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1. Java Map 
   In Java, a Map is a kind of generalized array. It provides a more general way of storing elements. It is an object that maps keys to values. Map is not an extension of the collection interface rather it has own interface hierarchy. Map provides a more general way for storing elements and does not contain duplicate keys.
    

2. Java Mail
   JavaMail includes APIs and provides implementations that allows us to develop fully functional email client applications. We can write our own Outlook replacement as "Email client applications" invokes thoughts of Microsoft Outlook.
     

3. Java Multithreading
   Multithreading allows two parts of the same program to run concurrently. In Java, the Java Virtual Machine (JVM) allows an application to have multiple threads of execution running concurrently independently. 
     

4. Method Overloading in java
   Polymorphism is the capability of an object to respond uniformly to achieve specific behavior to the method calls of the same name but with different implementations.
     

5. Method Signatures
   Like other languages, java also permits to work with the methods declarations and method definitions. To define a method in java we need to abide by the certain java syntax known as the method signature to create a method within a class. 
   

6. MVC Design Pattern
   In today's software applications that need to deal with the  presentation of a huge amount of data to the end user, a software developer tries to make the application loosely coupled via separating the data (model) from the user interface (view) concerns.