Java basics

Java basics
OOPS concenpts

• Class

        is a blue print from which the objects are implemented

• Objects

        is a instance of class

• Inheritance

        By which one object recieves all the properties and methods 
           One class can only extend only one class
          
          Overriding
            when a child class and super class have a function and if you want to redefine the same in the child class then it is called overriding

• Polymorphism
• Abstraction 

            Hiding all the internal details and showing only implementation details.
          Ex:Interface
                one has to implement all the methods of interface which implements it.
                one cant create object of interface.
                An abstract class which implements interface but not all the methods from the interface.

• Encapsulation 

Scope of Variables

   Local variable

     

1. Local variables are declared in methods, constructors, or blocks.
2. Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor or block.
3. Access modifiers cannot be used for local variables.
4. Local variables are visible only within the declared method, constructor or block.
5. Local variables are implemented at stack level internally.
6. There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.

   Instance variable     

1. Instance variables are declared in a class, but outside a method, constructor or any block.
2. When a space is allocated for an object in the heap, a slot for each instance variable value is created.
3. Instance variables are created when an object is created with the use of the keyword 'new' and destroyed when the object is destroyed.
4. Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object's state that must be present throughout the class.
5. Instance variables can be declared in class level before or after use.
6. Access modifiers can be given for instance variables.
7. The instance variables are visible for all methods, constructors and block in the class. Normally, it is recommended to make these variables private (access level). However visibility for subclasses can be given for these variables with the use of access modifiers.
8. Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor.
9. Instance variables can be accessed directly by calling the variable name inside the class. However within static methods and different class ( when instance variables are given accessibility) should be called using the fully qualified name . ObjectReference.VariableName.

   Class/Static variables

1. Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
2. There would only be one copy of each class variable per class, regardless of how many objects are created from it.
3. Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final and static. Constant variables never change from their initial value.
4. Static variables are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.
5. Static variables are created when the program starts and destroyed when the program stops.
6. Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
7. Default values are same as instance variables. For numbers, the default value is 0; for Booleans, it is false; and for object references, it is null. Values can be assigned during the declaration or within the constructor. Additionally values can be assigned in special static initializer blocks.
8. Static variables can be accessed by calling with the class name ClassName.VariableName.
9. When declaring class variables as public static final, then variables names (constants) are all in upper case. If the static variables are not public and final the naming syntax is the same as instance and local variables.     

Data types

•  Primitive data type

           - Defined by programming language

            Byte(number,1byte)

            Short(number,2 bytes)

            Int(number,4 bytes)

            Long(number,8 bytes)

                  long l=12456l

            Float(Float number,4 bytes)

            Double(Float number,8 bytes)
            Char(Character,2 bytes)
            Boolean (True or false,1 byte)

• Non primitive data type/Refernece data type

        -Class objects,array variable come under this categeory.

            -Default value of any refernce variable is null.

            -These varaiable are of specific type and cannot be changed.                 Test t=New Test(),

                 New Test() will create a new object,but there will be no reference to the object

                 t is a reference variable.

• String concatenation

            String s1="hello";

            String s2="world";

            String s3=s1+s2;

• Conditional operator

        -Ternary operator
            - &&,||,?:
            -variable x = (expression) ? value if true : value if false

• Method

         -  is a group of statements which are used to do particular operation.
         -AcessModifier ReturnType MethodName(Parameters)
         -Method overloading
                in this methods are having the same name,but parameter data types or parameter are different                       then it is called method overloading. 

•  Super keyword 

           Used to call super class contructor.

           Used to call super class methods when the subclass and super class have a method by same name

           Super.display()

• Static

          Are created in heap memory

          Static methods can only call static methods, similarly main can call only static functions.

          Value persists between objects.

          Static methods can be called using classname.methodname(),or methodname()

          Can be called before creating object i.e methods, variables.

• Constructor

            whenever you call the object of a class constructor is called.

            when you want initialize every object with parameters

            You can also do constructor overloading.

•  this keyword

          always refers to the current object being called.

•  Access modifier

     Private

         Only accessible in the class where it is defined.    

     Public

         Accessible to whole world

     Protected

         Default access modifier

         Acess in the same class ,within the classes in the same package and in the classes extending the class