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