java - 当找到几个集合的交集是使用 retainAll() 的最快顺序时

Question

我试图找到三个不同大小的哈希集的交集。通过改变集合相交的顺序，找到交集的速度是否有任何差异。示例程序如下:

public class RetainTest {
    static Set<Integer> large =new HashSet<>();
    static Set<Integer> medium =new HashSet<>();
    static Set<Integer> small =new HashSet<>();

    static int largeSize=10000;
    static int midSize=5000;
    static int smallSize=1000;      

    public static void main(String[] args){
        preamble()
        large.retainAll(medium);
        large.retainAll(small);

        System.out.println(large.size());
    }


    public static void preamble(){
        large =new HashSet<>();
        medium =new HashSet<>();
        small =new HashSet<>();

        Random rnd=new Random(15);
        for(int i=0;i<largeSize;i++){
            large.add(rnd.nextInt(largeSize*10));
        }

        for(int i=0;i<midSize;i++){
            medium.add(rnd.nextInt(largeSize*10));
        }
        for(int i=0;i<smallSize;i++){
            small.add(rnd.nextInt(largeSize*10));
        }

    }

}

Answer 1

分析表明，组合多个集合的最快方法是将较大的集合retainAll 合并到较小的集合中。此外，这些保留的顺序也应该从最小到最大。所以

    small.retainAll(medium);
    small.retainAll(large);

分析表明差异显着:对于此数据集，最慢的订单花费的时间大约是最慢的订单的 10 倍

测试程序

这些结果是使用以下测试程序创建的，该程序运行了 20 分钟

public class RetainTest {
    
    static Set<Integer> large =new HashSet<>();
    static Set<Integer> medium =new HashSet<>();
    static Set<Integer> small =new HashSet<>();
    
    static int largeSize=10000;
    static int midSize=5000;
    static int smallSize=1000;      
    
    public static void main(String[] args){
        while(true){
            preamble();
            int size1=largeMediumSmall().size();
            preamble();
            int size2=largeSmallMedium().size();
            preamble();
            int size3=smallMediumLarge().size();
            preamble();
            int size4=smallLargeMedium().size();
            preamble();
            int size5=mediumSmallLarge().size();
            preamble();
            int size6=mediumLargeSmall().size();
            
            //sanity check + ensuring the JIT can't optimise out
            if (size1!=size2 || size1!=size3 || size1!=size4 || size1!=size5 || size1!=size6){
                System.out.println("bad");
            }
        }
        

    }
    
    public static Set<Integer> largeMediumSmall(){
        large.retainAll(medium);
        large.retainAll(small);
        
        return large;
    }
    
    public static Set<Integer> smallMediumLarge(){
        small.retainAll(medium);
        small.retainAll(large);
        
        return small;
    }
    public static Set<Integer> smallLargeMedium(){
        small.retainAll(large);
        small.retainAll(medium);
        
        return small;
    }
    public static Set<Integer> mediumSmallLarge(){
        medium.retainAll(small);
        medium.retainAll(large);
        
        return medium;
    }
    public static Set<Integer> mediumLargeSmall(){
        medium.retainAll(large);
        medium.retainAll(small);
        
        return medium;
    }
    public static Set<Integer> largeSmallMedium(){
        large.retainAll(small);
        large.retainAll(medium);
        
        return large;
    }
    
    
    public static void preamble(){
        large =new HashSet<>();
        medium =new HashSet<>();
        small =new HashSet<>();
        
        Random rnd=new Random(15);
        for(int i=0;i<largeSize;i++){
            large.add(rnd.nextInt(largeSize*10));
        }
        
        for(int i=0;i<midSize;i++){
            medium.add(rnd.nextInt(largeSize*10));
        }
        for(int i=0;i<smallSize;i++){
            small.add(rnd.nextInt(largeSize*10));
        }
        
    }
    
}