Basic object oriented concepts (1)

Basic Object-Oriented Concepts
4-Dec-17

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
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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)
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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
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Concept: Classes describe objects
 Every object belongs to (is an instance of) a class
 An object may have fields, or variables
 The class describes those fields
 An object may have methods
 The class describes those methods
 A class is like a template, or cookie cutter
 You use the class’s constructor to make objects
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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
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Example of a class
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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
}

Approximate Terminology
 instance = object
 field = instance variable
 method = function
 sending a message to an object =
calling a function
 These are all approximately true
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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
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Example of (part of) a hierarchy
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A FileDialog is a Dialog is a Window is a Container
Container
Panel ScrollPane Window
Dialog Frame
FileDialog

C++ is different
 In C++ there may be more than one root
 but not in Java!
 In C++ an object may have more than one parent
(immediate superclass)
 but not in Java!
 Java has a single, strict hierarchy
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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!
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Example of inheritance
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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.

Concept: Objects must be created
 int n; 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
 Employee secretary; also does two things
 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
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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
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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!
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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
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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
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Example: Assignment of subclasses
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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

Concept: Methods can be overridden
 So birds can fly. Except penguins.
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class Bird extends Animal {
void fly (String destination) {
location = destination;
}
}
class Penguin extends Bird {
void fly (String whatever) { }
}

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
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Sneaky trick: How to use overridden methods
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class FamilyMember extends Person {
void birthday () { // override birthday() in Person
super.birthday (); // call overridden method
givePresent (); // and add your new stuff
}
}

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 your own
 The syntax for writing constructors is almost like that for
writing methods
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Syntax for constructors
 Do not use a return type and a name; use only the
class name
 You can supply arguments
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Employee (String theName, double theSalary) {
name = theName;
salary = theSalary;
}

Trick: Give field and parameter the same name
 A parameter overrides a field with the same name
 But you can use this.name to refer to the field
 class Person {
String name;
int age;
Person (String name, int age) {
this.name = name;
this.age = age;
}
}
 Using the same name is a common and useful convention
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Internal workings: Constructor chaining
 If an Employee is a Person, and a Person is an
Object, then when you say new Employee ()
 The Employee constructor calls the Person constructor
 The Person constructor calls the Object constructor
 The Object constructor creates a new Object
 The Person constructor adds its own stuff to the Object
 The Employee constructor adds its own stuff to the Person
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The case of the vanishing constructor
 If you don't write a constructor for a class, Java provides
one (the default constructor)
 The one Java provides has no arguments
 If you write any constructor for a class, Java does not
provide a default constructor
 Adding a perfectly good constructor can break a
constructor chain
 You may need to fix the chain
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Example: Broken constructor chain
class Person {
String name;
Person (String name) {
this.name = name;
}
}
class Employee extends Person {
double salary;
Employee ( ) {
salary = 12.50;
}
}
 cannot resolve symbol – constructor Person()
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Java tries to execute
an implicit super()
at this point
super();

Fixing a broken constructor chain
 Special syntax: super(...) calls the superclass constructor
 When one constructor calls another, that call must be first
class Employee {
double salary;
Employee (String name) {
super(name); // must be first
salary = 12.50;
}
}
 Now you can only create Employees with names
 This is fair, because you can only create Persons with names
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Trick: one constructor calling another
 this(...) calls another constructor for this same class
 It is poor style to have the same code more than once
 If you call this(...), that call must be the first thing in your
constructor
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class Something {
Something (int x, int y, int z) {
// do a lot of work here
}
Something ( ) { this (0, 0, 0); }
}

Concept: You can control access
 Each object is responsible for its own data
 Access control lets an object protect its data and
its methods
 Access control is the subject of a different lecture
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class Person {
public String name;
private String age;
protected double salary;
public void birthday { age++; }
}

Concept: Classes can have fields and methods
 Usually a class describes fields (variables) and
methods for its objects (instances)
 These are called instance variables and instance methods
 A class can have its own fields and methods
 These are called class variables and class methods
 There is exactly one copy of a class variable, not
one per object
 Use the special keyword static to say that a field
or method belongs to the class instead of to objects
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Example of a class variable
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class Person {
String name;
int age;
static int population;
Person (String name) {
this.name = name;
this.age = 0;
population++;
}
}

Advice: Restrict access
 Always, always strive for a narrow interface
 Follow the principle of information hiding:
 the caller should know as little as possible about how the
method does its job
 the method should know little or nothing about where or why
it is being called
 Make as much as possible private
 Your class is responsible for it’s own data; don’t
allow other classes to screw it up!
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Advice: Use setters and getters
 This way the object maintains control
 Setters and getters have conventional names: setDataName,
getDataName, isDataName (booleans only)
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class Employee extends Person {
private double salary;
private boolean male;
public void setSalary (double newSalary) {
salary = newSalary;
}
public double getSalary () { return salary; }
public boolean isMale() { return male; }
}

Kinds of access
 Java provides four levels of access:
 public: available everywhere
 protected: available within the package (in the same
subdirectory) and to all subclasses
 [default]: available within the package
 private: only available within the class itself
 The default is called package visibility
 In small programs this isn't important...right?
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