Keywords

Reserved words with special meaning which is already defined in the Java language.

Keywords can't be used as name for variables, methods or classes. Keywords usually will be in lower case

E.g. public, return, final, static

final

final keyword serves different purpose depends upon the place it's been used

In methods: final methods cannot be overridden

In variables (Fields): final variables will be treated as constants

In class: final class can't be inherited further

E.g. String, all wrapper classes (Integer, Float) are final classes in Java

super

The super keyword in java is generally used to refer immediate parent class object

super is used to refer immediate parent class variable & methods

super() is used to invoke immediate parent class constructor

Whenever we call the overridden method, we can only call the sub class implementation of the method. If we want to call super class implementation, use super keyword

package

A package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer

Default imported package in all Java class is java.lang.*

To use a package, we use the keyword import.

static

static keyword serves different functionality depends on the place it's been used

In methods: When a method is declared as static, it can be accesses using the class name even without creating an object for the class. These methods are called as class methods.

In variables (Fields): When number of objects are created from the same class, they each have their own distinct copies all variables. If we have to create variables that are common to all objects, we should declare them as static. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object

In class: Only nested class (A class declared inside another class) can be declared as static in Java. Normal classes can't be declared as static

Just to recall, to access a method lies inside a Class, we need to create Object for that class as the first step.

But What if, We don't want to create an object for a class ? Is there a way to access the methods without Object ?

Yes, we can, by declaring the method as static

static methods can be accessed using the Class Name itself. We don't need to create Object

public class VoidMethodSample {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        // Running a School

        System.out.println("Please Enter Student Name");
        String name = scanner.next();

        System.out.println("Please Enter Student age");
        int age = scanner.nextInt();

        System.out.println("Please Enter your ZipCode");
        int pinCode = scanner.nextInt();

        VoidMethodSample.validateStudentInput(name, age, pinCode);
    }

    static void validateStudentInput(String name, int age, int pinCode) {
        if (name.length() <= 10 && age > 7 && pinCode >= 100000 && pinCode <=999999) {
            System.out.println("Thank you for your input");
        } else {
            System.out.println("Something's wrong");
        }
    }
}

Focus on the below lines taken from above program

VoidMethodSample.validateStudentInput(name, age, pinCode);

Since the validateStudentInput method is static, we used the class name VoidMethodSample to access the validateStudentInput method

• REMEMBER: Methods that are declared as static can be accessed with ClassName as well by creating Object

class StudentInfo {
  static void validateName(String name) {
  }
}

// Accessing using ClassName
StudentInfo.validateName("Vicky");

// Accessing using Object
StudentInfo obj = new StudentInfo();
obj.validateName("Vicky");

instanceof

instanceof operator is used to check whether the object is an instance of the specified type. It will return either true or false.

class Student{

}

class Hostel_Student extends Student{

}

class Library_Member extends Student{

}

Student s1 = new Student();
Student s2 = new Hostel_Student();
Student s3 = new Library_Member();

if(s1 instanceof Student) // Result is true
if(s2 instance of Hostel_Student) //Result is true