Polymorphism Outline 8 Explanation of polymorphism 8 Use

Polymorphism

Outline 8 Explanation of polymorphism 8 Use polymorphism to make generic data structures CS 221 - Computer Science II 2

Data Type 8 All class variables have: – a declared type: • Also called the static type. – a dynamic type. • What is the actual type of the object pointed to at run time or when a particular statement is executed? CS 221 - Computer Science II 3

Polymorphism 8 Another key feature of OOP. – Literally “having many forms. ” 8 Declared types in Java are polymorphic. 8 Declared types can refer to: – the declared/static type of the variable OR – any descendants of the declared type. Rectangle r = new Rectangle(); r = new Square(); //sub-class legal! CS 221 - Computer Science II 4

Dynamic Types 8 Declared types refer to any valid class type, including abstract classes and interfaces. 8 Dynamic types can only refer to instantiable classes (i. e. any class that’s not abstract or an interface). Shape s; // abstract class s = new Shape(); //illegal! CS 221 - Computer Science II 5

Question 8 Consider the following class declarations: public class Board. Space Property extends Board. Space Street extends Property Railroad extends Property Which of the following statements would cause a syntax error, assuming all classes have a default constructor? A. Object obj = new Railroad(); B. Street s = new Board. Space(); C. Board. Space b = new Street(); D. Railroad r = new Street(); E. More than one of these CS 221 - Computer Science II 6

Question 8 Consider the following class declarations: public class Board. Space Property extends Board. Space Street extends Property Railroad extends Property Which of the following statements would cause a syntax error, assuming all classes have a default constructor? A. Object obj = new Railroad(); B. Street s = new Board. Space(); C. Board. Space b = new Street(); D. Railroad r = new Street(); E. More than one of these. CS 221 - Computer Science II 7

Legality of Method Calls 8 Method calls are syntactically legal if the method is in the declared type or any ancestor of the declared type 8 The actual method that is executed at runtime is based on the dynamic type. – dynamic dispatch CS 221 - Computer Science II 8

What’s the Output? Shape s = new Rectangle(1, 2); System. out. println(s. to. String()); s = new Rectangle(2, 3, 4, 5); System. out. println(s. to. String()); s = new Circle(4, 5, 10); System. out. println(s. to. String()); s = new Shape(); System. out. println(s. to. String()); CS 221 - Computer Science II 9

What’s the Output? Shape s = new Rectangle(1, 2); System. out. println(s. to. String()); x: 0 y: 0 width: 1 height: 2 s = new Rectangle(2, 3, 4, 5); System. out. println(s. to. String()); x: 2 y: 3 width: 4 height: 5 s = new Circle(4, 5, 10); System. out. println(s. to. String()); x: 4 y: 5 radius: 10 s = new Shape(); System. out. println(s. to. String()); Error: won’t compile CS 221 - Computer Science II 10

Method Look. Up 8 To determine if a method is legal, the compiler looks in the class based on the declared type. – If it finds it, great. – If not, goes to the super class and looks there. – Continues until the method is found, or the Object class is reached, then throws a compilation error. 8 To determine which method is actually executed at runtime: – – – Starts with the actual run-time class of the object calling the method. Searches the class for that method. If found, executes it. Otherwise, goes to the super class and keeps looking. Repeats until a version is found. 8 Is it possible the runtime system won’t find a method? CS 221 - Computer Science II 11

Question 2 8 What is output by the code to the right? A. !!live B. !eggegg C. !egglive D. !!! E. eggegglive public class Animal { public String talk(){ return "!"; } } public class Mammal extends Animal { public String talk(){ return "live"; } } public class Platypus extends Mammal { public String talk(){ return "egg"; } } Animal a 1 = new Animal(); Animal a 2 = new Platypus(); Mammal m 1 = new Platypus(); System. out. print(a 1. talk()); System. out. print(a 2. talk()); System. out. print(m 1. talk()); CS 221 - Computer Science II 12

Question 2 8 What is output by the code to the right? A. !!live B. !eggegg C. !egglive D. !!! E. eggegglive public class Animal { public String talk(){ return "!"; } } public class Mammal extends Animal { public String talk(){ return "live"; } } public class Platypus extends Mammal { public String talk(){ return "egg"; } } Animal a 1 = new Animal(); Animal a 2 = new Platypus(); Mammal m 1 = new Platypus(); System. out. print( a 1. talk() ); System. out. print( a 2. talk() ); System. out. print( m 1. talk() ); CS 221 - Computer Science II 13

Why Bother? 8 Polymorphism allows code reuse in another way. 8 Inheritance and polymorphism allow programmers to create generic data structures. CS 221 - Computer Science II 14

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