COMP 401 INITIALIZATION AND INHERITANCE. Instructor: Prasun Dewan
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1 COMP 401 INITIALIZATION AND INHERITANCE Instructor: Prasun Dewan
2 PREREQUISITE Inheritance Abstract Classes. 2
3 MORE INHERITANCE 3
4 AREGULARCOURSE public class ARegularCourse extends ACourse implements Course { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public int getnumber() { return coursenum; 4
5 ALTERNATIVE AREGULARCOURSE public class ARegularCourse extends ACourse implements Course { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { coursenum = thecoursenum; super (thetitle, thedept); public int getnumber() { return coursenum; Super call must be first statement in constructor but not other methods. Subclass may want to override initialization in super class Superclass vars initialized before subclass vars, which can use the former Subclass vars not visible in superclass 5
6 ALTERNATIVE AREGULARCOURSE public class ARegularCourse extends ACourse implements Course { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { coursenum = thecoursenum; public int getnumber() { return coursenum; No super call()! 6
7 ACOURSE public abstract class ACourse extends Object { String title, dept; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; Missing super call Every class including Object has constructor 7
8 EQUIVALENT ACOURSE public abstract class ACourse extends Object { String title, dept; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; Automatically inserted 8
9 ALTERNATIVE AREGULARCOURSE public class ARegularCourse extends ACourse implements Course { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { coursenum = thecoursenum; public int getnumber() { return coursenum; No super call()! 9
10 ALTERNATIVE AREGULARCOURSE public class ARegularCourse extends ACourse implements Course { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super(); coursenum = thecoursenum; public int getnumber() { return coursenum; Automatically inserted Java complains superclass does not have parameterless constructor 10
11 ADVANCED INITIALIZATION 11
12 ABSTRACT METHODS IN CONSTRUCTORS public abstract class ACourse implements Course { String title, dept; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; // innocuous debugging statement added to ACourse System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()) ; public String gettitle() { return title; public String getdepartment() { return dept; abstract public int getnumber(); 12
13 ABSTRACT METHODS IN CONSTRUCTORS getnumber() called before subclass constructor in ARegularCourse has initialized variables. 13
14 ABSTRACT METHODS IN CONSTRUCTORS Beware of abstract methods being called in constructors. They may access uninitialized variables in subclasses! 14
15 INITIALIZING DECLARATION public class ARegularCourse extends ACourse { int coursenum = 99; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public int getnumber() { return coursenum; 15
16 INITIALIZING DECLARATIONS Initializations in declarations processed after superclass constructor returns Try stepping into a constructor in a debugger. Why correct value? 16
17 CONSTANT INITIALIZATIONS Done when constants allocated memory. (Should be) shared by all instances. When class is loaded in memory. Not when an instance created. 17
18 PROCESSING OF NEW (new ARegularCourse ("Intro. Prog.", "COMP", 14)); The variables declared in ARegularCourse and Course are allocated space but not initialized. The constructor of ARegularCourse is called. It calls the constructor of ACourse in its first statement. 18
19 PROCESSING OF NEW The initializations in the declarations of the variables of ACourse are processed. In this case there are no initializations. The constructor of ACourse is started. The two instance variables of ACourse are assigned parameter values, Intro. Prog. And COMP. The values of these variables are printed. The method getnumber() of ARegularCourse is called and the default value of coursenum is printed. 19
20 PROCESSING OF NEW The constructor of ACourse returns. The initializations in the declarations of the variables of ARegularCourse are processed. In this case, coursenum is assigned the value 0. Execution resumes in the constructor of ARegularCourse.The coursenum variable is assigned the parameter value, 14. The constructor of ARegularCourse returns. The new statement completes, returning the new instance to its caller. 20
21 MULTIPLE CONSTRUCTORS public abstract class ACourse implements Course { String title = COMP ; String dept = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); How to remove code duplication? 21
22 CONSTRUCTOR CALLING CONSTRUCTOR public abstract class ACourse implements Course { String title = COMP ; String dept = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (title, dept); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); This implies constructor of same class 22
