Engr 691 Special Topics in Engineering Science Software Architecture Spring Semester 2004 Lecture Notes

Understanding Inheritance (Budd's UOOPJ, Ch. 8) This is a set of slides to accompany chapter 8 of Timothy Budd's textbook Understanding Object-Oriented Programming with Java, Updated Edition (Addison-Wesley, 2000) Created: 14 August 2004

Motivation for Inheritance Use inheritance to create new software structures from existing software units to:  improve productivity  enhance quality 1

Generality and Specialization in Software Development Conflict:  Specific projects usually require very specialized software  Reusability usually requires very general software Resolution?  Inheritance allows general software to be specialized for a project 2

Abstract Idea of Inheritance 3 Material Object Non-Living Thing Rock Air Living Thing Plant Animal Reptile Mammal Human Being DentistRoy Shopkeeper Flo WriterJohn Cat Dog Platypus

Practical Meaning of Inheritance In programming languages, inheritance means  Data and behavior of parent class are part of child  Child class may include data and behavior not in parent With respect to the parent class, a child class is, in some sense  An extension – larger set of properties  A contraction – more specialized (restricted) objects 4

Idealized Image of Inheritance Consider  Subclass instances must possess all data areas of the parent  Subclass instances must implement all functionality of the parent Thus  Subclass instance should be indistinguishable from parent class instance—child can be substituted for parent 5

Principle of Substitutability If C is a subclass of P, instances of C can be substituted for instances of P in any situation with no observable effect. 6

Subclass and Subtype Subtype  Class that satisfies principle of substitutability Subclass  Something constructed using inheritance, whether or not it satisfies the principle of substitutability. The two concepts are independent  Not all subclasses are subtypes  Sometimes subtypes are constructed without being subclasses 7

Forms of Inheritance Specialization  Child class is a special case (subtype) of parent Specification  Parent class defines behavior implemented in the child, but not parent Construction  Parent class used only for its behavior -- child class is not subtype -- no is-a relationship to parent Generalization  Child class modifies or overrides some methods of parent, extends the behavior to more general kind of object Extension  Child class adds new functionality to parent, but does not change any inherited behavior Limitation  Child class limits some of the behavior of parent Variance  Child and parent class are variants of each other -- inheritance to allow code sharing -- arbitrary relationship Combination  Child class inherits features from more than one parent -- multiple inheritance 8

Forms of Inheritance Specialization Child class is a special case (subtype) of parent  Most common form of inheritance  Example: Professor is specialized form of Employee  Child may override behavior of parent to specialize  Child satisfies specification of parent in all relevant aspects  Preserves substitutability 9

Forms of Inheritance Specification behavior Parent class defines behavior implemented in the child, but not parent  Second next most common form of inheritance  Example: class StackInArray gives implementations for method signatures defined in abstract class Stack Java: StackInArray extends Stack  Example: class ArrayRankedSeq gives implementations for method signatures defined in interface RankedSequence Java: ArrayRankedSeq implements RankedSequence  Subclasses are realizations of incomplete abstract specification  Defines common interface for group of related classes  Preserves substitutability 10

Forms of Inheritance Construction Parent class used only for its behavior — child class is not subtype — no is-a relationship to parent  Sometimes used for convenience, but discouraged  Example: extending List class to develop Set, without "hiding" unneeded methods  Example: extending a byte-based I/O stream to a stream for handling other objects  Often violates substitutability  More common in dynamically typed languages (e.g., Smalltalk) than in statically typed (e.g., Java)  Can sometimes use aggregation instead 11

Forms of Inheritance Generalization Child class modifies or overrides some methods of parent, extends the behavior to more general kind of object  Sometimes used for convenience (or necessity), but discouraged  Example: graphics Window generalized to ColorWindow (with background color)  Opposite of specialization—violates substitutability  Used when must build from fixed, difficult-to-modify set of classes  Where possible, invert class hierarchy or use aggregation 12

Forms of Inheritance Extension Child class adds new functionality to parent, but does not change any inherited behavior  Useful technique to give new behaviors to existing base class that cannot be modified  Example: StringSet extends Set, adding string-related methods (e.g, prefix search)  Preserves substitutability 13

Forms of Inheritance Limitation Child class limits some of the behavior of parent  Sometimes used for convenience, but strongly discouraged  Example: extends, replacing unneeded methods to give error messages  Example: Stack extends DoubleEndedQueue, replacing unneeded methods to give error messages  Violates substitutability  Used when must build from fixed, difficult-to-modify set of classes  Avoid when possible, perhaps use aggregation 14

