restriction on using enum

restriction on using enum

Let’s provide explanations followed by examples for each of the restrictions on using enums in Java:

restriction on using enum

1. No Inheritance:

Enumerations cannot extend any other class in Java. While enums implicitly extend the `Enum` class, they cannot explicitly extend any other class.

Example
  ```java
  // This code will result in a compilation error
  class MyEnum extends Enum<MyEnum> { // Enums cannot extend other classes explicitly
      // Enum constants and methods...
  }
  ```

2. No Instantiation:

Enumerations cannot be instantiated using the `new` keyword. Enums are implicitly instantiated as constants within the enum type itself.

Example
  ```java
  // This code will result in a compilation error
  MyEnum instance = new MyEnum(); // Enums cannot be instantiated using the 'new' keyword
  ```

3. No Abstract Methods:

Enumerations cannot declare abstract methods. Each constant in the enum implicitly becomes an instance of the enum type and must provide concrete implementations for any abstract methods declared in the enum.

Example
  ```java
  // This code will result in a compilation error
  enum MyEnum {
      // Enum constants...
      
      // This abstract method declaration is not allowed
      abstract void myMethod(); // Enums cannot declare abstract methods
  }
  ```

4. Fixed Set of Instances:

The set of instances of an enum is fixed at compile time and cannot be modified at runtime. Enum constants are defined at compile time and cannot be dynamically added, removed, or modified during program execution.

Example
```java
  enum Day {
      MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
  }
  
  // This code is invalid because enum constants cannot be modified at runtime
  Day.MONDAY = Day.SUNDAY; // Attempting to modify enum constants will result in a compilation error
```

5. Compile-Time Constants:

Enum constants are compile-time constants and cannot be changed or reassigned at runtime. Once defined, enum constants retain their values for the duration of the program’s execution.

Example
  ```java
  enum Color {
      RED, GREEN, BLUE
  }
  
  // This code is invalid because enum constants are compile-time constants
  Color color = Color.RED;
  color = Color.GREEN; // This reassignment is not allowed as enum constants are immutable
  ```

6. Immutability:

Enum constants are immutable, meaning their values cannot be modified after they are defined. Enums are designed to represent fixed sets of constants with immutable values.

Example
  ```java
  enum State {
      ACTIVE, INACTIVE
  }
  
  // This code is invalid because enum constants are immutable
  State state = State.ACTIVE;
  state = State.INACTIVE; // This reassignment is not allowed as enum constants are immutable
  ```

7. Type Safety:

Enums provide type safety by ensuring that only valid enum constants can be assigned to variables of the enum type. Attempts to assign invalid values to enum variables result in compilation errors.

Example
  ```java
  enum Season {
      SPRING, SUMMER, AUTUMN, WINTER
  }
  
  // This code will result in a compilation error
  Season season = Season.FALL; // Attempting to assign an invalid enum constant will result in a compilation error
  ```

These explanations with examples demonstrate the various restrictions on using enums in Java, ensuring the integrity and consistency of enum types in Java programs.