Introduction to Microservice Architecture Using SpringBoot - Part 1

Java Classes

Java is an object-oriented programming language.

Everything in Java is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.

A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can't be physical.

Declaring a class

You can declare a class by writing the name of the next to the class keyword, followed by the flower braces. Within these, you need to define the body (contents) of the class i.e. fields and methods.

To make the class accessible to all (classes) you need to make it public.

Constructor in Java

Whenever we use new keyword to create an instance of a class, the constructor is invoked and the object of the class is returned. Since constructor can only return the object to class, it’s implicitly done by java runtime and we are not supposed to add a return type to it.

public Employee() {

System.out.println("Employee Constructor");

}

public Employee Employee() {

System.out.println("Employee Method");

return new Employee();

}

Argument Passing

Pass-by-Value

When a parameter is pass-by-value, the caller and the callee method operate on two different variables which are copies of each other. Any changes to one variable don't modify the other.

It means that while calling a method, parameters passed to the callee method will be clones of original parameters. Any modification done in callee method will have no effect on the original parameters in caller method.

Pass-by-Reference

When a parameter is pass-by-reference, the caller and the callee operate on the same object.

It means that when a variable is pass-by-reference, the unique identifier of the object is sent to the method. 

What is the use of static keyword with example?

The static variable can be used to refer to the common property of all objects (which is not unique for each object), for example, the company name of employees, college name of students, etc. The static variable gets memory only once in the class area at the time of class loading.

Java Inner Classes (Nested Classes)

Java inner class or nested class is a class that is declared inside the class or interface.

We use inner classes to logically group classes and interfaces in one place to be more readable and maintainable.

Method overloading

Method Overloading allows different methods to have the same name, but different signatures where the signature can differ by the number of input parameters or type of input parameters, or a mixture of both. 

Different Ways of Method Overloading in Java

1. Changing the Number of Parameters.
2. Changing Data Types of the Arguments.
3. Changing the Order of the Parameters of Methods

1. Changing the Number of Parameters

Method overloading can be achieved by changing the number of parameters while passing to different methods.

2. Changing Data Types of the Arguments

In many cases, methods can be considered Overloaded if they have the same name but have different parameter types, methods are considered to be overloaded.

3. Changing the Order of the Parameters of Methods

Method overloading can also be implemented by rearranging the parameters of two or more overloaded methods

Usage of Java super Keyword

super can be used to refer immediate parent class instance variable.

super can be used to invoke immediate parent class method.

super() can be used to invoke immediate parent class constructor.

Method Overriding in Java

If subclass (child class) has the same method as declared in the parent class, it is known as method overriding in Java.

In other words, If a subclass provides the specific implementation of the method that has been declared by one of its parent class, it is known as method overriding.

Rules for Java Method Overriding

1. The method must have the same name as in the parent class
2. The method must have the same parameter as in the parent class.
3. There must be an IS-A relationship (inheritance).

Abstract class in Java

A class which is declared as abstract is known as an abstract class. It can have abstract and non-abstract methods. It needs to be extended and its method implemented. It cannot be instantiated.

Points to Remember

1. An abstract class must be declared with an abstract keyword.
2. It can have abstract and non-abstract methods.
3. It cannot be instantiated.
4. Dynamic Method Dispatch or Runtime Polymorphism in Java

Dynamic method dispatch

Dynamic method dispatch is the mechanism in which a call to an overridden method is resolved at run time instead of compile time.

Advantages of dynamic method dispatch

1. It allows Java to support overriding of methods, which are important for run-time polymorphism.
2. It allows a class to define methods that will be shared by all its derived classes, while also allowing these sub-classes to define their specific implementation of a few or all of those methods.

What is the use of final keyword in java answer?

In Java, we can use final keyword with variables, methods, and classes. When the final keyword is used with a variable of primitive data types such as int, float, etc), the value of the variable cannot be changed.