第二种方式实现自定义收集器把实现核心逻辑的三个函数传给collect方法的重载版本来获得同样的结果。

package com.java8.CustomCollector;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.function.Predicate;

import java.util.stream.IntStream;

import static java.util.stream.Collectors.partitioningBy;

public class Test {

public static void main(String[] arg){



    long fastest  = Long.MAX_VALUE;



    for(int i = 0; i < 10 ;i++){



        long start = System.nanoTime();



        partitionPaimesWithCustomCollector(1000000);



        long duration = (System.nanoTime() - start) / 1000000;



        if (duration < fastest ) fastest = duration;



    }



    System.out.println("CustomCollector Fastest execution done in " + fastest +" msecs");



}















/**



 * 自定义收集器调用



 * @param n



 * @return



 */



public static  Map<Boolean,List<Integer>> partitionPaimesWithCustomCollector(int n){



    return IntStream.rangeClosed(2,n).boxed().collect(



            ()->new HashMap<Boolean,List<Integer>>(){{



                put(true,new ArrayList<>());



                put(false,new ArrayList<>());



            }},



            (acc,candidate) ->{



                acc.get(isPrime(acc.get(true)



                        ,candidate)



                ).add(candidate);



            },



            (map1,map2) ->{



                map1.get(true).addAll(map2.get(true));



                map1.get(false).addAll(map2.get(false));



            });



}



public static <A> List<A> takeWhile(List<A> list, Predicate<A> p){



    int  i = 0;



    for(A item : list){



        if(!p.test(item)){



            return list.subList(0,i);



        }



        i++;



    }



    return list;



}



public static boolean isPrime(List<Integer> primes,int candidate){



    int candidateRoot = (int)Math.sqrt((double)candidate);



    return takeWhile(primes,i -> i <= candidateRoot).stream().noneMatch(p ->candidate % p == 0);



}

}

收获：可以自己需要的时候开发自己的收集器 （虽然比较麻烦），可以用于追求性能的优化