Forms of Inheritance Variance Child and parent class are variants of each other—inheritance to allow code sharing— arbitrary relationship  Sometimes used for convenience, but discouraged  Example: graphics class extending to share similar control code  Example: graphics Tablet class extending Mouse to share similar control code  Violates substitutability  Better to define more general parent class like  Better to define more general parent class like PointingDevice 15

Forms of Inheritance Combination Child class inherits features from more than one parent—multiple inheritance  Example: might inherit from both and  Example: GraduateInstructor might inherit from both GraduateStudent and Faculty  Often difficult to understand and to implement language  Often use to "mix-in" specification of another role or protocol  Java has single inheritance via subclassing (extends), but multiple inheritance for specification via interface implementations (implements) 16

Inheritance and Assertions Suppose C is a subtype of P   P and C have interface invariants I and IC, respectively   meth() is a public method of P with precondition Q and postcondition R   meth() in C has precondition QC and postcondition RC Subtype C should not violate I, Q, and R   IC implies I – may strengthen invariant – extend interface and data   Q implies QC – may weaken precondition – expand valid inputs   RC implies R -- may strengthen postcondition – restrict valid outputs Abstract preconditions can enable controlled "strengthening" of precondition   Consider BoundedStack inheriting from an unbounded Stack class   Give method push() a "not full" precondition – always true in Stack   Refine "not full" in subclass BoundedStack to be true or false 17

Inheritance and Assertions Interface invariants meth() Pre- condition Post- condition P (Parent) IQR C (Child) ICQCRC IC=>IQ=>QCRC=>R strengthenweakenstrengthen 18

Trees versus Forests Two common views of class hierarchies Tree  All classes are part of single class hierarchy  Advantage: root's functionality inherited by all objects – all have basic functionality  Disadvantage: tight coupling of classes, large libraries for an application  Languages: Java's classes, Smalltalk, Objective C, Delphi Object Pascal Forest  Classes only placed in hierarchies if they have a relationship – many small hierarchies.  Advantage: smaller libraries of classes for application, less coupling possible  Disadvantage: no shared functionality among all objects  Languages: Java's interfaces, C++, Apple Object Pascal 19

Tree versus Forest A B C D EFG Tree 20

Trees versus Forests Forest A B C D EFG X Y Z 21

Inheritance in Java Tree-structured class hierarchy Forest-structured interface hierarchy Modifiers for classes/interfaces Visibility modifiers for class/interface features 22

Inheritance in Java Tree-structured Class Hierarchy Root class is Object Other classes extend exactly one other class   default is Object Declaration uses keyword extends after class name 23 Object Material_Object Non_living_ThingLiving_Thing ……

Inheritance in Java Forest-structured Interface Hierarchy interface defines an interface specification implements after class name to promise implementation of interface – inheritance for specification An interface extends zero or more other interfaces A class implements zero or more interfaces public interface Queue { // signatures of public methods // Queues must provide } public class QueueAsLinkedList implements Queue { // includes implementations of // the Queue methods } 24

Inheritance in Java Modifiers for Classes/Interfaces abstract classes cannot be instantiated – interfaces are abstract by default final classes cannot be extended public classes/interfaces are visible everywhere, otherwise only visible within current package 25

Inheritance in Java Visibility Modifiers for Class/Interface Features public features accessible from anywhere in program private features accessible from inside class only Default-access (i.e., "friendly") features accessible from inside the current Java package protected features accessible in package or inside any child class 26

Inheritance in Java public abstract class Stack { // extends Object by default // data definitions plus signatures and // data definitions plus signatures and // possibly implementations of methods // possibly implementations of methods} public class StackInArray extends Stack { // extended features plus overridden implementations } public interface Queue { // signatures of public methods Queues must provide } public class QueueAsLinkedList implements Queue { // includes implementations of the Queue methods } 27

Facilities of Root Class Object Minimum functionality for all objects include equals(Object obj) is obj the same as receiver? toString() converts the object to a string value hashCode() return a default hashcode for the object getClass() return an identifier for the class of the object First three above are often overridden in classes. 28

Benefits of Inheritance Software reusability (among projects) Code sharing (within a project) Increased reliability (resulting from reuse and sharing of well-tested code) Consistency of interface (among related objects) Rapid prototyping (quickly assemble from pre- existing components) Polymorphism and frameworks (high-level reusable components) Information hiding 29

Costs of Inheritance Execution speed Program size Message-passing overhead Program complexity 30

Acknowledgement 31 This work was supported by a grant from Acxiom Corporation titled “The Acxiom Laboratory for Software Architecture and Component Engineering (ALSACE).” This work was supported by a grant from Acxiom Corporation titled “The Acxiom Laboratory for Software Architecture and Component Engineering (ALSACE).”