Ben Cheng

Primitive Date Type is the date type which just specifies data size and type of variable value and no additional methods.

In Java, there are 8 primitive Data Type.

  • int
  • double
  • short
  • long
  • float
  • boolean
  • char
  • byte

Wrapper Class

Wrapper Class is a class whose object wraps primitive data type.


Autoboxing is Automatic conversion of primitive data type to object of their corresponding wrapper classes. unboxing is the reverse process of autoboxing.




Java Virtual Machine is a virtual machine to enable to computer to run Java Program (compile to java bytecode)

Class Loader

  • Loading
  • Linking
  • Initializing


Class Loader read .class file and generate corresponding binary data and then save it in memory area.

Execution Engine

Execution Engine executes .class file (bytecode). It reads byte-code line by line, and uses data presented in various memory area and execute instructions.

  • Intercepter
  • Just-In Compiler
  • Garbage Collector



  • Abstraction
  • Encapsulation
  • Inheritance
  • Polymorphism


Abstraction is that only essential detail would be displayed to user.

Java achieved this by Abstract Class and Interface.


Encapsulation is wrapping data in singe unit.


Inheritance is a class acquiring properties another class.


Polymorphism allow you to perform actions in different ways.



Ben Cheng

Ben Cheng

A developer in Hong Kong. Learning and rethinking as a developer. Welcome to contact me and make friend with me. Cooperation is welcome.