1CMSC 131 Spring 2010Jan Plane & Ben BedersonLecture Set #11:Polymorphism Introduction1. Wrappers2. InterfacesCMSC 131 Fall 2009Jan Plane (Adapted from Bonnie Dorr)1Wrappers 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 Boolean2CMSC 131 Fall 2009Jan Plane (Adapted from Bonnie Dorr)2The 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 2009Jan Plane (Adapted from Bonnie Dorr)3Code 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 code3CMSC 131 Fall 2009Jan Plane (Adapted from Bonnie Dorr)4Polymorphism 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 interfacesCMSC 131 Fall 2009Jan Plane (Adapted from Bonnie Dorr)5In 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 Cats4CMSC 131 Fall 2009Jan Plane (Adapted from Bonnie Dorr)6Adapting 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 objectCMSC 131 Fall 2009Jan Plane (Adapted from Bonnie Dorr)7What 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
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