Chapter 6

1. Chapter 6 Object-Oriented Design Part 2

2. The this Reference • The this reference allows an object to refer to itself •  That is, the this reference, used inside a method, refers to the object through which the method is being executed • Remember: instance variables are unique to each object; but the methods are shared!! • Suppose the this reference is used in a method called tryMe, which is invoked as follows: obj1.tryMe(); obj2.tryMe(); • In the first invocation, the this reference refers to obj1; in the second it refers to obj2 © 2004 Pearson Addison-Wesley. All rights reserved

3. The this reference • The this reference can be used to distinguish the instance variables of a class from corresponding method parameters with the same names • The constructor of the Account class (from Chapter 4) could have been written as follows: public Account (Sring name, long acctNumber, double balance) { this.name = name; this.acctNumber = acctNumber; this.balance = balance; } Here this.name refers to the instance variable for ‘this’ object… (don’t need different formal parameter names) © 2004 Pearson Addison-Wesley. All rights reserved

4. Outline Software Development Activities Identifying Classes and Objects Static Variables and Methods Class Relationships Interfaces Enumerated Types Revisited Method Design Testing GUI Design and Layout © 2004 Pearson Addison-Wesley. All rights reserved

5. Interfaces • A Java interface is a collection of abstract methods and constants • An abstract method is a method header without a method body • An abstract method can be declared using the modifier abstract, but because all methods in an interface are abstract, usually it is left off • An interface is used to establish a set of methods that a class will implement. • Note: the Java interface is itself NOT a class! © 2004 Pearson Addison-Wesley. All rights reserved

6. interfaceis a reserved word (note: ‘class’ is not used) A semicolon immediately follows each method header This ‘prevents’ there being a body for the method. Interfaces None of the methods in an interface are given a definition (body) public interface Doable { public void doThis(); public int doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num); } These are all abstract methods because they are declared within an ‘interface.’ They could be preceded with ‘abstract’ but this is not necessary! © 2004 Pearson Addison-Wesley. All rights reserved

7. Note: • In defining ‘classes,’ we say: public class whatever • In defining an ‘interface,’ we say public interface whatever (we do Not say ‘class.’) © 2004 Pearson Addison-Wesley. All rights reserved

8. Interfaces • An interface cannot be instantiated. • Recall, it is NOT a class! (thus cannot be instantiated!) • Methods in an interface have public visibility by default •  A class formally implements an interface by: • stating so in the class header • providing implementations for each abstract method in the interface • If a class asserts that it “implements an interface,” it must define all methods in the interface © 2004 Pearson Addison-Wesley. All rights reserved

9. implements is a reserved word Each method listed in Doable is given a definition Interfaces public class CanDo implements Doable { public void doThis () { // whatever } public void doThat () { // whatever } // etc. } Note: this class ‘implements’ the interface. All abstract methods in the interface must be defined here in the class that ‘implements’ the interface. If all the methods are not defined, then this class is itself abstract and cannot be instantiated. © 2004 Pearson Addison-Wesley. All rights reserved

10. Interfaces • A class that implements an interface can implement other methods as well • See Complexity.java (ahead) • See Question.java (ahead) • See MiniQuiz.java (ahead) • In addition to (or instead of) abstract methods, an interface can contain constants • When a class implements an interface, it gains access to all its constants © 2004 Pearson Addison-Wesley. All rights reserved

11. //************************************************ // Complexity.java Author: Lewis/Loftus // // Represents the interface for an object that can be assigned // an explicit complexity. //************************************************ public interface Complexity { public void setComplexity (int complexity); public int getComplexity(); } Note the semicolon after the abstract methods! Note: public  interface… This interface allows many classes to ‘implement’ it in different ways. © 2004 Pearson Addison-Wesley. All rights reserved

12. // Question.java Author: Lewis/Loftus // Represents a question (and its answer). public class Question implements Complexity { private String question, answer; private int complexityLevel; // Constructor: Sets up the question with a default complexity. public Question (String query, String result) // Of course, we usually need a Constructor…. { question = query; answer = result; complexityLevel = 1; } // Sets the complexity level for this question. public void setComplexity (int level) // Must have this one { complexityLevel = level; } // Returns the complexity level for this question. public int getComplexity() // Must have this one. { return complexityLevel; } // Returns the question. public String getQuestion() { return question; } // Do NOT code like I have the next two methods. Only for space here! // Returns the answer to this question. public String getAnswer() { return answer; } // Returns true if the candidate answer matches the answer. public boolean answerCorrect (String candidateAnswer) { return answer.equals(candidateAnswer); } // Returns this question (and its answer) as a string. public String toString() { return question + "\n" + answer; } © 2004 Pearson Addison-Wesley. All rights reserved

