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Why does hashCode() function generate an error


Why does hashCode() function generate an error

By : GGenom
Date : October 17 2020, 06:10 PM
hop of those help? Here is the general contract of Object.hashCode. The question is basically asking you which of those choices fulfils the general contract.
code :


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How likely is it to get a HashCode collision with this hashcode function?

How likely is it to get a HashCode collision with this hashcode function?


By : user2846366
Date : March 29 2020, 07:55 AM
Hope that helps How likely is it to get a HashCode collision with the function below in following scenarios. , I wrote a quick script to test this.
code :
import random

def hash(key):
    hashKey = 0
    hashKey += 2047 * key[0]
    hashKey += 8191 * key[1]
    hashKey += 32767 * key[2]
    hashKey += 131071 * key[3]
    return hashKey

seen = set()
collisions = 0
for i in range(0,10000000):
    x = hash([random.randint(0,1000000), random.randint(0,10000), random.randint(0,1000), random.randint(0,1000)])
    if x in seen:
        collisions += 1
    else:
        seen.add(x)

print collisions
Why in Enum hashCode() refers to the Object hashCode() implementaion, instead of ordinal() function?

Why in Enum hashCode() refers to the Object hashCode() implementaion, instead of ordinal() function?


By : Alex Ceb
Date : March 29 2020, 07:55 AM
hop of those help? I always thought that enum hashCode was referring to ordinal in Java, since ordinal seems to be a perfect candidate for hashCode, but it turns out it enum hashCode actually refers to default hashCode object implementation. , Imagine this trivial, completely made up scenario:
code :
interface MediaType {}

enum BookTypes implements MediaType {
    HARDCOVER;
}

enum MagazineTypes implements MediaType {
    MONTHLY;
}

Map<MediaType, MediaItem> itemMap = new HashMap<MediaType, MediaItem>();
itemMap.put(BookTypes.HARDCOVER, new Book());
itemMap.put(MagazineTypes.MONTHLY, new Magazine());
Why does Netbeans generate hashCode() the way that it does?

Why does Netbeans generate hashCode() the way that it does?


By : SurfaceNote
Date : March 29 2020, 07:55 AM
it should still fix some issue This is an optimization aiming to better distribute the hash values. Eclipse does it similarly. Have a look at Why use a prime number in hashCode? and Why does Java's hashCode() in String use 31 as a multiplier?.
This is in no way required. Even return 0; is sufficient in order to fulfill the equals/hashcode contract. The only reason is that hash based data structures perform better with good distributed hash values.
Why hashcode does not generate unique hashcode?

Why hashcode does not generate unique hashcode?


By : user2681047
Date : March 29 2020, 07:55 AM
I hope this helps you . Because it can't.
Since there are only 2^32 different ints and there may be more than 2^32 live objects in any VM instance, it is technically impossible to guarantee a unique hash code for each object.
why not to use nonfinal fields to generate hashCode in method HashCode ()

why not to use nonfinal fields to generate hashCode in method HashCode ()


By : delta3506
Date : March 29 2020, 07:55 AM
I wish did fix the issue. The idea is that if two objects are equal they must have the same hashcode. So taken to the next step you could say that an object should always have the same hashcode. Obviously, if you use non-final fields in the hashcode calculation, then the hashcode will not remain the same.
For a more concrete example, if you put an object in a hashmap and then change the object such that the hashcode changes the object in now in the wrong bucket of the hashmap. BAD!
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