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Core Java Concepts

Java Applications 

There are mainly four types of Java applications.
  • Standalone
  • Web 
  • Enterprise 
  • Mobile

Java Features 

There are several features in Java, but in here listed some important ones
  • Platform independent
  • Object oriented
  • Multi threaded
  • Distributed
  • Robust (automatic garbage collection, exception handling, type checking)

Java Security 

Java has 3 built-in security components;
  • Classloader: Separate class files packages from imported packages.
  • BytecodeVerifier: check the illegal access to objects
  • Security Manager: check what resources can access the object.

C++ vs Java



Methods

  • Static - No need to create an object to invoke the static method.
  • Constructor - use to initialize objects
    • Constructor name must same as the class name
    • Constructor has no return type
    • There are two types of constructors
      • Default - no parameter
      • Parameterized
    • If there is no constructor defined in the class, compiler automatically create default constructor
    • Constructor return value: current class instance

Compiling

At the compiler stage, Java code is converted to a bytecode.
JVM: Java Virtual Machine, used to execute the Java bytecode.Platform dependent.

Variables

There are three types of variables in Java
  • Local variable: variables define inside the method
  • Instance variable: variables define outside the method but inside the class
  • Static variable: variable declared as static

Data types

There are two kinds of data types in Java
  • Primitive: Byte, char, int, short, long, boolean, double
  • Non-primitive: String, array

Loops

  • While loop: If number of iterations are not fixed use this one
  • DoWhile loop: the loop is at least execute once.

Static

  • Variable: 
    • Refer common property for all objects (save memory)
  • Method
    • Belong to the class rather than object
    • Can invoke without creating object
    • this and super keyword can't use inside the static method

Types of inheritance (is-a)







In Java, multiple and hybrid interfaces are supported by interface only.If those are used in class level Java throws compile time error.

Aggregation (has-a)

Aggregation is based on entity reference





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