OO terminology & objectives §Encapsulation l don’t touch my private data l get/set it using...

OO terminology & objectivesEncapsulation

don’t touch my private data get/set it using my public/package methods

Inheritance a parent provides for her/his children children add to a parent’s knowledge

Polymorphism Java figures out family relationships:

when the parent knows best or when the child knows more than the parent

Inheritance in Java super class or parent, subclass or

child adds functionality to an existing class class Child extends Parent C++ multiple inheritance … NOT!

single parent families only in Java use of the keyword super

not this object’s constructor/variable/method,but the one from the super class.

The Object Class

“The Ultimate Superclass”- all classes in Java extend the

Object class Example (an array of generic objects):

ObjectDemo.java "is a" test for inheritance/super class

a String is an Object (true) an Object is a String (false)

How to override a parent’s method in the child class? use the same …

method name signature (parameter list) return type

different code in the child’s method overrides parent's logic

e.g. equals(), toString() access level can be increased (e.g. private to public) a different signature: overloads the method a different return type: compiler error Example: Inherit.java, ShapeTest & docs

Polymorphism the same method call on two objects in the same

“family” (inheritance branch) can give different results Which method is actually called at run time? method(signature) in this object’s class else look up the super (parent) class tree run-time/dynamic binding determines which one compile time: the declared type of an object reference

Object x = new BigDecimal("123.45"); run-time: the actual type of the object referred to

determines behaviour. It can be the declared class type or one of its sub classes (inheritance).x.toString() calls BigDecimal class method, not Object'sbut it compiles because Object x has toString().

Polymorphism a final method: dynamic binding is turned off sub (child) classes can inherit but not override

any final super (parent) method like when your parent said, “…and that’s

final!”. a method of limiting inheritance

Casting and Inheritance casting up is automatic:

Object object; String string;object = string; // because string IS AN object

casting down is up to the programmer:string = (String) object;

promises compiler that, at run time, object will refer to a String type.

compile-time error: subclass = superclass;

Abstract Class: An Example

public abstract class Aclass { private int m; // instance variable to be inherited

// implemented and to be inherited public void set( int t ) { m = t; } // abstract methods declared, not implemented // must be defined by extending class public abstract void show();

public abstract void change(); }

The Subclasses of an Abstract Class

public class Bclass extends Aclass { … // inherit Class Aclass’s set method public void show ( ) { … } // implement details public void change ( ) { … } // implement details}

public class Cclass extends Aclass { … // inherit Class Aclass’s set method public void show( ) { … } // class C version public void change ( ) { … } // class C version}

What is an abstract class? an abstraction(i.e. generalization) of subclasses- a

class hierarchy is enforced abstract methods: the developers of the subclasses

MUST customize the code for these methods cannot be used to create an object instance

Aclass aClassRef; // object reference is OK aClassRef = new Aclass(); // but cannot construct

in practice: an abstract class has a mix of instance variables, implemented and abstract

methods

Interfaces in Java What is an interface? How to declare an interface? How to implement an interface? How to use an interface?

What is an interface? syntax: a declaration of methods

(that are not implemented yet) software development viewpoint

- an agreement on how a “specific” object can be used by other objects - a promise to other software developers that all the methods will be implemented by a class

cannot be used to create an object static final variables may be included

(e.g. the Adjustable interface)

How is an interface used? one or more interfaces can be implemented by a classclass MyClass implements Comparable<MyClass>

other classes can tell if your class has implemented an interfaceif (myClass instanceof Comparable){

then other classes know they can call interface methodsint compare=myClass.compareTo(otherMyClass)}

only your class knows how to implement those methods

see API for Comparable interface event handling(GUI programming) - Example: the

ActionListener interface an interface can extend one or more interfaces

Inner Classes a class that is defined inside another class,

called the outer or enclosing class. access privilege to members of the outer

class: fields, methods, other inner classes can access outer class’s private instance

members mostly used for event-driven programs

i.e. GUI programming Example: Outer.java & OuterInnerTest.java

Anonymous Inner Classes

anonymous inner class definition an inner class that does not have a name notational convenience: event handling code

can be put close to where GUI objects are created

not recommended because code is messy and dense can only implement behaviour, i.e. override methods must use default constructor, i.e. cannot have state