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Abstract Classes

Partial blueprint - some methods, some holes

Abstract Classes

Abstract classes define a contract with some implementation. Subclasses fill in the rest.

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Deep dive

The Big Picture

Abstract classes sit between interfaces and concrete classes. They can mix implemented and abstract methods, but can't be instantiated directly.

Use when subclasses share common code
Template Method pattern: skeleton with overridable steps
Examples: AbstractList, HttpServlet, InputStream

How It Works

Compiler blocks new on abstract classes. Subclasses must implement all abstract methods or be abstract themselves.

Abstract methods have no body (just signature)
Concrete methods work like regular class methods
Can have fields, constructors, static methods
Constructors run normally for inheritance chain

In the Java Landscape

Abstract classes bridge interfaces and full inheritance.

Java 8+ default methods blur the line with interfaces
Single inheritance limit applies (one superclass)
Often combined with interfaces for flexibility
Common in collection and UI frameworks
Sealed abstract classes (Java 17+) restrict hierarchy

Pitfalls and Best Practices

Prefer interfaces + composition over abstract classes
Use abstract classes when you need state or constructors
Don't make everything protected - be selective
Document the subclass contract clearly
Keep small to avoid fragile base class problem

#java#oop#abstract