Basic Object Oriented Concepts Jan 14 2019 Concept An object has behaviors In old style programming you had data which was completely passive functions which could manipulate any data An object contains both data and methods that manipulate that data An object is active not passive it does things An object is responsible for its own data But it can expose that data to other objects 2 Concept An object has state An object contains both data and methods that manipulate that data The data represent the state of the object Data can also describe the relationships between this object and other objects Example A CheckingAccount might have A balance the internal state of the account An owner some object representing a person 3 Example A Rabbit object You could in a game for example create an object representing a rabbit It would have data How hungry it is How frightened it is Where it is And methods eat hide run dig 4 Concept Classes describe objects Every object belongs to is an instance of a class An object may have fields or variables An object may have methods The class describes those fields The class describes those methods A class is like a template or cookie cutter You use the class s constructor to make objects 5 Concept Classes are like Abstract Data Types An Abstract Data Type ADT bundles together some data representing an object or thing the operations on that data The operations defined by the ADT are the only operations permitted on its data Example a CheckingAccount with operations deposit withdraw getBalance etc Classes enforce this bundling together If all data values are private a class can also enforce the rule that its defined operations are the only ones permitted on the data 6 Example of a class class Employee Fields private String name Can get but not change private double salary Cannot get or set Constructor Employee String n double s name n salary s Methods void pay System out println Pay to the order of name salary public String getName return name getter 7 Approximate Terminology instance object field instance variable method function sending a message to an object calling a function These are all approximately true 8 Concept Classes form a hierarchy Classes are arranged in a treelike structure called a hierarchy The class at the root is named Object Every class except Object has a superclass A class may have several ancestors up to Object When you define a class you specify its superclass If you don t specify a superclass Object is assumed Every class may have one or more subclasses 9 Example of part of a hierarchy Container Panel ScrollPan e Windo w Dialo g FileDialo g Fram e A FileDialog is a Dialog is a Window is a Container 10 C is different In C there may be more than one root In C an object may have more than one parent immediate superclass but not in Java but not in Java Java has a single strict hierarchy 11 Concept Objects inherit from superclasses A class describes fields and methods Objects of that class have those fields and methods But an object also inherits the fields described in the class s superclasses the methods described in the class s superclasses A class is not a complete description of its objects 12 Example of inheritance class Person String name int age void birthday age age 1 class Employee extends Person double salary void pay Every Employee has name and age fields and birthday method as well as a salary field and a pay method 13 Concept Objects must be created int n does two things Employee secretary also does two things It declares that n is an integer variable It allocates space to hold a value for n For a primitive this is all that is needed It declares that secretary is type Employee It allocates space to hold a reference to an Employee For an object this is not all that is needed secretary new Employee This allocate space to hold a value for the Employee Until you do this the Employee is null 14 Notation How to declare and create objects Employee secretary declares secretary secretary new Employee allocates space Employee secretary new Employee does both But the secretary is still blank null secretary name Adele dot notation secretary birthday sends a message 15 Notation How to reference a field or method Inside a class no dots are necessary class Person age age 1 Outside a class you need to say which object you are talking to if john age 75 john birthday If you don t have an object you cannot use its fields or methods 16 Concept this object Inside a class no dots are necessary because you are working on this object If you wish you can make it explicit class Person this age this age 1 this is like an extra parameter to the method You usually don t need to use this 17 Concept A variable can hold subclass objects Suppose B is a subclass of A A objects can be assigned to A variables B objects can be assigned to B variables B objects can be assigned to A variables but A objects can not be assigned to B variables Every B is also an A but not every A is a B You can cast bVariable B aObject In this case Java does a runtime check 18 Example Assignment of subclasses class Dog class Poodle extends Dog Dog myDog Dog rover new Dog Poodle yourPoodle Poodle fifi new Poodle myDog rover ok yourPoodle fifi ok myDog fifi ok yourPoodle rover illegal yourPoodle Poodle rover runtime check 19 Concept Methods can be overridden class Bird extends Animal void fly String destination location destination class Penguin extends Bird void fly String whatever So birds can fly Except penguins 20 Concept Don t call functions send messages Bird someBird pingu someBird fly South America Did pingu actually go anywhere You sent the message fly to pingu If pingu is a penguin he ignored it Otherwise he used the method defined in Bird You did not directly call any method You cannot tell without studying the program which method actually gets used The same statement may result in different methods being used at different times 21 Sneaky trick How to use overridden methods class FamilyMember extends Person void birthday override birthday in Person super birthday call overridden method givePresent and add your new stuf 22 Concept Constructors make objects Every class has a constructor to make its objects Use the keyword new to call a constructor secretary new Employee You can write your own constructors but if you don t Java provides a default constructor with no arguments It sets all the fields of the new object to zero If this is good enough you don t need to write
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