static keyword

• In order to share the same method or variable of any given class, we use the static keyword.
• We can use it for blocks, variables, methods, and nested classes.
• Static variable
  • A static variable is nothing but the class variable.
  • It is common for all objects of a class, or let’s say it is shared by all objects of a class, whereas the non-static variable is different for each object of a class.
  • E.g., static String name=” John”;
• Static block
  • To initialize static variables, we use a static block.
  • It gets executed only once after class is loaded or an object is created.
  • A class can contain multiple static blocks.
  • E.g. – static String name; static{ name = “Java”; }
• Static method
  • A static method is one that can be invoked without creating object of a class.
  • It can access static variables without using the object of the class.
  •  It can access static and non-static methods directly. A non-static method can access the static method, while a static method can access the non-static method only through the object of a class.
  • Eg – public static void main(String args[]) { }
• Static class
  • We can declare a class as static only if it is a nested class.
  • Non-static members of the Outer class are not accessible by the static class.
  • Eg – class Vehicle{ //Static class static class Car{ } }

this keyword

• “this” is basically just a reference variable referring to the current object.
• It eliminates the confusion between class attributes and parameters with the same name. 
• We can use it to –
  • refer class variable
  • invoke class method or constructor
  • passed as an argument in method or constructor
  • return current class object
• E.g. –
  • this.name = name;
  • this.setName();
  • setName(this);
  • Person p = new Person(this);  
  • return this;

super keyword

• The “super” keyword is a reference variable in java, used to refer parent class objects.
• We can use the super keyword in three ways –
  • with variable
    • It is used when a derived class and base class has the same data members. There is a possibility of ambiguity in such a case. 
    • This keyword helps us refer to the immediate parent class instance variable.
    • Eg – System.out.println(super.speed);
  • with method
    • When we want to call the parent class method, and parent & child class have same-named methods; then, to resolve ambiguity, we use the super keyword to invoke the immediate parent class method.
    • E.g., super.message();
  • with constructor
    • To invoke the immediate parent class constructor, we can use it with a constructor.
    • It can call both parametric as well as non-parametric constructors depending upon the situation.
    • Call to super() must be the first statement in a derived class constructor.
    • Eg – Student() { super(); System.out.println(“constructor”); }

final keyword

• To apply restrictions on user access, the final keyword is used.
• There are three ways to use the final keyword –
  • final variable 
    • Its value cannot be changed once initialized.
    • The compiler cannot assign a default value to a final variable.
    • Eg – final int count = 10;
  • final method 
    • A final method cannot​ be overridden, nor can it be hidden by a subclass.
    • Since the final method cannot be overridden so core functionalities must be declared.
    • Eg – final void manageCount() { //code }
  • final class 
    • It cannot be inherited.
    • We cannot declare a final class as abstract at the same time as they are antonyms.
    • Eg – final class Car { //methods and fields }