Why a constructor cannot be final in Java?

Ans: If a method is marked as final it means we do not want any class to override it. As per Java Language Specification, a constructor cannot be overridden. So, there is no use in declaring a constructor as final.

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What is method overriding in java ?
If we have methods with same signature (same name, same arguments, same return type) in super class
and subclass then we say
subclass method is overridding method in superclass.
When to use overriding in java
If we want same method with different behaviour in superclass and subclass then we go for overriding.
When we call overridden method with subclass reference subclass method is called hiding the superclass
method.

What is functional Interface ?

An Interface that contains exactly one abstract method is known as functional interface. It can have any number of default, static methods but can contain only one abstract method. It can also declare methods of object class. Functional Interface is also known as Single Abstract Method Interfaces or SAM Interfaces. It is a new feature in Java, which helps to achieve functional programming approach.