Polymorphism and access control

What is polymorphism? In general: the ability of some concept to take on different forms In Java: the ability to make identical method calls invoke different behaviors This is one of the fundamental concepts in Object-Oriented programming! Enables code reuse, encapsulation, etc. DCS – SWC

What is polymorphism? We have already seen it – more or less Can be achieved either through interfaces or inheritance (no fundamental difference) The principle is the same – a method is defined in a fundamental class, and several other classes can contain different implementations of that method DCS – SWC

What is polymorphism? Animal + makeSound() Duck Mouse + makeSound() DCS – SWC

What is polymorphism? public class Duck extends Animal { public void makeSound() { print(”Quack”);} } public class Mouse extends Animal public void makeSound() { print(”Squeak”);} DCS – SWC

What is polymorphism? public void makeAnimals() { ArrayList<Animal> zoo; zoo = new ArrayList<Animal>(); zoo.add(new Duck()); zoo.add(new Mouse()); makeNoise(zoo); } DCS – SWC

What is polymorphism? public void makeNoise(ArrayList<Animal> zoo) { for (Animal a : zoo) a.makeSound(); } Output: ”Quack” ”Squeak” DCS – SWC

What is polymorphism? A bit strange, when you think of it… The method makeNoise knows nothing at all about the subclasses… …but still, the subclass-specific methods are invoked! How is that possible? DCS – SWC

What is polymorphism? ”In Java, method calls are always determined by the type of the actual object, not the type of the object reference” This is polymorphism DCS – SWC

What is polymorphism? The method makeNoise will still work, even if we add new subclasses! (”closed for modification, open for extension”) It is the Java Virtual Machine (JVM), which picks the correct method to call The JVM knows the true type of any object This principle is know as late binding DCS – SWC

Choosing methods Multiple versions of the same method can also be created through overloading println(String output) println(int output) However, method ”signatures” are different Compiler can pick correct method at compile-time, this is early binding DCS – SWC

Abstract classes An interface class only provides method definitions, no implementation at all A super-class provides an implementation for all its methods, which can be over-written or used as-is Not the entire truth… We can choose only to provide an imple-mentation for some of the methods DCS – SWC

Abstract classes public class DefaultShape { public void draw() { // do nothing } public double getArea() { return 0.0;} } DCS – SWC

Abstract classes This is known as an abstract class public abstract class DefaultShape { public void draw() { // do nothing } public abstract double getArea(); } This is known as an abstract class getArea is an abstract method DCS – SWC

Abstract classes You can never create an instance of an abstract class (just like interface) You can have a variable of this type You force the user to provide an implementation of the abstract methods DCS – SWC

Abstract classes Is an abstract class just an interface? No, because they can have Instance fields Methods with implementation Constructors Why constructors…? Cannot be called by a user, but… …can be called from a sub-class DCS – SWC

Preventing subclassing Maybe we do not want a user to be able to create a sub-class, or override a method Could be for security reasons, etc. We can prevent this by declaring a class or method as being final DCS – SWC

Preventing subclassing public class SecureAccount extends BankAccount { ... public final boolean checkPassword(String pwd) } public final class String {...} DCS – SWC

Access control Why could we not do this…? // Overridden method in CheckingAccount public void deposit(double amount) { balance = balance + amount; transactionCount++; } private! DCS – SWC

Access control Even a sub-class does not gain access to private methods or instance fields Private really means private! Sub-classes must use get/set-methods, just like any other user Not specifying public or private will provide access at package level DCS – SWC

Access control public class Person { // Access private String name; // class only public int height; // everybody int weight; // all in package } Unless a very good reason exists, stick to public and private… DCS – SWC

The Object class The system class Object is a superclass of all classes in Java! Polymorphism to the extreme!? Which methods could be meaningful to all Java classes? DCS – SWC

The Object class Not many… String toString() – returns a string representation of the object boolean equals(Object other) – tests if the object equals another object Object clone() – makes a full copy of the object DCS – SWC

The Object class String toString() Mostly used for debugging If not overwritten, outputs the class name plus an object identifier (memory location) BankAccount@a122c09 Usually overwritten to print out the values of the instance fields, i.e. the state of the object Subclasses can print their own part DCS – SWC

The Object class boolean equals(Object other) Often useful to know if two objects are ”equal” Equal by content, not by reference! We must first cast argument to correct class, then check if instance fields are equal NOTE: For instance fields of non-primitive types, we need to call their equals(...) methods… DCS – SWC

The Object class Object clone() Useful, if we want a real copy of an object, not just a copy of the reference to the object Clone() method should return a new object, with the same state as the given object Not trivial to clone an object, if the object contains references to other objects Can make a shallow copy or deep copy DCS – SWC

The Object class Deep copy Shallow copy Original Copy Org. Org. Copy DCS – SWC