Designing and Implementing Classes Topic 6 Don’t know much geography

Designing and Implementing Classes Topic 6 Don't know much geography Don't know much trigonometry Don't know much about algebra Don't know what a slide rule is for -Sam Cooke AP Computer Science Designing and Implementing Classes 1

Object Oriented Programming Object Oriented Programming is not new. – Simula 1 (1962 - 1965) and Simula 67 (1967) Norwegian Computing Center, Oslo, Norway by Ole-Johan Dahl and Kristen Nygaard. – Smalltalk (1970s), Alan Kay's group at Xerox PARC – C (early 1980s), Bjarne Stroustrup, Bell Labs – Eiffel, Bertrand Meyer – Java and others Becoming the dominant style for implementing complex programs with large numbers of interacting components and in education AP Computer Science Designing and Implementing Classes 2

Main Tenets of OO Programming Encapsulation – abstraction, information hiding, responsibility driven programming Inheritance – code reuse, specialization Polymorphism – allows old code to use new code, do X, where X is different depending on the type of object AP Computer Science Designing and Implementing Classes 3

Tenants of Object Oriented Programming - Encapsulation information hiding, using something without knowing (or caring or having to worry) about how it works. abstraction achieved via classes and objects. Useful in breaking up large problems based on a study of the data involved first Don Knuth, Turing award winner algorithms data structures programming AP Computer Science Designing and Implementing Classes 4

Class and Objects What are classes? – classes are user defined data types. (as opposed to the primitive data types) – classes specify the operations that may be carried out on variables of this type. Behaviors – They also specify what sub data the variable consists of and have the implementation of the behaviors listed above. – objects are simply variables of a given class It is easier to describe how to use an object than it is to create a class – easier to teach some one to drive car or teach someone how to design and build a car? AP Computer Science Designing and Implementing Classes 5

Using Objects An object is a model (abstraction of) a realworld object. – it is a collection of variables (fields) and methods A method is an operation on an object. Objects interact with messages. A message is sent to an object and specifies the method that object should perform. – The sender (normally another object) doesn't know anything about how the method is performed AP Computer Science Designing and Implementing Classes 6

Using Objects Objects are variables, they have a declared type and a set of behaviors or operations String name1 new String("Olivia A. Scott"); String name2 new String("Isabelle M. Scott"); All object variables are reference variables (i.e. pointers) Objects must be dynamically allocated using the new operator and calling a constructor – special cases for Strings and arrays To determine the various behaviors of an object must look at the class documentation AP Computer Science Designing and Implementing Classes 7

The dot Operator Once an object is created the dot operator ( . ) is used to perform operations on the object or access sub data in the object String name1 new String("Olivia A. Scott"); char first name1.charAt(0); char second name2.charAt(1); Any public part of the object may be accessed. normally only methods are public. – Must call a method that actually exists and match one of the parameter signatures or syntax error the dot operator calls or invokes the method flow of control shifts to that code AP Computer Science Designing and Implementing Classes 8

Object Data occasionally fields of an object are public, but this is generally considered bad style array length vs. String length int[] list new int[x]; for(int i 0; i list.length; i ) Only a good idea if the data is constant. Good object oriented programming style calls for any changes to an object to be done via a method call Normally a data's object is hidden (encapsulated) and if I am using an object I don't know or care about the details of that implementation AP Computer Science Designing and Implementing Classes 9

Object Methods Object methods either – tell you something about the object accessors, have a return value the charAt and length methods in String – change the object in some way mutators, no return value. Changes the state of the object the setSize method in Rectangle – do both, some people consider this bad style mutators that return a value, return value and a change to the object's state AP Computer Science Designing and Implementing Classes 10

Finding Object Methods JavaTM 2 Platform, Standard Edition, v 1.4.x API Specification – Your best friend in the world http://java.sun.com/j2se/1.4.1/docs/api/ Has specification for all classes in the Java standard library Only documents the public portion, what we can use! A product of javadoc java source code in - easy to read html out AP Computer Science Designing and Implementing Classes 11

Classes fundamental unit of OO programming Java programs of any significance are a collection of classes This is a rather smallish program and it contains 6 classes not counting another 6 classes from the Java Standard Library AP Computer Science Designing and Implementing Classes 12

What are Classes? classes are blueprints – they specify what new objects will contain and what can be done to the objects – classes are analogous to data types and objects are the variables of that type classes consist of an interface (what is it?) and an implementation (how it is done?) – interface describes the ways an object of this type may be created via constructors and how a client may use or manipulate an object of this type via methods – implementation is hidden and of no concern to the client. implementation can be changed without affecting any client code AP Computer Science Designing and Implementing Classes 13

More on Classes Each class in Java is contained in its own file with the name matching the class name and with the .java extension The basics elements of a class are – constructors used in the creation of objects. Similar to methods (procedures and functions). Normally several overloaded for the convenience of the class users – methods instance methods and class (static) methods – variables (or fields) instance variables and class variables (static) – constants instance (rare) and class (static) AP Computer Science Designing and Implementing Classes 14