23 EQUIVALENT CODE public abstract class ACourse implements Course { String title = COMP ; String dept = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (title, dept); No super public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Super may have side effects, so cannot call super() before this() Java complains that instance variables not initialized 23
24 CONSTRUCTOR CALLING CONSTRUCTOR public abstract class ACourse implements Course { String title = COMP ; String dept = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; this(); public ACourse () { System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Java complains that this() must be first call, as it results in super() being called 24
25 CONSTRUCTOR CALLING CONSTRUCTOR public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this ( COMP, Topics in Computer Science ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); No complaints. 25
26 CONSTRUCTOR CALLING CONSTRUCTOR ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Java complains that instance vars cannot be accessed. 26
27 POSSIBLE CONSTRUCTOR CALL SEMANTICS Rule so far: each constructor calls superclass constructor. Problem: then super class initialized multiple times. Not a problem if each initialization yields the same result. Can have multiple constructors doing different initializations. Same constructor may do different things on different invocations or may have side effects. 27
28 ACTUAL SEMANTICS A constructor may call another constructor as first statement. Super not called in that case. Only last call in constructor chain calls super. Hence no danger of super being initialized multiple times. But danger that constructor parameters may be uninitialized. Literal parameters ok. Constructor call must be first statement. Otherwise code executed before superclass initialized 28
29 SOLUTION: INIT METHOD ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { init (thetitle, thedept) public ACourse () { init (DEFAULT_DEPT, DEFAULT_TITLE); void init (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); 29
30 ALLOWING INIT AFTER CONSTRUCTION ; public abstract class ACourse implements Course { String title = Topics in Computer Science ;; String dept = COMP ; public ACourse (String thetitle, String thedept) { init (thetitle, thedept) public ACourse () { init (DEFAULT_DEPT, DEFAULT_TITLE); public void init (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); 30
31 AREGULARCOURSE ; public class ARegularCourse extends ACourse { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public ARegularCourse () { public int getnumber() { return coursenum; coursenum uninitialized, as init method only in super class Course course =new ARegularCourse(); course.init( Meaning of Life, PHIL ); 31
32 AREGULARCOURSE WITH INIT ; public class ARegularCourse extends ACourse { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public ARegularCourse () { public int getnumber() { return coursenum; public void init (String thetitle, String thedept, int thecoursenum) { coursenum = thecoursenum; Super class init not called Course course =new ARegularCourse(); Course.init( Meaning of Life, PHIL, 999); 32
33 AREGULARCOURSE WITH INIT ; public class ARegularCourse extends ACourse { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public ARegularCourse () { public int getnumber() { return coursenum; public void init (String thetitle, String thedept, int thecoursenum) { init(thetitle, thecoursenum); coursenum = thecoursenum; Course course =new ARegularCourse(); Course.init( Meaning of Life, PHIL, 999); Super class init called. 33
34 TWO WAYS TO CONSTRUCT Initialized construction new ARegularCourse ( Intro. Prog, COMP, 14); Construction and then initialization (new ARegularCourse()).init( Intro Prog., COMP, 14) 34
35 PARAMETERIZED CONSTRUCTORS Can create multiple parameterized constructors To initializes some subset of instance variables. Use default values assigned by initializing declarations for the rest. public ACourse (String thetitle) { title = thetitle; Add an init method for each parameterized constructor 35
36 INIT METHODS Allows initialization after object is created. Initializer can be different from creator Abstract class may initialize Concrete factory method may instantiate. Init methods can be in interfaces Init method(s) recommended but not required 36
37 AFRESHMANSEMINAR public class AFreshmanSeminar extends ACourse { public AFreshmanSeminar (String thetitle, String thedept) { super (thetitle, thedept); title = thetitle; public int getnumber() { return SEMINAR_NUMBER; Allowed since in same package 37
38 ABSTRACT COURSE ; public abstract class ACourse { String title, dept; public ACourse (String thetitle, String thedept) { //title = thetitle; dept = thedept; // innocuous debugging statement added to ACourse System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber() public String gettitle() { return title; public String getdepartment() { return dept; abstract public int getnumber(); No longer necessary 38
