1 Inheritance in Java CS 3331 Fall 2009

2 Outline  Overloading  Inheritance and object initialization  Subtyping  Overriding  Hiding

3 Overloading of Methods and Constructors  Def. Overloading The ability to allow different methods or constructors of a class to share the same name. public class Point { public Point() { /* … */ } public Point(int x, int y) { /* … */ } public double distance(Point other) { /* … */ } public double distance(int x, int y) { /* … */ } public double distance() { /* … */ } // … } constructor overloading method overloading

4 Overloading (Cont.)  Which overloaded method to invoke? Resolved at compile-time with signature matching, where signature is name and parameter types. Constructors/MethodsSignatures 1: Point() Point() 2: Point(int x,int y) Point(int,int) 3: double distance(Point other) distance(Point) 4: double distance(int x,int y)distance(int,int) 5: double distance()distance() Point p1 = new Point(); // which constructor? Point p2 = new Point(10,20); p2.distance(p1); // which method? p2.distance(20,30); p2.distance();

5 When to Overload? When there is a general, nondiscriminative description of the functionality that fits all the overloaded methods. public class StringBuffer { public StringBuffer append(String str) { /* … */ } public StringBuffer append(boolean b) { /* … */ } public StringBuffer append(char c) { /* … */ } public StringBuffer append(int i) { /* … */ } public StringBuffer append(long l) { /* … */ } // … }

6 When to Overload? (Cont.) When all the overloaded methods offer the same functionality, with some of them providing default arguments. public class String { public String substring(int i, int j) { // base method: return substring from index i to j - 1. } public String substring(int i) { // provide default argument return substring(i, length()); } // … }

7 Outline Overloading  Inheritance and object initialization  Subtyping  Overriding  Hiding

8 Inheritance  Inheritance models the is-a relationship.  If class S extends class T, then all objects of S can act-like an object of T.  Only single inheritance is allowed among classes.  All public and protected members of a superclass are accessible in the subclasses.* *All protected members are also accessible within the package.

9 Constructors of Subclasses  Can invoke a constructor of the direct superclass. super(…) must be the first statement. If the super constructor call is missing, by default the no-arg super() is invoked implicitly.  Can also invoke another constructor of the same class. this(…) must be the first statement.

10 Example of “this” Calls public class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public Point() { // default constructor this.x = 0; this.y = 0; } } this(0,0);

11 Example of “super” Calls public class ColoredPoint extends Point { private Color color; public ColoredPoint(int x, int y, Color color) { this.x = x; this.y = y; this.color = color; } public ColoredPoint(int x, int y) { this(x, y, Color.BLACK); // point with default value } public ColoredPoint() { color = Color.BLACK; // what will be the values of x and y? } } super(x,y); Point -x: int -y: int +Point() +Point(x: int, y: int)

12 Default Constructor public ClassName() { super(); } Q. What would be the use of default constructor?  If no constructor is defined, the following form of no-arg default constructor is automatically generated by the compiler.

13 Execution Order of Constructors public class T { int x = 10; // 1: first public T() { x = 20; // 2: second } //... } Rule: Superclass first and field initialization first - Instance variable initializer vs. instance initializer (or initialization block) Example: S x = new S(); public class S extends T { int y = 30; // 3: third public S() { super(); y = 40; // 4: fourth } //... }

14 Exercise class Super { protected int x = 100; } class Sub extends Super { { x = 200; } public static void main(String[] args) { System.out.println(new Sub().x); }  What value will be printed and why?

15 Exercise class ClassRoom extends Room { private int capacity = 60; public ClassRoom(int c) { capacity = c; } public ClassRoom(int n, int c) { super(n); } { if (number % 2 == 1) capacity *= 2; }  What are the room number and capacity of the following two objects, respectively? ClassRoom c1 = new ClassRom(100); ClassRoom c2 = new ClassRoom(300, 100); class Room { private int number = 100; public Room() { this(++nextNum); } public Room(int n) { number = n; } private static int nextNum = 100; static { nextNum += 100; } { number += 2; } }

16 Outline Overloading Inheritance  Subtyping  Overriding  Hiding

17 Types  What’re types? Sets of values  Why types? To statically detect program errors, called type errors. Typed- vs. untyped-languages (also, static vs. dynamic typing)  Types in Java Primitive types (e.g., boolean, int, long, etc.) Classes (e.g., Object, String, etc.) Interfaces (e.g., Runnable, List, etc.) Array types (e.g., int[], Object[], List[][], etc.)

