Java - Equals and Hashcode

By -
Every object has the following two properties:
1. An ability to compare itself with any other object
2. A int value associated with it, which can have a business logic

There are a few basic rules to be applied for two objects to be equal:
  1. Consistency : If two objects are equal they should always be equal provided none of their values(which determine their equality) change
  2. Symmetry : If object1 equals object2, then object2 should equal object1, Same rule for non-equality
  3. Reflexive : If you compare an object with itself, it should always return true, under all circumstances
  4. Transitivity : If object1 equals object2, and object2 equals object3, then object1 should equal object3.
Why do we need equals method when we have ==?
== checks for reference equality (if two reference are same) equals method checks if two objects are equal or not

Just be sure of overriding equals method before using it. Else be ready for false, except when comparing same references :D

It is a good practice to override HashCode and Equals in your POJOs. Hashcode should have some business logic associated with it. Should be unique.
I really love using code which has comments, equals and hashcode overriden.

If two objects are equal (according to the equals methods), they need to have the same hashcode.
BUT the reverse is not always true. Two objects with the same hashcode need not be same.

Method Signatures
Equals Method and Hashcode overridden in a class:



Bottomline: Your POJOs should have equals and hashCode method overridde. Just define the parameters which determine the equality of two objects. And keep a check on the rules.

Coming soon: Hashcode and HashMaps.

Comments

Post a Comment

Popular posts from this blog

Writing your own ejabberd Module

By -
Coming soon: What is ejabberd? What is erlang? Installation details. I will start with writing the ejabberd module first. Once you have done all the installations: sudo make sudo make install sudo ejabberdctl start sudo ejabberdctl stop You can check the status using: sudo ejabberdctl status You can locate logs at: (With default installation) /var/log/ejabberd All config related changes will be mainly done at: /etc/ejabberd/ejabberd.yml You will need a basic understanding of erlang. You can take a short crash course at: http://www.erlang.org/course/course.html This is a good reference. Go to your ejabberd-15.07/src directory. This is where you will write the hook for your ejabberd server. Create a file, say mod_changeBody.erl. Your module name and file name should be same. Add the following basic contents into your erl file: -module(mod_changeBody). -behavior(gen_mod). -include("ejabberd.hrl"). -include("logger.hrl"). -expo
Read more

npm ECONNREFUSED error

By -
If you face error which says ECONNREFUSED while doing npm install <<package name>>, you are probably behind a proxy which is not allowing you to connect to the server and download the packages required. Just follow the below steps: npm config set proxy  http://your_proxy_name:port npm config set https-proxy https://your_proxy_name:port And then try npm install How to get npm config? npm get proxy will do the work :)
Read more

Conditional Flow - Spring Batch Part 6

By -
Jump to Conditional Flow using Custom Status There can be muliple scenario where you want your jobs to proceed depending on the outcome of your steps. For example, in case of our Corporate gifts shop, in case the automatic packaging of the gifts fails, we want some manual intervention to be done i.e. somebody should manually package the item.  IF-ELSE IF loop for packaging:     if packageStep fails                              then manualPackageStep                         else if packageStep executes successfully                              then sendOutForDelivery Entire code available at git repo:   https://github.com/ricsr/spring-batch-demo/tree/exercise_06 Using SpringBatch you can achieve it as follows: Create you manualPackageStep, which is very similar to your packageStep, except that it does the packaging manually. @Bean public Step manualPackageStep () { return this . stepBuilderFactory .get( "manualPackageStep" ).tasklet( new Tasklet() { @Override pu
Read more