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"immutability" Nature of an Object


A Java object is considered to be immutable when its state (properties and contents) cannot change after it is created.

Use of immutable objects is widely accepted as a sound strategy for creating simple, reliable code. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state. java.lang.String and java.lang.Integer classes are the Examples of immutable

Immutable objects are simple to use test and construct.
Immutable objects are thread-safe by default.
Immutable objects are good Map keys and Set elements (Since state of these objects must not change while stored in a collection). This is the reason most of the times we prefer String objects as Key in many Map Collection objects.
Immutable objects do not require an implementation of clone.
Immutable objects allow hashCode to use lazy initialization, and to cache its return value.

To create a object immutable

Need to make the class final and all its member final so that once objects gets crated no one can modify its state.
By making member as non final but private and not modifying them except in constructor. Also its NOT necessary to have all the properties final since you can achieve same functionality by making member as non final but private and not modifying them except in constructor.


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Maven Build Life Cyles

Following are the maven build life cycles.

Clean - Removes the generated files in 'target' folder.

command : mvn clean

Validate - It validates your project and verify all the necessary information is there in project or not.

mvn  valiate

Compile - It compiles your project code.

mvn compile

Package - It takes the compiled code and packaged to JAR/WAR/EAR. It also compiles your code if your code is not compiled.

mvn package

Install -   It takes the packaged code and puts in your local maven repository (M2_HOME). It also does the compile and package if these are not already been done.

mvn install

Deploy - It takes the package and puts in remote repository for sharing accross team members or for other projects.

mvn deploy

FYI - Above commands can be used as variety of combinations.
e.g. mvn clean install
mvn clean package
mvn clean compile package install

To skip the tests to run 
mvn clean install -DskipTests

NO SQL Types and vendors in market


KEY - VALUE Store  - Similar to a  MAP.
   e.g. Dynamo DB or REDIS

DOCUMENT Sore  - Similar to KEY-VALUE, value can be JSON/XML and key will be unqiuely idnetified this document.
   e.g. Coutch DB or mongoDB.

Column Store - Multi timentional table, identifying data with row and column numbers.
  e.g.  Cassandra or Apache HBase

Graph Store - Store relation between nodes (record entities). It has better transaction management
e.g. Neo4J and Orient DB.