18 Subtyping  What’s subtyping? Subset relationship among types (e.g., Manager is a subtype (or subset) of Employee) Subtype’s values are legitimate values of supertypes.  Why subtyping? Can organizes types into hierarchy, called subype hierarchy Can extend programs by adding subtypes  Subtyping in Java Between classes (subclassing) Between interfaces (subinterfacing) Between classes and interfaces (implementation or realization) Between arrays

19 Substitution Property  Def. Substitution property A value of subtype can appear where a value of its supertype is expected, e.g., in arguments, results, receivers, and assignments. // 1. argument // public void register(Student s); register(new PhDStudent(“Joe”)); // 2. return value public Student find(String name) { // I am lazy, so I am going to return Joe every time. return new PhDStudent(“Joe”); } // 3. receiver of method call // toString in the class Object new PhDStudent(“Joe”).toString();

20 Substitution Property (Cont.)  Rules of (polymorphic) assignment The type of expression at the right-hand side of an assignment must be a subtype of the type of the variable at the left-hand side of the assignment. class Student { … } class Undergraduate extends Student { … } class Graduate extends Student { … } Student s1, s2; s1 = new Undergradute(); // polymorphic assignment s2 = new Graudate(); // polymorphic assignment Graduate s3 = s2; // is this OK? Graduate s3 = (Graduate) s2; // explicit casting

21 Widening and Narrowing  Def. widening and narrowing The conversion of a subtype to one of its supertype is called widening, and the conversion of a supertype to one of its subtype is called narrowing (or downcasting). // s is a stack of strings Stack s = new Stack(); s.push(“Hello”); // widening: void push(Object) … // Stack defines a method top, i.e., “public Object top()”. s.top().size(); // okay? ((String) s.top()).size(); // downcasting

22 Outline Overloading Inheritance Subtyping  Overriding  Hiding

23 Overriding Methods  Def. Overriding Refers to the introduction of an instance method in a subclass that has the same name, signature, and return type of a method declared in the superclass.  Consequences Implementation of the method in the subclass replaces the implementation of the method in the superclass

24 Overriding Methods (Cont.) public class T { public void m() { … } } public class S extends T { public void m() { … } } T t = new T(); S s = new S(); t.m(); // invoke m of class T s.m(); // invoke m of class S

25 Overriding Methods (Cont.)  Dynamic dispatch (binding): The method to be invoked is determined at runtime by the runtime type of the object, not by the declared type (static type). class Student { public int minCredits() { return 12; } … } class GraduateStudent extends Student { public int minCredits() { return 9; } … } Student s; // … s.minCredits(); // which minCredits method?

26 Overriding Methods (Cont.)  Q. How overriding differ from overloading?

27 Implementation of Dynamic Binding  Storage structure of instance variables Class instance records (CIRs) store the state of an object The CIR for a subclass adds its new fields to the parent CIR.  Dynamic bindings of messages to methods Virtual Method Tables (VMTs) are used for dynamic binding.

28 Dynamic Binding (Cont.) class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public int getX() { return x; } public int getY() { return y; } } class ColoredPoint extends Point { private Color color; public ColoredPoint(int x, int y, Color c) { super(x, y); color = c; } public Color getColor() { return color; } } xy…xy… x y color … CIR for ColoredPoint CIR for Point

29 super: vmt: … Point.class getX: getY: … class: x: 10 y: 20 color: p: Color.RED super: vmt: … ColoredPoint.class Dynamic Binding (Cont.) ColoredPoint p = new ColoredPoint(10,20,Color.RED); p.getColor(); p.getX(); Object.class super: vmt: … getColor: … Q: What if getX is overridden in ColoredPoint?

30 Outline Overloading Inheritance Subtyping Overriding  Hiding

31 Hiding Fields and Class Methods  Def. Hiding Refers to the introduction of a field (instance or class) or a class method in a subclass that has the same name as a field or class method declared in the superclass.  Hiding vs. overriding Statically resolved (bound) at compile-time vs. dynamically dispatched at run-time

32 Example class Student { protected String description = “Student”; public String getDescription() { return description; } class Undergraduate extends Student { protected String description = “Undergraduate”; } new Student().getDecription(); // what value is returned? new Undergraduate().getDescription(); // what value? Q. How to refer to hidden fields? hiding of field