Lecture Set 11 Polymorphism Introduction 1 2 Wrappers Interfaces CMSC 131 Spring 2010 Jan Plane Ben Bederson Wrappers We may want to treat primitives as though they were objects For example generic routines can be implemented using interfaces but they are not usable on primitive types To overcome this problem Java provides wrappers for primitive types Wrappers classes whose objects contain single values of the wrapped type Wrappers convert easily to and from that wrapped type Wrappers also contain other useful conversion operations to from String etc Wrappers included in java lang Byte Short Integer Long Float Double Character Boolean CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 1 1 The Integer Wrapper The documentation is on line at http java sun com j2se 1 5 0 docs api Notes Immutable Constructors Implements Comparable Documentation says Comparable Integer Comparable in Java 5 0 is a interface Has compareTo method CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 2 Code Re use Many operations recur in programming sorting max min These operations may apply to strings numbers etc Desirable one implementation Less coding Less likely to have typos Easier maintenance of code CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 3 2 Polymorphism Using an interface we can create one variable that can reference objects different types i e Comparable variable referencing Integers Strings or Cats UMStudent variable referencing CSMajor CEMajor or PsychMajor This form of generalization is called polymorphism Hallmark of OO languages Allows application of same code to objects of different types Polymorphism A variable that takes on many shapes Interfaces one mechanism Java provides for polymorphism a collection of prototypes method prototypes but no bodies aka abstract methods A class C implements an interface I If and only if C provides implementations of all of I s abstract methods A class implementing an interface can also provide other methods or implement other interfaces CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 4 In class Demo Implementing a method using the Integer class Create objects of type Integer using the constructor can be based on int type values or variables Create an array of Integer type object references and those objects of type Integer Use the API to access information about the data in the Integer class Expand this example to Strings Expand this example to Cats CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 5 3 Adapting Cat to Implement Comparable The Comparable Interface insists that I must implement compareTo method which has the following prototype int compareTo Object o it must return a negative if the current object is less a positive if the current object is greater or a 0 if they are the same What is Object Type of all possible objects in any class Shortcoming of earlier Java no good way to say same type as this Instead Implementation must take any object CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 6 What about int char Polymorphic findMin can be used on any class implementing Comparable What about primitive types int char double etc They are not classes So they do not implement Comparable Hence findMin cannot be used on them That s why we use wrappers CMSC 131 Fall 2009 Jan Plane Adapted from Bonnie Dorr 7 4
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