Week 4 Content - Objects

Week 4 Content – Objects 05 March 2021 15:01 Video 1 – Objects What is Object-Oriented Programming? 1. Program a model of the world in which 'objects' interact 2. Run a simulation of the model to bring the model to life Objects An object represents a specific, identifiable part of the world-model • It incorporates passive attributes (variables) • It has active behaviours & services (methods) • Objects interact by sending messages (calling methods) Example: Shapes All three of these shapes are objects with attributes to show their number of corners, colour and whether or not they are currently filled. They can also send messages to one another through calling methods of another object. Example: ATM Usage Russ checks the ATM near the guild to check on his bank account. Note how we specify a specific ATM and specific bank account, so only the balance of Russ' bank account is accessed and it is only sent to the ATM besides the guild building. When Russ uses the ATM, he sends a message requesting his bank account balance to the ATM – represented by requestBalance(). The ATM sends a message to the account to retrieve the balance – getBalance() – and the account returns the balance to the ATM which is passed onto Russ. Russ has no idea what happens inside the ATM & bank network, and simply makes a request for his bank balance which is duly returned to him by the ATM. This concept is called information hiding. Video 2 – Classes How do we create objects? 1. By copying & adjusting existing ones 2. By instantiating a template – a class Classes A class is a template or 'blueprint' for objects • Classes specify which attributes and methods should exist • An object is an instance of a class Aside: Class Diagrams The rectangle class can be represented in this form: a UML class diagram representing the attributes and methods within a class. • The first box contains the name of the class • The second box lists all attributes • The third box lists all methods within the class Java Objects & Classes To create objects in Java we need to: 1. Define a class 2. Instantiate a new object of that class Defining a Class Instantiating an Object of this Class • Every class defines a type, with the type name corresponding to that of the class • Variables to hold objects of our new class is defined as such: • However – to instantiate the new object – we need to use the keyword new (similarly to how we create arrays), noting that parameters need to be included between the brackets: Video 3 – Constructors Recall that classes are templates for creating objects, and that we can instantiate an object using the keywork new. But what happens internally? 1. The JVM allocates memory to store the new object 2. A constructor method is called to initialise the new object Constructors A constructor is a special method that gets called when an object is created. It is intended to set up the object for later use. The syntax for constructor methods is almost identical to normal methods except: • It must be named just like the class • It has no declared return type Example Multiple Constructors • A class can have more than one constructor • As with any overloaded method (that is to say, multiple methods with the same name), each constructor requires them to have different signatures (i.e. different amounts of arguments, or arguments with different data types) • If we do not define any constructors, the class will automatically be assigned a default constructor with no arguments Example One constructor can call another. Example: Note that the this keyword refers to the current object in the constructor so – in this context – the statement on line 10 calls the constructor on line 5 with the parameters s and "blue". This creates a rectangle which is blue by default. Video 4 – Encapsulation Introductory Example An alarm clock has: • Attributes – current time, alarm time, set/unset • Methods – buttons and switches to set the alarm or snooze However, to use the alarm clock, we only need to know its outward interface: not its internal workings. Private and Public Content Attributes and methods of Java classes can be public or private. • Private members of a class cannot be accessed from outside the object (recall that an object is just an instance of a class, so the definitions of public/private in a class transfer unto objects) • Public members can be accessed from outside an object using 'dot notation': • The public members form the objects interface: the messages it can receive and services it offers Information Hiding • It's good practise to declare attributes private to avoid accidental outside interference or messy dependencies • This principle is called Information Hiding or Encapsulation • But what if we needed to read/write private attributes? Accessors & Mutators • We can introduce public methods whose sole job it is to get or set the value of private attributes • Methods which modify attributes are called mutators • Methods which retrieve the value of an attribute are called accessors • It is conventional to call these methods getX and/or setX for private attribute X Here, time and alarm are private attributes and the methods getTime and setAlarm are used as accessors and mutators for time and alarm respectively. Video 5 – Class Variables/Methods Instance Variables • Ordinarily, attribute values belong to individual objects and methods describe the object's individual response to messages • e.g. alarmClock.time and alarmClock.setAlarm() are attributes and methods which pertain specifically to a single, particular object (alarmClock) • More simply, each clock has its own alarm time. Class Variables • Using the keyword static, we can declare that an attribute or method belongs to the class • Static variables are shared among all instances • e.g. we could make every alarm clock share the attribute make = "B&Q" Static Variables & Methods • The values of static variables can be read and modified by all instances of the class (e.g. all objects of type AlarmClock), as well as from within static methods • Static methods cannot access instance variables or instance methods (i.e. attributes or methods within a specific object), but only other static members • Static methods can be called directly on the class without instantiating an object: e.g. AlarmClock.ring() Example: Counting Instances • The static attribute totalClocks is shared amongst all objects of type AlarmClock (all instances) • Everytime a new AlarmClock object is instantiated, the value of totalClocks is incremented for all clocks.