6. Explain Enum.

An Enum, short for Enumeration, is a special data type that enables for a variable to be a set of predefined constants. The variable must be equal to one of the values that have been predefined for it. Enumerations are used when we know all possible values at compile time, such as choices on a menu, rounding modes, command line flags, etc.

Here is a step-by-step explanation of Enum:

1. Definition: Enum is a keyword, a feature, used in many programming languages, including Java, C++, C#, etc. It represents a group of named constants.

2. Declaration: Enum is declared using the keyword 'enum'. The general form to declare an enumeration is:

enum EnumName {
    CONSTANT1,
    CONSTANT2,
    ...
    CONSTANTN
}

3. Usage: Once Enum is declared, we can use it to declare variables, similar to how we declare a variable of a class type. For example:

EnumName variable;

4. Assigning Values: We can assign a value to the variable from the set of constants. For example:

variable = EnumName.CONSTANT1;

5. Enum in Switch Statement: Enum can be used in a switch statement to check for corresponding values.

6. Enum Methods: Enum comes with some built-in methods like values(), which returns an array of enum type containing all the constants defined inside the enum.

7. Enum Fields: Enum can have fields, constructors, and methods.

8. Enum Constructors: Enum can have constructors, which are called separately for each constant at the time of enum class loading.

9. Enum in Collections: Enum can be used in collections like EnumSet, EnumMap, etc.

10. Enum with interfaces: Enum can implement many interfaces.

Remember, Enum improves type safety and can be easily used in switch statements. It provides a fixed set of constants and reduces bugs in your code.