1 CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) Lecture Set #16: Inheritance Inheritance Conceptual Is-A relationship compared to contains-a Terminology Overloading compared to Overriding super isInstanceOf and getClass() CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 1 Inheritance A crucial feature of object-oriented programming languages One class (derived class, subclass, child class) is constructed … … by including (extending, inheriting) information … … from another (base class, superclass, parent class) … … and adding new information / redefining existing Example Base class: Clock setTime getTime tick Derived class: Alarm Clock Same methods as Clock plus a few additional ones: setAlarm, ring CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 2 Can We Avoid Code Copying and therefore redundancy? Alarm Clock “IS-A” Clock Operations on Clock (e.g. setTime) should be inherited by Alarm Clock Alarm Clock should only have to add information specific to alarm clocks setAlarm ring Inheritance provides just this capability2 CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 3 Inheritance One class (derived class, subclass, child class) is constructed by including (extending, inheriting) information from another (base class, superclass, parent class) then also adding new information and/or redefining existing information To derive a class D from a base class B, use: public class D extends B { … } Example: Base class: public class Shape Derived class: public class Circle extends Shape Derived class inherits all instance variables, methods from base class. It can also define new instance variables, methods Polymorphism: object in derived class can be used anywhere base class is expected (an alarmClock “is a” Clock!) CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 4 Inheritance vs. Composition Inheritance: a way to build new classes out of old ones Objects in subclass inherit data, methods from superclass Object in a subclass “is-a”(n) object in superclass Association: another way to build new classes out of old Class definitions may include instance variables which are objects of other class types Object in a new class “has-a”(n) object in the original class Composition: the strongest form of association – when the lifetime of the enclosed object is completely dependant on the lifetime of the object that contains it CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 5 Implements vs. Extends When Defining a Class implements: Keyword followed by the name of an interface Interfaces only have method prototypes Can’t create on object of an interface type Can have a reference of the interface type point to an object of the class that implements it extends: Keyword followed by the name of a class That class contains full method definitions Can create objects of that base class type Can have reference of the base class type point to an object of the class that extends it3 CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 6 Inheritance More Generally Classes / objects have a natural “is-a” hierarchy Object-oriented programming provides mechanisms for exploiting this for Code re-use Common operations implemented in super classes Polymorphism Objects in subclasses can be used wherever superclass objects are needed Shape Circle Rectangle Triangle Right-Triangle Equilateral-Triangle Animal Insect Reptile Mammal Cat Dog Primate Human Ape Homer CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 7 Example: People at University Base class: person Derived classes: student, faculty, administrator Derived from those: undergrad, grad, instructor, professor,… Person Student Faculty Administrator Undergrad GradStudent Instructor Professor … … Person Student Faculty CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 8 University Person Example class: Person instance variables: String name String idNum methods: Person( … ) [various] String getName( ) String getIdNum( ) void setName( String ) void setIdNum( String ) String toString( ) boolean equals( Person ) class: Student instance variables: int admitYear double gpa methods: Student( … ) [various] int getAdmitYear( ) double getGpa( ) void setAdmitYear( int ) void setGpa( double ) String toString( ) boolean equals( Student ) extends Person class: Faculty instance variables: int hireYear methods: Faculty( … ) [various] int hireYear( ) void setHireYear( int ) String toString( ) boolean equals( Faculty ) extends Person4 CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 9 Method Overriding A derived class can define new instance variables and methods (e.g. hireYear and getHireYear( ) ) A derived class can also redefine (override) existing methods public class Person { … public String toString( ) { … } } public class Student extends Person { … public String toString( ) { … } } Student bob = new Student("Bob Goodstudent","123-45-6789",2004,4.0 ); System.out.println( "Bob's info: " + bob.toString( ) ); Overrides base-class definition of this method Since bob is Student, Student toString used CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 10 Overriding vs. Overloading Overriding: a derived class defines a method with same name, parameters as base class Overloading: two or more methods have the same name, but different parameters Example public class Person { public void setName( String n ) { name = n; } … } public class Faculty extends Person { public void setName( String n ) { super.setName( “The Evil Professor ” + n ); } public void setName( String first, String last ) { super.setName( first + “ ” + last ); } } Base class setName( ) Overriding Overloading CMSC 131 Fall 2009 Jan Plane (adapted from Bonnie Dorr) 11 Early vs. Late Binding Consider: Faculty carol = new Faculty("Carol Tuffteacher","999-99-9999", 1995); Person p = carol; System.out.println( p.toString() ); Which version of toString – Person or Faculty – is called? Early (static) binding p is declared to be of type Person Therefore, the Person version of toString is used Late (dynamic) binding The object to
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