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I am going through the Introduction of Algorithms by Cormen et al video and it discusses several hashing functions . I want to know what hashing function does Java use by default?Does the hashing function actually differ for different types of objects that are used as keys? Is there an api in the Collections framework which let us write our own hashing algorithm ?
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Types of Hashing Algorithms in Java There are several hashing algorithms – the most common ones are: MD5, SHA-1, and SHA-256. These algorithms are used to generate a hash of a given piece of data, which can then be used to verify the integrity of that data.
Here hashcode()and other methods are defined in the Object class. So all java classes have the hashcode() method by default. We can override these methods in our classes.
In the simplest usage, the hash function returns the 32-bit or 64-bit hash of a data buffer or string. The default underlying hash function is murmur3, chosen because it has good hash function properties and offers a 64-bit version.
A hash function is a way to create a compact representation of an arbitrarily large amount of data. In java with the hashcode method this means somehow describing the state of your object (no matter how large) in an int (4 bytes). And is usually written to be a fairly fast as explained below.
Each object in java has a public int hashCode() method that returns a hash. Each object is free to implement it in its own way by overriding that method. If the method is not overriden, the default Object#hashCode method is used.
You can have look at the source code of various objects to see how it is implemented in the JDK. This is String's hashCode for example (line 1494).
Some collections can add an additional layer of hashing on top of the objects' hashCode methods. For example, HashMap does that to improve performance when an object's hashCode is not well distributed.
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