1 Object Oriented Programming Lecture II Classes, Objects, Abstract Data Types, Representation Invariants, Mutables and Immutables

2 Terminology of Classes and Objects The Class construction –The structure for representation of objects of similar kind Methods – how to compute on data (Algorithms) Constructor – init the state of an object Variables – the state of an object (Data) Objects –An object has a unique identity, a state and a representation We instantiate objects out of classes and use them together in a program

3 Definition of Object Equality Objects are equal when they have the same state (usually comparing variables) Objects are identical when they share class identity For objects, the expression r1 == r2 tests identity, NOT equality –identity comparison made by ’ == ’ operator in Java Equality is tested by the equals() method –object equality is defined by the programmer

4 References in Java Java uses message passing –all objects are passed by reference –but primitive types are passed by value Advice –be careful when objects and especially data collections are shared (such ”problems” will appear in the lab project)

5 Wrapper Classes What is a wrapper and why is it needed? –Primitives are not objects, therefore object oriented techiques cannot be applied on primitive types –Wrapper classes are ”type” containers for primitives Examples are: Integer, Float, Byte... etc. –These are all of immutable type, and arithmetic operators can not directly be applied –To calculate, we need to unwrap... and then wrap again. If a, b, c are Integer type, then c = new Integer(a.intValue() + b.intValue());

6 Accessing Object Internals The state of an object can be mutable or immutable. –mutable, when internal values (variables) can be changed –immutable, when internal values are not changed Accessors –methods that doesn’t change any values, only return values public int getVal() {return value;} Mutators –Methods that can change the value of the state variables public void setVal(int scalar){value = scalar;}

7 Example: The Point Class Public Class Point{ int x,y; Point(int x1,int y1){ x = x1, y = y1; } public void move(int x1,int y1){ x = x1; y = y1; } public int getX(){return x;} public int getY(){return y;} }

8 Example Cnt’d Public Class Point{ int x,y; Point(int x1,int y1){ x = x1, y = y1; } public void move(int x1,int y1){ x = x1; y = y1; } public int getX(){return x;} public int getY(){return y;} public boolean repOk(){x != null && y != null;} } State representation State initializer Mutator Accessors

9 Representation Invariant The representation invariant of a class is –a state condition that always is guaranteed to be legal, for all object instances, whenever the object is in its stable state Conditions for the representation invariant –1. The invariant must be established when creating a new object –2. Mutators must always preserve the invariant For simple testing of an invariant –implement a method boolean repOk()

10 Abstract Data Types Consider the following problem: –we need to express and compute on rational numbers like: 1/3, 2/3, 11/6...etc. –we want to be able to formulate arithmetic expressions, such as 1/3+2/3 = 3/3 (= 1) 2/3+4/5 = (2*5/3*5) + (4*3/5*3) = 22/15 Question: How can we do this?

11 The Abstract Type Rational Constructors: Rational(), Rational(int), Rational(int,int); Arithmetic operations: plus(Rational a, Rational b) minus(Rational a, Rational b) div(Rational a, Rational b) times(Rational a, Rational b) Other methods: toString(),equals(Object x),compareTo(Object x)

12 Computing with Rational public static void main(String[] args){ Rational sum = 0; for(int i;i<100;i++){ sum += 1/i; } System.out.println(sum); } We need to define toString(); We can’t overload objects with primitive types!

13 A Second Try with Rational Public static void main(String[] args){ Rational sum = new Rational(); for(int i = 1;i<100;i++){ sum = Rational.sum(sum,new Rational(1,i)); } System.out.println(sum); }

14 Representation Invariant for Rational Before implementation, we need to make a few decisions –1. What are legal values for a Rational? all integer fractions a/b, where b ≠ 0 –2. We also decide to allow Rationals to have either positive or negative How do we handle sign representation? –3. Let the numerator carry the (-) sign

15 Lets do some implementation... Public Class Rational{ private num,den; public Rational(){ this(0,1); } public Rational(int num){ this.(num,1); } public Rational(int num, int den){ this.num = num; this.den = den; } The state of Rational is ”hidden” (private) Hey, wait a minute...

16 Taking Care of the Invariant Public Rational(int num,int den){ if(den == 0){ throw new ArithmeticException(”zero denominator is illegal”); }else{ this.num = den<0 ? –num:num; this.den = Math.abs(den); simplify(); } Numerator carries the sign! Auxillary method for deriving canonical representation of Rationals

17 What about this Simplify thing? Simplify() is a private help method to compute the Greates Common Divisor (GCD) of a rational number. Private void simplify(){ int d = gcd(num,den); num = num/d; den = den/d; } Private static gcd(int a, int b){ if (a==0) return b; else return gcd(b%a,a); }

18 Static Methods We want to use Rational Arithmetic anywhere in our programs. A solution is to make methods static. public static Rational plus(Rational a, Rational b){ return new Rational(...); } Rational Arithmetics can now be used anywhere without instantiating Rational.

19 Extending Our Number Represenation What about complex numbers? A complex number has two components – a real and an imaginary part Interesting observation, each component can be expressed by a rational...

20 Designing Complex Class What operators do we need to support? –{+, -, *, /} What is the representation invariant? –real and imaginary component ≠ null When are two complex numbers equal? –We define this to be when magnitude and direction of a = magnitude and direction of b

21 Complex Interface Constructors: Complex(); Complex(Rational re, Rational im); Arithmetic operations: plus(Complex a, Complex b); minus(Complex a, Complex b); times(Complex a, Complex b); div(Complex a, Complex b); Auxillary: conjugate(Complex c); Helpful when computing division!! Overload the arithmetic operators in Rational

22 The Complex Constructor class Complex{ private Rational real; private Rational imag; Complex(){ this(new Rational(), new Rational()); } Complex(Rational real, Rational imag){ this.real = real; this.imag = imag; } ?

23 Complex Arithmetic

24 Exercise 1 & 2 1. Implement the Class Rational 2. Implement the Class Complex