39 DISPLAY ABSTRACT COURSE 39
40 ABSTRACT COURSE ; public abstract class ACourse { String title, dept; public ACourse (String thetitle, String thedept) { //title = thetitle; dept = thedept; // innocuous debugging statement added to ACourse System.out.println("New course created: " + "Title:" + title+ " Dept:"+ dept + " Number: " + getnumber() public String gettitle() { return title; public String getdepartment() { return dept; Accesses Uninitialized variable 40
41 REDEFINING SUPERCLASS VARIABLES ; public class ARegularCourse extends ACourse { int coursenum; String title; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; title = thetitle; public int getnumber() { return coursenum; Refers to overriding variable Overriding variable legal in Java 41
42 ALTERNATIVE AFRESHMANSEMINAR ; public class AFreshmanSeminar extends ACourse { String title; public AFreshmanSeminar (String thetitle, String thedept) { super (thetitle, thedept); title = thetitle; public int getnumber() { return SEMINAR_NUMBER; 42
43 REDECLARING SUPERCLASS VARIABLES 43
44 ABSTRACT COURSE ; public abstract class ACourse implements Course { String title, dept; public ACourse (String thetitle, String thedept) { //title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber() public String gettitle() { return title; public String getdepartment() { return dept; abstract public int getnumber(); Equals accesses uninitialized variable 44
45 RE-DECLARING SUBCLASS VARIABLES Happens accidentally when class is re-factored manually to move subclass variables to superclasses. Original variable in subclass remains. Beware! Use Eclipse refactor move command when possible. 45
46 EXTRA SLIDES 46
47 CANNOT ALWAYS GET RID OF MANUAL DISPATCH/INSTANCOF static void print (Course course) { if (course instanceof ARegularCourse) printheader ((ARegularCourse) course); else if (course instanceof AFreshmanSeminar)) printheader ((AFreshmanSeminar) course); System.out.println( course.gettitle() + " " + course.getdepartment() + course.getnumbert() ); static void printheader (ARegularCourse course) { System.out.print( Regular Course: ); static void printheader (AFreshmanSeminar course) { System.out.print( Freshman Seminar: ); 47
48 MULTIPLE INITS ; public abstract class ACourse implements Course { String title = Topics in Computer Science ;; String dept = COMP ; public ACourse (String thetitle, String thedept) { init (thetitle, thedept); init(); public ACourse () { init (); public void init (String thetitle, String thedept) { title = thetitle; dept = thedept; void init() { System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); 48
49 SOME SUPERTYPES IN EXAMPLES Abstract AStringHistory ACourse AStringDatabase AFreshmanSeminar ARegularCourse AStringSet 49
50 ABSTRACT CLASSES IN JAVA.UTIL? Collection Set List HashSet Vector ArrayList LinkedList implements extends 50
51 ABSTRACT CLASSES IN JAVA.UTIL AbstractCollection AbstractSet AbstractList Collection AbstractSequentialList Set List HashSet Vector ArrayList LinkedList implements extends 51
52 POLYMORPHISM VS. OVERLOADING AND DYNAMIC DISPATCH Create polymorphic code when you can In overloading and dynamic dispatch, multiple implementations associated with each method name. In polymorphic case, single implementation. Use interfaces rather than classes as types of parameters Use supertypes rather than subtypes as types of parameters Polymorphism vs. Overloading Polymorphism: single print (Course course) Overloading: print (ARegularCourse course) and print (AFreshmanSeminar course) with same implementation. Polymorphism vs. Dynamic Dispatch Polymorphism: single gettitle() in ACourse Dynamic dispatch: gettitle() in AFreshmanSeminar() and gettitle() in ARegularCourse() with same implementation. 52
53 POLYMORPHISM VS. OVERLOADING AND DYNAMIC DISPATCH Cannot always create polymorphic method. getnumber() for ARegularCourse and AFreshmanSeminar do different things. print(course course) and print (CourseList courselist) do different things. When polymorphism not possible try overloading and dynamic dispatch. 53
54 OVERLOADING VS. DYNAMIC DISPATCH Overloading: Object is parameter Dynamic dispatch: Object is target Method in object class vs. external class Program decomposition issue print(course), print (CourseList) definitions overloaded AFreshmanSeminar.getCourseNumber(), ARegularCourse.getCourseNumber() dynamically dispatched Overload resolution looks at multiple parameter types More general and used for that reason also Dynamic dispatch is done at runtime Can be used for that reason 54
55 size array size array 43 James Dean 42 James Dean Joe Doe Jane Smith Jane Smith John Smith John Smith 55
56 array array Current window James Dean Joe Doe Jane Smith Current window James Dean Joe Doe Jane Smith John Smith John Smith 56
57 size array size array size array 43 James Dean 40 James Dean 41 Jane Smith Joe Doe Jane Smith John Smith Joe Doe Jane Smith John Smith Joe Doe Jane Smith John Smith 57
