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Constructor Overloading in Java is the concept of having different definitions of a constructor based on different parameters, so that each constructor can perform a different task from another. Consider an example,

public class Bank{ 
public Bank(){

System.out.println(“This is a default constructor”);
}


public Bank (String name){

System.out.println(“Bank name is  :” + name);

} 


public static void main(String[] args){ 

Bank bank = new Bank (); 
Bank bank2 = new Bank(“Bank of India”);

}

}

Output - 

This is a default constructor

Bank name is Bank of India


this() is used to call the current class constructor in Java. 


public class Bank{ 

public Bank(){

this(“hello”);

System.out.println(“This is a default constructor”);

}


public Bank (String name){

System.out.println(“Bank name is  :” + name);

} 

public static void main(String[] args){ 

Bank bank = new Bank (); 

Bank bank2 = new Bank(“Bank of India”);

}

}


Output - 
Bank name is Bank of India
This is a default constructor

• When a variable is final, its value cannot be changed, but when a variable is static only one copy of that variable is shared among all the objects of the class.
• A final method cannot be overridden by its child class whereas a static method can be invoked without creating the object of the class containing that method, it may/may not get overridden by its child class. For example, main method in Java  is static because it has to be loaded before the  instantiation of a class takes place.
• To restrict the inheritance of a class, final keyword is used with the class so its child class cannot extend it whereas a class can be declared static only when it is a nested class in Java, the advantage is that objects of a static class can be created without any need of creating the instance of its outer class.

For example, 

class Demo{ 

class A{} 

static class B{} 

public static void main(String[] args){ 

A a = new A(); /* it will show compile time error, you need an instance of Demo to instantiate A;*

B b = new B(); /*it will compile successfully, no instance of Demo needs to be created to instantiate B.*/

  }

}

Constructor call is always the first statement. Either you can use this() inside the constructor or super(), this() is used to invoke the current class constructor of a class, while super() is used to invoke the parent class / superclass constructor of that class.

• • public - The keyword public indicates that main method is accessible globally as it is the entry point of the program execution so JVM can invoke it from outside the class.

• • static - The main method has to be invoked even before instantiating that class, so it is declared as static.

• • • void - In C++, the return type of main method is declared as int, which indicates the exit code status of a program which is the process of Operating System, return 0 means program executed successfully and non-zero exit code means there is an error in the program. These programs are the sub-process of the main process which after receiving their exit code cleans up the sub-process from the process table and frees the memory allocated to that process, that is why it is compulsory to pass exit code for the C++ programs. But in Java, the program runs as the main thread and there is no direct allocation of resources to a Java program. That is why main method in Java does not return anything.
   
  • • main - main is the starting point the JVM looks for when executing a Java program.
   
  • String[] args - It is an array of java.lang.String class. String[] args can also be written as String...args or String args[], which accepts command line arguments.

• Class (Method) Area

• Heap

• Stack

• Program Counter Register

• Native Method Stack

• Class (Method) Area - This memory stores data at the class level like method data, runtime constant pool, and program code of methods.

• Heap - Heap is used for dynamic memory allocation.

• Stack - In JVM, the Stack is used to store frames. Every time a method is invoked, a new frame is created and destroyed when the invocation completion.

• Program Counter Register - A JVM thread that performs a specific task of a method has a program counter associated with it. 

this keyword in Java is used in - 

• Referring to class variable when both, class variable and local variable have same name.
• Invoking the current class constructor.
• Passing it as an argument in method call.
• Passing it as an argument in the constructor call.
• Returning the current class object from the method.

Aggregation represents one-to-many relationship and is unidirectional in nature. Aggregation defines has-a relationship. Both the objects in aggregation can exist independently which means ending one object doesn’t affect another entity.  For example, Bank has-a Customer, if Bank gets destroyed Customer will still remain. Aggregation is a weak association. Code resusability is best achieved by Aggregration. 

Composition defines a part-of relationship, i.e., child cannot exist independent of the parent. For example, engine is a part-of the car. If the car gets destroyed, then engine also gets destroyed. Composition is a strong association. Multiple Inheritance which cannot be achieved by Inheritance, can be achieved by Composition. 

• Java Server Pages (JSP) use Java inside HTML provides ease to the developer to write, debug, and edit the code whereas Servlets use HTML in Java which makes the code complicated. 
• Servlets use println statements for printing an HTML doucment whereas JSP does not.
• View logic and Business logic is separated in JSP which is not possible in Servlet.

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