Interfaces & Abstract Classes

What does an abstraction focus on?

• What behavior is available

• What can a client rely on

• What must remain stable

What does abstraction hide?

• What data structure is used

• Which algorithm is used

• Which service does the work

What can an abstract class contain?

• Instance fields

• Constructors

• Concrete methods

• Abstract methods

• Private/protected helpers

What can an abstract class cannot contain/do?

• Cannot be instantiated directly with new (cannot make objects)

• A concrete subclass must finish the missing pieces

When should an abstract class be used?

Used when related implementation truly share code, state, invariants, or an algorithm skeleton

Extends vs implements

• A class extends at most one class

• A class can implement many interfaces

• An interface can extend another interface

• Every runtime object is still a concrete class

What is the common pattern for interfaces and abstract classes?

• Interfaces for the public role

• Abstract class for shared code

• Concrete subclasses for the representation-specific details

Implements make a _____

promise

Extends inherits ______

From a parent type

Primary methods

core capability that depends on implementation details. Often abstract in an interface

Secondary methods

Behavior built from existing public operations. Often a default method

Declared type

Type of a variable from its declaration. Compile time: Which methods can be called

Dynamic type

Class of the reference object. Runtime: Which instance-method implementation runs?

What does the declared type determine?

Determines which methods can be called

What does the dynamic type determine?

Determines how the method runs