58 PHYSICAL AND COMPUTER INHERITANCE Physical Object Implicit Extension Object Animal Rock AString History String Mammal AString Database Primate Human 58
59 DIFFERENT TYPES public void addelement(object element) { if (isfull()) System.out.println("Adding item to a full history"); else { contents[size] = element; size++; public void addelement(string element) { if (member(element)) return; super.addelement(element); Signature of Overridden method should be identical in overriding class, otherwise considered different method 59
60 == FOR OBJECTS Point p1 = new ACartesianPoint(200, 200); ACartesianPoint@ Point p Point p2 = p1; p2 == p2 P1 true P2 Point p ACartesianPoint@8 But not necessary. 60
61 ABSTRACT COURSE ; public abstract class ACourse { String title, dept; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; Does not implement an interface 61
62 ANOTHER COURSE ; public abstract class ACourse implements Course { String title, dept; public ACourse (String thetitle, String thedept) { super(); ; title = thetitle; public interface Course { dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; abstract public int getnumber(); public String gettitle(); public String getdepartment(); public int getnumber(); Abstract Method 62
63 ABSTRACT METHOD Declared only in abstract classes Keyword: abstract No body Each (direct or indirect) subclass must implement abstract methods defined by an abstract class. Much like each class must implement the methods defined by its interface(s). 63
64 COURSE DISPLAYER USER INTERFACE 64
65 MAIN CLASS: FILLING LIST static CourseList courses = new ACourseList(); static void fillcourses() { courses.addelement(new ARegularCourse ("Intro. Prog.", "COMP", 14)); courses.addelement(new ARegularCourse ("Found. of Prog.", "COMP", 114)); courses.addelement(new AFreshmanSeminar("Comp. Animation", "COMP")); courses.addelement(new AFreshmanSeminar("Lego Robots", "COMP")); 65
66 JAVA VECTORS, ARRAY LISTS AND ITERATORS Just like collections we defined Except they can store and iterate arbitrary objects 66
67 VECTORS (JAVA.UTIL.VECTOR) Vector v; null Vector Instance James Dean Joe Doe Object Point Instance Vector Instance v = new Vector(); v.addelement( James Dean ) v.addelement( Joe Doe ) v.addelement(5) v.addelement(new ACartesianPoint(5, 5)) v.addelement(new Vector()) 67
68 IMPORTANT METHODS OF CLASS VECTOR public final int size() public final Object elementat(int index) public final void addelement(object obj) public final void setelementat(object obj, int index) public final void insertelementat(object obj, int index) public final boolean removeelement(object obj) public final void removeelementat(int index) public final int indexof(object obj) public final Enumeration elements() 68
69 METHODS OF INTERFACE ENUMERATION (JAVA.UTIL.ENUMERATION) public boolean hasmoreelements(); public Object nextelement(); Enumeration elements = vector.elements(); while ( elements.hasmoreelements()) System.out.println(elements.nextElement(); 69
70 METHODS OF INTERFACE ENUMERATION (JAVA.UTIL.ENUMERATION) public boolean hasmoreelements(); public Object nextelement(); for (Enumeration elements = vector.elements(); elements.hasmoreelements();) System.out.println(elements.nextElement(); 70
71 USING VECTOR DIRECTLY String history user Vector v = new Vector(); Vector null Instance size(): int PointModel Instance PointModel Instance Vector Instance v.addelemen( Joe Doe ); elementat(): index Object addelement(): Object void removeelementat (): Object void Violating Least Privilege v.addelement( John Smith ); v.addelement(new Vector()); v.removeelementat(0); 71
72 ENCAPSULATING VECTOR String history user StringHistory stringhistory = new AStringHistory() AStringHistory instance Vector v = new Vector(); size(): int Vector null Instance PointModel Instance size(): int elementat(): index Object addelement(): Object void elementat(): index String addelement(): String void removeelementat (): Object void stringhistory.addelement( Joe Doe ); stringhistory.addelement(new Vector()) 72
73 ENCAPSULATING VECTOR import java.util.vector; public class AStringHistory implements StringHistory { Vector contents = new Vector(); public void addelement (String s) { contents.addelement(s); public String elementat (int index) { return (String) contents.elementat(index); public int size() { return contents.size(); Simply converts types 73
74 ADAPTER PATTERN client adapter adaptee Degree of adaptation undefined. Methods offered to client Adapted name Adapted type 74
75 ADAPTER PATTERN client adapter adaptee Degree of adaptation undefined. Methods offered to client Adapted name Adapted type 75
76 PARSER STRUCTURE Each production associated with a parser method. Parser method returns object associated with LHS of production. Usually at start of method execution Prefix of unconsumed input should be legal phrase derived from LHS Unless calling method consumed one ore more tokens of the phrase to choose an alternative rule Such a parser called: recursive descent parser Illustrates top-down programming 76
