What is a constructor? When do we declare a class as 'final' keyword in JAVA? Explain with example.

Constructor:

• The constructor is a special method that is used to initialize an object.
• It is automatically invoked when an object is created.
• The name of the constructor is always the same as the name of the class.
• The constructor can have parameters or it can be parameterless.
• If the constructor has parameters, then the values passed to the constructor are used to initialize the object's fields.
• If the constructor has no parameters, then the object's fields are initialized with default values.

When to declare a class as 'final' keyword in Java:

• The final keyword in Java is used to prevent a class from being subclassed and to prevent a method or variable from being overridden.
• A class is declared as final using the final keyword before the class name, as shown in the following example:

final class MyClass {
    // ...
}

• A method is declared as final using the final keyword before the method name, as shown in the following example:

class MyClass {
    final void myMethod() {
        // ...
    }
}

• A variable is declared as final using the final keyword before the variable name, as shown in the following example:

class MyClass {
    final int myVariable = 10;
}

• There are a few reasons why you might want to declare a class as final:

  • To prevent the class from being subclassed. This can be useful for classes that represent a fundamental concept in your program, such as a String or a Number. You don't want these classes to be subclassed because that would allow other developers to change the way they work, which could lead to unexpected results.
  • To improve performance. A final class can be optimized by the compiler because the compiler knows that the class will never be subclassed. This can make the class run faster.
  • To make the class more secure. A final class is less likely to be attacked by malicious code because the attacker cannot subclass the class and override its methods. This makes the class more secure.

Here are some examples of when you might want to declare a class as final:

• Immutable classes: Classes that represent immutable data, such as strings, numbers, and dates, should be declared as final to prevent them from being modified.

• Utility classes: Utility classes that provide a collection of static methods, such as math functions or string manipulation functions, should be declared as final to prevent them from being instantiated.

• Security classes: Security classes that perform sensitive operations, such as encryption or authentication, should be declared as final to prevent them from being subclassed and overridden by malicious code.