1. Polymorphism and access control

2. 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.
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3. 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
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4. What is polymorphism? Animal + makeSound() Duck Mouse + makeSound()
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5. What is polymorphism? public class Duck extends Animal{
public void makeSound() { print(”Quack”);} }
public class Mouse extends Animal
public void makeSound() { print(”Squeak”);}
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6. What is polymorphism? public void makeAnimals() {
ArrayList<Animal> zoo;
zoo = new ArrayList<Animal>();
zoo.add(new Duck());
zoo.add(new Mouse());
makeNoise(zoo); }
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7. What is polymorphism?
public void makeNoise(ArrayList<Animal> zoo) {
for (Animal a : zoo)
a.makeSound(); }
Output:
”Quack”
”Squeak”
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8. 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?
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9. 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
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10. 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
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11. 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
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12. 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
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13. Abstract classes public class DefaultShape {
public void draw() { // do nothing }
public double getArea() { return 0.0;} }
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14. 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
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15. 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
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16. 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
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17. 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
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18. Preventing subclassing
public class SecureAccount extends BankAccount {
...
public final boolean checkPassword(String pwd) }
public final class String {...}
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19. Access control Why could we not do this…?
// Overridden method in CheckingAccount
public void deposit(double amount) {
balance = balance + amount;
transactionCount++; }
private!
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20. 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
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21. 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…
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22. 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?
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23. 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
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24. The Object class String toString() Mostly used for debugging
If not overwritten, outputs the class name plus an object identifier (memory location)
Usually overwritten to print out the values of the instance fields, i.e. the state of the object
Subclasses can print their own part
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25. 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…
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26. 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
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27. The Object class Deep copy Shallow copy Original Copy Org. Org. Copy
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