public boolean in(TransactionType... types)
    {
        if (types == null || types.length == 0)
            return false;

        for (int i = 0; i < types.length; ++i)
            if (types[i] != null && types[i] == this)
                return true;
        return false;
    }

我把它改成了这个

public boolean in(TransactionType... types)
    {
        if (types == null || types.length == 0)
            return false;

        for (int i = 0; i < types.length; ++i)
            if (types[i] == this)
                return true;
        return false;
    }

(TransactionType是一个包含大约30个值的枚举)

结果震惊了我.在我的所有测试中,第二个测试速度提高了一个数量级.我预计可能快2倍,但不是一个数量级.为什么不同？这是一个非常慢的nullcheck,或者是额外的数组访问会发生什么奇怪的事情？

我的基准代码看起来像这样

public class App
{
    public enum TransactionType
    {
        A(1,"A","A"),B(3,"B","B"),C(5,"C","C"),D(6,"D","D"),E(7,"E","E"),F(8,"F","F"),G(9,"G","G"),H(10,"H","H"),I(11,"I","I"),J(12,"J","J"),K(13,"K","K"),L(14,"L","L"),M(15,"M","M"),N(16,"N","N"),O(17,"O","O"),P(18,"P","P"),Q(19,"Q","Q"),R(20,"R","R"),S(21,"S","S"),T(22,"T","T"),U(25,"U","U"),V(26,"V","V"),W(27,"W","W"),X(28,"X","X"),Y(29,"Y","Y"),Z(30,"Z","Z"),AA(31,"AA","AA"),AB(32,"AB","AB"),AC(33,"AC","AC"),AD(35,"AD","AD"),AE(36,"AE","AE"),AF(37,"AF","AF"),AG(38,"AG","AG"),AH(39,"AH","AH"),AI(40,"AI","AI"),AJ(41,"AJ","AJ"),AK(42,"AK","AK"),AL(43,"AL","AL"),AM(44,"AM","AM"),AN(45,"AN","AN"),AO(46,"AO","AO"),AP(47,"AP","AP");

        public final static TransactionType[] aArray =
        {
            O,Z,N,Y,AB
        };

        public final static TransactionType[] bArray =
        {
            J,P,AA,L,Q,M,K,AE,AK,AF,AD,AG,AH
        };

        public final static TransactionType[] cArray =
        {
            S,U,V
        };

        public final static TransactionType[] dArray =
        {
            A,B,D,G,C,E,T,R,I,F,H,AC,AI,AJ,AL,AM,AN,AO
        };

        private int id;
        private String abbrev;
        private String name;

        private TransactionType(int id,String abbrev,String name)
        {
            this.id = id;
            this.abbrev = abbrev;
            this.name = name;
        }

        public boolean in(TransactionType... types)
        {
            if (types == null || types.length == 0)
                return false;

            for (int i = 0; i < types.length; ++i)
                if (types[i] == this)
                    return true;
            return false;
        }

        public boolean inOld(TransactionType... types)
        {
            if (types == null || types.length == 0)
                return false;

            for (int i = 0; i < types.length; ++i)
            {
                if (types[i] != null && types[i] == this)
                    return true;
            }
            return false;
        }
    }

    public static void main(String[] args)
    {
        for (int i = 0; i < 10; ++i)
            bench2();

        for (int i = 0; i < 10; ++i)
            bench1();
    }

    private static void bench1()
    {
        final TransactionType[] values = TransactionType.values();
        long runs = 0;
        long currTime = System.currentTimeMillis();
        while (System.currentTimeMillis() - currTime < 1000)
        {
            for (TransactionType value : values)
            {
                value.inOld(TransactionType.dArray);
            }
            ++runs;
        }
        System.out.println("old " + runs);
    }

    private static void bench2()
    {
        final TransactionType[] values = TransactionType.values();
        long runs = 0;
        long currTime = System.currentTimeMillis();
        while (System.currentTimeMillis() - currTime < 1000)
        {
            for (TransactionType value : values)
            {
                value.in(TransactionType.dArray);
            }
            ++runs;
        }
        System.out.println("new " + runs);
    }
}

以下是基准测试运行的结果

new 20164901
new 20084651
new 45739657
new 45735251
new 45757756
new 45726575
new 45413016
new 45649661
new 45325360
new 45380665
old 2021652
old 2022286
old 2246888
old 2237484
old 2246172
old 2268073
old 2271554
old 2259544
old 2272642
old 2268579

这是使用Oracle JDK 1.7.0.67