hash collision

[chench...@gmail.com]

public class HashCollisionTest {

    private int countA;

    public void setCountA(int countA) {

        this.countA = countA;

    }

    @Cacheable(cacheName = "LRUCache-1d")

    public int sum(Integer a, Integer b) {

        countA++;

        return a + b;

    }

    public int getCount() {

        return countA;

    }

}

public class Cacheable2Test {

    @Autowired

    private HashCollisionTest hashCollisionTest;

 

    @Test

    public void test()  {

        Integer _108 = 108, _109 = 109, _34 = 34, _3 = 3;

        //test passed

        Assert.assertEquals(hashCollisionTest.sum(_108, _34), hashCollisionTest.sum(_109, _3));

    }

}

Hi,

I met a hash collision problem with the code above. I use the default CacheKeyGenerator "HashCodeCacheKeyGenerator". I checked the source code and found the code below that cause the hash collision. Since 108*31+34=109*31+3, so the unit test passed. Is there a method to fix the problem or I have to write a custom CacheKeyGenerator?

HashCodeCacheKeyGenerator:

protected void append(LongGenerator generator, int[] a) {

        for (final int element : a) {

            generator.hash = MULTIPLIER * generator.hash + element;

        }

    }

@Override

    protected void appendHash(LongGenerator generator, Object e) {

        if (e instanceof Double) {

            generator.hash = MULTIPLIER * generator.hash + Double.doubleToLongBits(((Double)e).doubleValue());

        }

        else if (e instanceof Long) {

            generator.hash = MULTIPLIER * generator.hash + ((Long)e).longValue();

        }

        else {        

            generator.hash = MULTIPLIER * generator.hash + e.hashCode();

        }

    }

regards,

Chun Chen.

[David Karlsen]

Maybe you could try switching to:
http://code.google.com/p/ehcache-spring-annotations/wiki/MessageDigestCacheKeyGenerator
?

2013/2/17 <chench...@gmail.com>:

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David J. M. Karlsen - http://www.linkedin.com/in/davidkarlsen

[Eric Dalquist]

Or even better use http://code.google.com/p/ehcache-spring-annotations/wiki/ListCacheKeyGenerator

That is the key generation style that the JSR-107 caching spec will use by default.

[chenchun]

Thanks, I'll try.

-- 

chenchun

在 星期日, 17 2月, 2013，10:00 PM，Eric Dalquist 写道：

ListCacheKeyGenerator