1) What is a Constructor?
---> In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling constructor, memory for the object is allocated in the memory.
---> It is a special type of method which is used to initialize the object.
---> Every time an object is created using the new keyword, at least one constructor is called.
---> It calls a default constructor if there is no constructor available in the class. In such case, Java compiler provides a default constructor by default.

There are following rules for defining a constructor:

• Constructor name must be the same as its class name.
• A Constructor must have no explicit return type.
• A Java constructor cannot be abstract, static, final, and synchronized.[TBD]

2) Types of Constructors
There are three types of constructors in Java:

• Default Constructor
• Parameterized Constructor
• Copy Constructor

1. Default Constructor in Java

A constructor that has no parameters is known as default constructor. A default constructor is invisible. And if we write a constructor with no arguments, the compiler does not create a default constructor. It is taken out. It is being overloaded and called a parameterized constructor. The default constructor changed into the parameterized constructor. But Parameterized constructor can’t change the default constructor. The default constructor can be implicit or explicit.

Implicit Default Constructor: If no constructor is defined in a class, the Java compiler automatically provides a default constructor. This constructor doesn’t take any parameters and initializes the object with default values, such as 0 for numbers, null for objects.

Explicit Default Constructor: If we define a constructor that takes no parameters, it’s called an explicit default constructor. This constructor replaces the one the compiler would normally create automatically. Once you define any constructor (with or without parameters), the compiler no longer provides the default constructor for you.

2. Parameterized Constructor in Java
A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with our own values, then use a parameterized constructor.

3. Copy Constructor in Java [TBD]
Unlike other constructors copy constructor is passed with another object which copies the data available from the passed object to the newly created object.

3) What is Constructor Overloading?
The constructor overloading can be defined as the concept of having more than one constructor with different parameters so that every constructor can perform a different task.
Java supports Constructor Overloading in addition to overloading methods. In Java, overloaded constructor is called based on the parameters specified when a new is executed. Sometimes there is a need of initializing an object in different ways. This can be done using constructor overloading.

4) What is the use of "this" keyword?
The "this" keyword refers to the current object in a method or constructor.

The most common use of the "this" keyword is to eliminate the confusion between class attributes and parameters with the same name (because a class attribute is shadowed by a method or constructor parameter).

"this" can also be used to:

• Invoke current class constructor
• Invoke current class method
• Return the current class object
• Pass an argument in the method call
• Pass an argument in the constructor call

Reference
https://www.javatpoint.com/java-constructor
https://www.geeksforgeeks.org/constructors-in-java/
https://www.javatpoint.com/constructor-overloading-in-java
https://www.geeksforgeeks.org/constructor-overloading-java/
https://www.w3schools.com/java/ref_keyword_this.asp