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Inner classes and sub-classes are two different concepts in object-oriented programming, typically found in languages like Java, C++, and Python. They serve distinct purposes and have different characteristics:

1. Inner Class:

   - Inner classes are classes defined within another class.

   - They are often used for encapsulation and to logically group related classes together.

   - Inner classes have access to the members (fields and methods) of the outer class, including private members.

   - They are primarily used for improving code organization and readability.

   - Inner classes do not inherit from the outer class, and they are not a form of inheritance.

   - Inner classes can be instantiated within the context of an instance of the outer class, and they can access instance-specific data of the outer class.

   - Example (Java):

     ```java

     public class OuterClass {

         private int outerField;

         public class InnerClass {

             public void doSomething() {

                 // Inner class can access outerField

                 outerField = 10;

             }

         }

     }

     ```

2. Sub-Class (Inheritance):

   - A sub-class, also known as a derived class or child class, is a class that inherits attributes and behaviors from another class called the super-class or base class.

   - Sub-classes are created to extend or specialize the functionality of the super-class.

   - Inheritance is an "is-a" relationship, meaning a sub-class is a type of the super-class.

   - Sub-classes inherit the fields and methods (including constructors) of the super-class, and they can add new members or override existing ones.

   - Sub-classes can be used to create a hierarchy of classes, with each level of the hierarchy adding or modifying behavior as needed.

   - Example (Java):

     ```java

     class Animal {

         void eat() {

             System.out.println("Animal is eating");

         }

     }

     class Dog extends Animal {

         void bark() {

             System.out.println("Dog is barking");

         }

     }

     ```

In summary, inner classes are used for encapsulation and code organization within a single class, while sub-classes are used for inheritance and building class hierarchies. They serve different purposes and have different relationships with their enclosing or super-classes.