The Die Class Consider a class used to model a die What is the interface? What actions should a die be able to perform? The methods or behaviors can be broken up into constructors, mutators, accessors AP Computer Science Designing and Implementing Classes 15

The Die Class Interface Constructors (used in creation of objects) – default, single int parameter to specify the number of sides, int and boolean to determine if should roll Mutators (change state of objects) – roll Accessors (do not change state of objects) – getTopSide, getNumSides, toString Public constants – DEFAULT SIDES AP Computer Science Designing and Implementing Classes 16

Visibility Modifiers All parts of a class have visibility modifiers – Java keywords – public, protected, private, (no modifier means package access) – do not use these modifiers on local variables (syntax error) public means that constructor, method, or field may be accessed outside of the class. – part of the interface – constructors and methods are generally public private means that part of the class is hidden and inaccessible by code outside of the class – part of the implementation – data fields are generally private AP Computer Science Designing and Implementing Classes 17

The Die Class Implementation Implementation is made up of constructor code, method code, and private data members of the class. scope of data members / instance variables – private data members may be used in any of the constructors or methods of a class Implementation is hidden from users of a class and can be changed without changing the interface or affecting clients (other classes that use this class) – Example: Previous version of Die class, DieVersion1.java Once Die class completed can be used in anything requiring a Die or situation requiring random numbers between 1 and N – DieTester class. What does it do? AP Computer Science Designing and Implementing Classes 18

DieTester method public static void main(String[] args) { final int NUM ROLLS 50; final int TEN SIDED 10; Die d1 new Die(); Die d2 new Die(); Die d3 new Die(TEN SIDED); final int MAX ROLL d1.getNumSides() d2.getNumSides() d3.getNumSides(); for(int i 0; i NUM ROLLS; i ) { d1.roll(); d2.roll(); System.out.println("d1: " d1.getResult() " d2: " d2.getTopSide() " Total: " (d1.getTopSide() d2.getTopSide() ) ); } AP Computer Science Designing and Implementing Classes 19

DieTester continued int total 0; int numRolls 0; do { d1.roll(); d2.roll(); d3.roll(); total d1.getTopSide() d2.getTopSide () d3.getTopSide (); numRolls ; } while(total ! MAX ROLL); System.out.println("\n\nNumber of rolls to get " MAX ROLL " was " numRolls); AP Computer Science Designing and Implementing Classes 20

The Steps of Class Design Requirements – what is the problem to be solved – detailed requirements lead to specifications Nouns may be classes Verbs signal behavior and thus methods (also defines a classes responsibilities) walkthrough scenarios to find nouns and verbs implementing and testing of classes design rather than implementation is normally the hardest part – planning for reuse AP Computer Science Designing and Implementing Classes 21

Correctness Sidetrack When creating the public interface of a class give careful thought and consideration to the contract you are creating between yourself and users (other programmers) of your class Use preconditions to state what you assume to be true before a method is called – caller of the method is responsible for making sure these are true Use postconditions to state what you guarantee to be true after the method is done if the preconditions are met – implementer of the method is responsible for making sure these are true AP Computer Science Designing and Implementing Classes 22

Precondition and Postcondition Example /* pre: sides 1 post: getTopSide() 1, getNumSides() sides */ public Die(int numSides) { if(numSides 1) throw new IllegalArgumentException("numSides is " numSides ". numSides must be 1"); iMyNumSides numSides; iMyResult 1; assert getTopSide() 1 && getNumSides() numSides; } AP Computer Science Designing and Implementing Classes 23

Object Behavior - Instantiation Consider the DieTester class Die d1 new Die(); Die d2 new Die(); Die d3 new Die(10); When the new operator is invoked control is transferred to the Die class and the specified constructor is executed, based on parameter matching Space(memory) is set aside for the new object's fields The memory address of the new object is passed back and stored in the object variable (pointer) After creating the object, methods may be called on it. AP Computer Science Designing and Implementing Classes 24

Creating Dice Objects memory address d1 DieTester class. Sees interface of Die class memory address d2 memory address d3 AP Computer Science a Die object 6 1 iMyResult Die class. Sees a Die object implementation. (of Die class.) iMySides 6 iMySides 1 iMyResult a Die object 10 iMySides 1 iMyResult Designing and Implementing Classes 25

Objects Every Die object created has its own instance of the variables declared in the class blueprint private int iMySides; private int iMyResult; thus the term instance variable the instance vars are part of the hidden implementation and may be of any data type – unless they are public, which is almost always a bad idea if you follow the tenets of information hiding and encapsulation AP Computer Science Designing and Implementing Classes 26

Complex Objects What if one of the instance variables is itself an object? add to the Die class private String myName; memory address d1 a Die object 6 1 memory address iMySides iMyResult myName d1 can hold the memory address of a Die object. The instance variable myName inside a Die object can hold the memory address of a String object AP Computer Science Designing and Implementing Classes a String object implementation details not shown 27