77 77
78 ALTERNATIVE AREGULARCOURSE ; public class ARegularCourse extends ACourse { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public int getnumber() { return coursenum; Implementation of abstract method Implicitly implements interfaces implemented by superclass 78
79 ALTERNATIVE AFRESHMANSEMINAR ; public class AFreshmanSeminar extends ACourse { public AFreshmanSeminar (String thetitle, String thedept) { super (thetitle, thedept); public int getnumber() { return SEMINAR_NUMBER; Implementation of abstract method Implicitly implements interfaces implemented by superclass 79
80 ALTERNATIVE AFRESHMANSEMINAR ; public class AFreshmanSeminar extends ACourse { public AFreshmanSeminar (String thetitle, String thedept) { super (thetitle, thedept); title = thetitle; public AFreshmanSeminar () { public int getnumber() { return SEMINAR_NUMBER; new AFreshmanSeminar ( Lego Robots, COMP ); new AFreshmanSeminar(); 80
81 MULTIPLE CONSTRUCTORS ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber() public ACourse () { dept = DEFAULT_DEPT; title = DEFAULT_TITLE; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()) ; public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); 81
82 CODE DUPLICATION ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber() public ACourse () { dept = DEFAULT_DEPT; title = DEFAULT_TITLE; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()) ; public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); How to remove code duplication? 82
83 CONSTRUCTOR CALLING CONSTRUCTOR ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); this implies constructor of same class. 83
84 EQUIVALENT CODE ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); super()? public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Super may have side effects, so cannot call super() before this() 84
85 CONSTRUCTOR CALLING CONSTRUCTOR ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Java complains that instance vars cannot be accessed. 85
86 MULTIPLE CONSTRUCTORS ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber() public ACourse () { dept = DEFAULT_DEPT; title = DEFAULT_TITLE; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()) ; public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); 86
87 CODE DUPLICATION ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber() public ACourse () { dept = DEFAULT_DEPT; title = DEFAULT_TITLE; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()) ; public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); How to remove code duplication? 87
88 CONSTRUCTOR CALLING CONSTRUCTOR ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); this implies constructor of same class. 88
89 EQUIVALENT CODE ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); super()? public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Super may have side effects, so cannot call super() before this() 89
90 CONSTRUCTOR CALLING CONSTRUCTOR ; public abstract class ACourse implements Course { String title, dept; final String DEFAULT_DEPT = COMP ; final String DEFAULT_TITLE = Topics in Computer Science ; public ACourse (String thetitle, String thedept) { title = thetitle; dept = thedept; System.out.println("New course created: " + "Title:" + title + " Dept:"+ dept + " Number: " + getnumber()); public ACourse () { this (DEFAULT_DEPT, DEFAULT_TITLE ); public String gettitle() {return title; public String getdepartment() {return dept; abstract public int getnumber(); Java complains that instance vars cannot be accessed. 90
91 ABSTRACT METHOD EXAMPLE ; public abstract class ACourse { String title, dept; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; abstract public int getnumber(); Abstract Method 91
92 FACTORY ABSTRACT METHOD abstract <T> <M>( ) ; extends <T> <M>( ) { return new <C1> ( ) <T> <M> ( ){ return new <C2> ( ) Abstract method that returns an instance of some type T like a factory, it creates and initializes an object. 92
93 INIT METHODS Allows initialization after object is created. Initializer can be different from creator Init methods can be in interfaces Init method(s) recommended but not required 93
94 extends VISIBILITY OF SUPERCLASS MEMBERS AFreshmanSeminar getnumber() extends title dept ACourse gettitle() getdepartment() Should non public identifiers be visible in subclasses? coursenum ARegularCourse getnumber() 94
95 ACCESS CONTROL ON VARIABLES AND INHERITANCE, PACKAGES public: accessible in all classes. protected: accessible in all subclasses of its class and all classes in its package. default: accessible in all classes in its package. private: accessible only in its class. 95
96 ACCESSING SUPERCLASS VARIABLES ; public class ARegularCourse extends ACourse { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; title = thetitle; public int getnumber() { return coursenum; Access allowed since in same package 96
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