How does java implement hash tables

We seek a hash function that is both easy to compute and uniformly distributes the keys. Typical example. Suppose that we have an application where the keys are U. A social security number such as is a 9-digit number divided into three fields. The first field identifies the geographical area where the number was issued for example number whose first field are are from Rhode Island and numbers whose first field are are from Maryland and the other two fields identify the individual.

One possible approach to implementing a hash function is to use three digits from the key. Using three digits from the field on the right is likely to be preferable to using the three digits in the field on the left since customers may not be equally dispersed over geographic areas , but a better approach is to use all nine digits to make an int value, then consider hash functions for integers, described next.

Positive integers. The most commonly used method for hashing integers is called modular hashing : we choose the array size M to be prime, and, for any positive integer key k, compute the remainder when dividing k by M. Floating-point numbers. If the keys are real numbers between 0 and 1, we might just multiply by M and round off to the nearest integer to get an index between 0 and M Although it is intuitive, this approach is defective because it gives more weight to the most significant bits of the keys; the least significant bits play no role.

One way to address this situation is to use modular hashing on the binary representation of the key this is what Java does. Modular hashing works for long keys such as strings, too: we simply treat them as huge integers. For example, the code below computes a modular hash function for a String s, where R is a small prime integer Java uses If the key type has multiple integer fields, we can typically mix them together in the way just described for String values.

For example, suppose that search keys are of type USPhoneNumber. Java helps us address the basic problem that every type of data needs a hash function by requiring that every data type must implement a method called hashCode which returns a bit integer. The implementation of hashCode for an object must be consistent with equals. That is, if a. If the hashCode values are the same, the objects may or may not be equal, and we must use equals to decide which condition holds.

Converting a hashCode to an array index. User-defined hashCode. Client code expects that hashCode disperses the keys uniformly among the possible bit result values. That is, for any object x , you can write x. Java provides hashCode implementations that aspire to this functionality for many common types including String , Integer , Double , Date , and URL , but for your own type, you have to try to do it on your own.

Program PhoneNumber. Program Transaction. We have three primary requirements in implementing a good hash function for a given data type: It should be deterministic —equal keys must produce the same hash value. It should be efficient to compute. It should uniformly distribute the keys.

To analyze our hashing algorithms and develop hypotheses about their performance, we make the following idealized assumption. Assumption J uniform hashing assumption. The hash function that we use uniformly distributes keys among the integer values between 0 and M This method works similarly to the other two.

If there are no objects no LinkedList in the array location specified by the hash function, the method simply returns null.

If this key is found, the associated value is returned. Note that the get method functionality is split into get and getObj in the code below. Save my name, email, and website in this browser for the next time I comment. By continuing to use this website, you consent to the use of cookies in accordance with our Cookie Policy. Skip to content. Basic Hash Table Methods The three basic operations of a hash table include adding, removing, and getting using a key to get the associated value.

The Get Method The get method is used to return the value of a provided key. The Code package com. GZip files with. Sun's implementation initialises the value on first use, using a slight rehash of the memory address at the time of initialisation. I've edited the answer.

Show 1 more comment. In my own words: An Entry object is created to hold the reference of the Key and Value. The HashMap has an array of Entry 's. The index for the given entry is the hash returned by key. That's how two objects with the same hash could be stored into the collection. Community Bot 1 1 1 silver badge. OscarRyz OscarRyz k gold badges silver badges bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook.

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