Inheritance in Java

Inheritance in Java CS 3331 Fall 2009

Outline • Overloading • Inheritance and object initialization • Subtyping • Overriding • Hiding

constructor overloading • 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() { /* … */ } • // … • } method overloading Overloading of Methods and Constructors • Def. Overloading The ability to allow different methods or constructors of a class to share the same name.

Overloading (Cont.) • Which overloaded method to invoke? Resolved at compile-time with signature matching, where signature is name and parameter types. • Constructors/Methods Signatures • 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();

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) { /* … */ } // … }

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()); } // … }

Outline • Overloading • Inheritance and object initialization • Subtyping • Overriding • Hiding

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.

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.

this(0,0); 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; } }

Point -x: int -y: int +Point() +Point(x: int, y: int) super(x,y); 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? } }

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

Execution Order of Constructors 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 } // ... } public class T { int x = 10; // 1: first public T() { x = 20; // 2: second } // ... }

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

Exercise • 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 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; } } 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; } }

Outline • Overloading • Inheritance • Subtyping • Overriding • Hiding

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.)

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

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(Students); 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();

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

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

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

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

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?

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

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.

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; } } CIR for Point x y … CIR for ColoredPoint x y color …

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

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

hiding of field 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?

Inheritance in Java