The Implicit Parameter Consider this code from the Die class public void roll() { iMyResult ourRandomNumGen.nextInt(iMySides) 1; } Taken in isolation this code is rather confusing. what is this iMyResult thing? – – – – – It's not a parameter or local variable why does it exist? it belongs to the Die object that called this method if there are numerous Die objects in existence Which one is used depends on which object called the method. AP Computer Science Designing and Implementing Classes 28

The this Keyword When a method is called it may be necessary for the calling object to be able to refer to itself – most likely so it can pass itself somewhere as a parameter when an object calls a method an implicit reference is assigned to the calling object the name of this implicit reference is this this is a reference to the current calling object and may be used as an object variable (may not declare it) AP Computer Science Designing and Implementing Classes 29

this Visually // in some class other than Die Die d3 new Die(); d3.roll(); memory address d3 // in the Die class public void roll() { iMyResult ourRandomNumGen.nextInt(iMySides) 1; /* OR this.iMyResult a Die object */ memory } address this AP Computer Science 6 iMySides Designing and Implementing Classes 1 iMyResult 30

An equals method working with objects of the same type in a class can be confusing write an equals method for the Die class. assume every Die has a myName instance variable as well as iMyNumber and iMySides AP Computer Science Designing and Implementing Classes 31

A Possible Equals Method public boolean equals(Object otherObject) { Die other (Die)otherObject; return iMySides other.iMySides && iMyResult other.iMyResult && myName.equals( other.myName ); } Declared Type of Parameter is Object not Die override (replace) the equals method instead of overload (present an alternate version) – easier to create generic code we will see the equals method is inherited from the Object class access to another object's private instance variables? AP Computer Science Designing and Implementing Classes 32

Another equals Methods public boolean equals(Object otherObject) { Die other (Die)otherObject; return this.iMySides other.iMySides && this.iMyNumber other.iMyNumber && this.myName.equals( other.myName ); } Using the this keyword / reference to access the implicit parameters instance variables is unnecessary. If a method within the same class is called within a method, the original calling object is still the calling object AP Computer Science Designing and Implementing Classes 33

A "Perfect" Equals Method From Cay Horstmann's Core Java public boolean equals(Object otherObject) { // check if objects identical if( this otherObject) return true; // must return false if explicit parameter null if(otherObject null) return false; // if objects not of same type they cannot be equal if(getClass() ! otherObject.getClass() ) return false; // we know otherObject is a non null Die Die other (Die)otherObject; return iMySides other.iMySides && iMyNumber other.iMyNumber && myName.equals( other.myName ); } AP Computer Science Designing and Implementing Classes 34

the instanceof Operator instanceof is a Java keyword. part of a boolean statement public boolean equals(Object otherObj) { if otherObj instanceof Die { //now go and cast // rest of equals method } } Should not use instanceof in equals methods. instanceof has its uses but not in equals because of the contract of the equals method AP Computer Science Designing and Implementing Classes 35

Class Variables and Class Methods Sometimes every object of a class does not need its own copy of a variable or constant The keyword static is used to specify class variables, constants, and methods private static Random ourRandNumGen new Random(); public static final int DEFAULT SIDES 6; The most prevalent use of static is for class constants. – if the value can't be changed why should every object have a copy of this non changing value AP Computer Science Designing and Implementing Classes 36

Class Variables and Constants the Die class 6 memory address DEFAULT SIDES ourRandNumGen All objects of type Die have access to the class variables and constants. a Random object implementation details not shown A public class variable or constant may be referred to via the class name. AP Computer Science Designing and Implementing Classes 37

Syntax for Accessing Class Variables public class UseDieStatic { public static void main(String[] args) { System.out.println( "Die.DEFAULT SIDES " Die.DEFAULT SIDES ); // Any attempt to access Die.ourRandNumGen // would generate a syntax error Die d1 new Die(10); System.out.println( "Die.DEFAULT SIDES " Die.DEFAULT SIDES ); System.out.println( "d1.DEFAULT SIDES " d1.DEFAULT SIDES ); // regardless of the number of Die objects in // existence, there is only one copy of DEFAULT SIDES // in the Die class } // end of main method } // end of UseDieStatic class AP Computer Science Designing and Implementing Classes 38

Static Methods static has a somewhat different meaning when used in a method declaration static methods may not manipulate any instance variables in non static methods, some object invokes the method d3.roll(); the object that makes the method call is an implicit parameter to the method AP Computer Science Designing and Implementing Classes 39

Static Methods Continued Since there is no implicit object parameter sent to the static method it does not have access to a copy of any objects instance variables – unless of course that object is sent as an explicit parameter Static methods are normally utility methods or used to manipulate static variables ( class variables ) The Math and System classes are nothing but static methods AP Computer Science Designing and Implementing Classes 40

static and this Why does this work (added to Die class) public class Die { public void outputSelf() { System.out.println( this ); } } but this doesn't? public class StaticThis { public static void main(String[] args) { System.out.println( this ); } } AP Computer Science Designing and Implementing Classes 41