Abstract Classes and Interfaces Flashcards

Abstract Classes

• An abstract class can be declared without any abstract methods; it can still provide complete method implementations.
• A compilation error occurs if an abstract method contains a body; abstract methods must be declared without implementation.

Instantiation and Implementation

• Only non-abstract classes can be instantiated; thus, Class B (subclass of abstract Class A) is instantiable.
• When extending an abstract class, the subclass must implement all inherited abstract methods or also be declared abstract.

Between Abstract Classes and Interfaces

• Abstract classes can contain constructors; they are not limited in the number of constructors they can define.
• Abstract methods in an abstract class cannot be private; they must be accessible for implementation by subclasses.

Interfaces

• Protected methods cannot be declared in interfaces; only public methods are permitted.
• An interface generates a separate .class file after the compilation of each interface in Java.

Method Implementations

• Interface methods must always be declared public; reducing visibility during implementation results in a compilation error.

Field Modifiers

• Interface fields are inherently public, static, and final, meaning their values cannot be modified after initialization.
• Declaring a private field in an interface is prohibited due to legal modifier restrictions.

Example Code Behavior

• In the provided abstract class demonstration, calling methods results in the output: "FIRST", "THIRD", "SECOND" based on method chaining.
• In another example, constructing an instance of Class Y from abstract Class X outputs "ONE", "TWO", and executing the abstract method outputs "THREE".

General Knowledge

• A class can implement multiple interfaces, promoting polymorphism and code reusability.
• Ensuring method signatures maintain public visibility when implementing interface methods is crucial to avoid compilation errors.