13. // MiniQuiz.java Author: Lewis/Loftus // Demonstrates the use of a class that implements an interface. import java.util.Scanner; public class MiniQuiz { // Presents a short quiz. public static void main (String[] args) { Question q1, q2; // Creates two references to Question objects. String possible; Scanner scan = new Scanner (System.in); q1 = new Question ("What is the capital of Jamaica?", "Kingston"); // Creates object, q1 q1.setComplexity (4); // then sets the complexity to 4. q2 = new Question ("Which is worse, ignorance or apathy?", “I don't know and I don't care"); q2.setComplexity (10); // Creates a second object, q2, and its complexity. System.out.print (q1.getQuestion()); System.out.println (" (Level: " + q1.getComplexity() + ")"); possible = scan.nextLine(); if (q1.answerCorrect(possible)) System.out.println ("Correct"); else System.out.println ("No, the answer is " + q1.getAnswer()); System.out.println(); System.out.print (q2.getQuestion()); System.out.println (" (Level: " + q2.getComplexity() + ")"); possible = scan.nextLine(); © 2004 Pearson Addison-Wesley. All rights reserved

14. Interfaces • A class can implement multiple interfaces • The interfaces are listed in the implements clause • The class must implement all methods in all interfaces listed in the header class ManyThings implements interface1, interface2 { // all methods of both interfaces } © 2004 Pearson Addison-Wesley. All rights reserved

15. Interfaces • The Java standard class library contains many helpful interfaces • The Comparable interface contains one abstract method called compareTo, which is used to compare two objects public interface Comparable { public int compareTo (Object obj2 ); } • We discussed the compareTo method of the String class in Chapter 5 • The String class implements Comparable, giving us the ability to put strings in lexicographic order •  Note: If the Java API provides the interface (fine), WE must provide the “implementation” of that interface. This means we can tailor it to whatever! © 2004 Pearson Addison-Wesley. All rights reserved

16. The Comparable Interface • Any class can implement Comparable to provide a mechanism for comparing objects of that type. • Thus WE can decide exactly how we want to determine how two “things” are to be compared! if (obj1.compareTo(obj2) < 0) System.out.println ("obj1 is less than obj2"); • The value returned from compareTo should be negative is obj1 is less that obj2, 0 if they are equal, and positive if obj1 is greater than obj2 • When a programmer designs a class that implements the Comparable interface, it should follow this intent © 2004 Pearson Addison-Wesley. All rights reserved

17. Implementing the Comparable Interface • It's up to the programmer to determine what makes one object less than another • If we are comparing two … Strings, (this is already done for us in String), we decide ‘how’ to compare. •  If we are comparing two, say, air filters on cars, we may decide to compare by comparing the volume of air that each allows through the filter… •  The implementation of the method can be as straightforward or as complex as needed for the situation © 2004 Pearson Addison-Wesley. All rights reserved

18. The Iterator Interface • As we discussed in Chapter 5, an iterator is an object that provides a means of processing a collection of objects one at a time •  An iterator is created formally by implementing the Iterator interface, which contains three methods • The hasNext method returns a boolean result – true if there are items left to process • The next method returns the next object in the iteration • The remove method removes the object most recently returned by the next method • So, if we implement the Iterator interface, WE must define HOW we implement the Iterator! © 2004 Pearson Addison-Wesley. All rights reserved

19. The Iterator Interface • By implementing the Iterator interface, a class formally establishes that objects of that type are iterators (that is, whatever we are iterating thru…, whether the ‘collection’ is a list of Strings, objects, integers, etc.) • The programmer, as stated, must decide how best to implement the iterator functions • Once established, the for-each version of the for loop can be used to process the items in the iterator (See last slides in 5.3 lecture slides) © 2004 Pearson Addison-Wesley. All rights reserved

20. Interfaces • You could write a class that implements certain methods (such as compareTo) without formally implementing the interface (Comparable) • However, formally establishing the relationship between a class and an interface allows Java to deal with an object in certain ways • Interfaces are a key aspect of object-oriented design in Java • We discuss this idea further in Chapter 9 © 2004 Pearson Addison-Wesley. All rights reserved