Core Java

1. what is the access modifiers/specifiers or what is the difference between them?

A) Access modifiers/specifiers:
               








         JVM(java virtual machine) Architecture 


            
                   

 Reference 

                   https://www.youtube.com/watch?v=dncpVFP1JeQ





Q) Difference between array and ArrayList in java?
Sol:
   Refer:
      
http://javahungry.blogspot.com/2015/03/difference-between-array-and-arraylist-in-java-example.html



Q)  What is Generics in java and what is use of the Generics?

sol:

 Reference:    
http://www.javatpoint.com/generics-in-java

Q) Difference between DAO and DTO in Java?

Sol:

      Data Access Object (DAO)

      DAO is a class that usually has the CRUD operations like save, update, delete.

     Data Transfer Object (DTO).

     The DTO is just an object that holds data. It is really a bean with instance variables and setter and getters.

 Reference:
   https://coderanch.com/t/99834/difference-DTO-DAO


Q) AutoBoxing and AutoUnboxing in Java?

AutoBoxing(wrapping):

·         Converting  primitive data types into reference type or corresponding object wrapper class is called AutoBoxing

·         Java compiler performs automatic conversation.

·         Ex:      int a= 10;

          Integer b = a:

·         Here we are assigning primitive value to object type. Internally java compiler converts below conversation operation.

       Integer b = Integer.valueOf (a);

AutoUnBoxing(unwrapping):

·         Converting  reference type or object  of wrapper class into primitive data types is called AutoUnBoxing

·         Java compiler performs automatic conversation.

·         Ex:      Integer a= 100;

             int b = a:

·         Here we are assigning object to primitive type. Internally java compiler converts below conversation operation.

       int b =a.intValue();

·         Using reference type will reduces performance because it has to recreate the object first then memory created in the heap.

·         Primitive types will have default values so need of checking null check on the primitive types.

Ex: int a;

Means int default value is 0

·         Object types of default values are null. We have to apply null check object/reference types.

·         All primitive types will have corresponding wrapper class.

Q) Generics  in Java ?

Generics:

·         The purpose of Generics is to provide Type Safety and to resolve typecasting.

·         Arrays are type safety that means it will store only specified data type in the array but the size is fixed.

Ex: Array[String ]  a = new [String](10);

·         Collections are not type safety which means it can store any kind of data type.

·         To achieve  above  two draw backs in arrays and collections Generics are introduced which generics is type safety and size is not fixed.

Ex:  Box<Integer> integerBox = new Box<Integer>();

·         Generics will accept only object type but not primitive types.

Ex: List<Integer> a = new Arraylist<>();  correct

       List<int> a = new Arraylist<>();  is wrong because int is primitive type.   

Q)  How to print the  SQL query which is used in hibernate or spring in the console using log4j?

 A) log4j.logger.org.hibernate.SQL=debug

 

Q) how to add jar to M2 repository manually ?
A)
mvn install:install-file -Dfile=<physical location of the jar file with name> -DgroupId=<groupId> -DartifactId=<artifactId> -Dversion=<version> -Dpackaging=jar