What is the difference between abstract class and interface in Java

In Java, both abstract classes and interfaces are used to achieve abstraction, but they have some key differences. Here are the main distinctions between abstract classes and interfaces:

### Abstract Class:

1. **Definition:**
   - An abstract class is a class that cannot be instantiated on its own and is typically meant to be subclassed by other classes. It may contain abstract methods (methods without a body) and concrete methods (methods with a body).

2. **Keyword:**
   - Declared using the `abstract` keyword.

3. **Constructor:**
   - Can have constructors, and the constructor of an abstract class is called when an instance of a concrete subclass is created.

4. **Fields:**
   - Can have instance variables (fields), including both regular variables and constants.

5. **Access Modifiers:**
   - Can have various access modifiers for methods and fields (public, private, protected, etc.).

6. **Inheritance:**
   - Supports single inheritance, meaning a class can extend only one abstract class.

7. **Method Body:**
   - Abstract methods (methods without a body) must be implemented by concrete subclasses, and concrete methods can have an implementation.

### Interface:

1. **Definition:**
   - An interface is a collection of abstract methods (methods without a body) that a class implements. It defines a contract for classes that implement it to provide specific behavior.

2. **Keyword:**
   - Declared using the `interface` keyword.

3. **Constructor:**
   - Cannot have constructors. Interfaces cannot be instantiated, and they don't have instance variables.

4. **Fields:**
   - Can only have constants (public, static, and final fields).

5. **Access Modifiers:**
   - All methods declared in an interface are implicitly public and abstract. Fields are implicitly public, static, and final.

6. **Inheritance:**
   - Supports multiple inheritance, meaning a class can implement multiple interfaces.

7. **Method Body:**
   - Methods in interfaces are implicitly abstract and don't have a method body. Starting from Java 8, interfaces can have default methods (methods with a default implementation) and static methods.

### When to Use Each:

- Use an abstract class when you want to provide a common base class for multiple subclasses and when you need to share code and state among those subclasses.

- Use an interface when you want to define a contract that multiple unrelated classes can adhere to. Interfaces are often used to achieve multiple inheritance in Java.

In some cases, you may use a combination of both abstract classes and interfaces to achieve a specific design goal. The choice between an abstract class and an interface depends on the specific requirements and design considerations of your application.