Department of Computer Engineering Creating Classes 2140101 Computer

Department of Computer Engineering Creating Classes 2140101 Computer Programming for International Engineers 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY

Department of Computer Engineering Objectives Students should: • Recall the meaning of classes and objects in Java • Know the components in the definition of a Java class • Understand how constructors work • Be able to create class and object methods • Be able to create new Java classes and use them correctly 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 2

Department of Computer Engineering Define Your Own Data Type • There are two categories of data in Java – primitive data type – class • We cannot create a new primitive data type. • they can create new data types – by creating new classes containing attributes and behaviors of the desired data types. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 3

Department of Computer Engineering Define Your Own Data Type • Creating a new class – write a class definition associated with that class in specific Java syntax. – save the definition in a separated. java file named after the name of the class. • Once the definition is created, other programs can utilize the newly created data type or class in a similar fashion to the primitive data types or other existing classes. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 4

Department of Computer Engineering Example • we would like to create a new data type for representing points in a Cartesian co-ordinate – create a new class called My. Point. public class My. Point } //a blank class definition //there’re no details yet { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 5

Department of Computer Engineering Example • This definition has to be saved using the name My. Point. java. • we can write another program that makes use of this class. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 6

Department of Computer Engineering Example public class Test. My. Point 1 { public static void main(String[] args) { My. Point p, q; p = new My. Point(); q = new My. Point(); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 1 2 3 4 5 6 7 8 9 7

Department of Computer Engineering Example • variables p and q are declared as variables of the type My. Point on line 5. My. Point p, q; • On line 6 and line 7, p and q are assigned with, or in other words, are made to refer to, new instances, or objects, of the class My. Point using the keyword new. p = new My. Point; () q = new My. Point; () 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 8

Department of Computer Engineering Define Your Own Data Type • Notice that source codes of Java programs that we have written so far, they take the same structure as class definitions. • They are in fact class definitions. • Java programs are classes that contain the methods named main() which make the class executable. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 9

Department of Computer Engineering Components of Class Definitions • The main functionality of a class definition is to define attributes and behaviors of that class. • Attributes are entities defining properties of an object. • Behaviors are actions (or reactions) of an object. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 10

Department of Computer Engineering example attributes and behaviors of some objects. Object type Point in a 2 D space Attributes • The x coordinate • The y coordinate • etc. Behaviors • Moving the point a specified location • Calculating distance from the point to a specified location • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 11

Department of Computer Engineering example attributes and behaviors of some objects. Object type Graphical line in a 3 D space Attributes • Location of the starting • point • Location of the ending • point • Color • etc. Behaviors • Calculating the length of the line • Moving the starting point to a specified location • Moving the ending point to a specified location • Changing the color of the line • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 12

Department of Computer Engineering example attributes and behaviors of some objects. Object type Complex number Attributes Behaviors • Value of the real part • Value of the imaginary part • etc. • Adding the object with another complex object, • Multiplying the object with another complex object • Finding the conjugate of the object • Setting the real part to a specific number • Showing String representation of the object • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 13

Department of Computer Engineering example attributes and behaviors of some objects. Object type Attributes Matrix • Members • etc. Behaviors • Adding elements to the object • Finding determinant • Adding the object with another matrix object • Finding the inverse of the object • Raising to the power of n • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 14

Department of Computer Engineering example attributes and behaviors of some objects. Object type Car Attributes • Body color • Dimensions • Weight • Number of doors • Manufacturer • Engine status • etc. Behaviors • Starting the engine • Shutting down the engine • Showing the name of its manufacturer • Accelerating • Decelerating • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 15

Department of Computer Engineering example attributes and behaviors of some objects. Object type Bank account Attributes • Account name • Owner • Account type • Balance • etc. Behaviors • Showing the current balance • Showing all info associated with the account • Withdrawing money from the account • Depositing to the account • Closing the account • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 16

Department of Computer Engineering example attributes and behaviors of some objects. Object type Customer Attributes • Customer ID • First name • Family name • Credit line • Gender • Favorite products • etc. Behaviors • Showing all info of the customer • Changing the credit line • Checking whether the customer’s favorite product consists of a • specified product • etc. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 17

Department of Computer Engineering Components of Class Definitions • To describe attributes and behaviors of objects of the class, a class definition can consist of – data member or fields – methods – constructors 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 18

Department of Computer Engineering Components of Class Definitions • An object’s attribute is represented using a data member. • Variables used for storing data members are called instance variables. • The behaviors of an object are described using methods. • Constructors are special methods invoked whenever objects of the class are created. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 19

Department of Computer Engineering Components of Class Definitions 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 20

Department of Computer Engineering Data Members • Instance variables are used for storing data members. – can be declared, and possibly initialized, using the same syntax used with variables in methods – such as int x; , String s; , double [] d = {1. 0, 2. 0}; , and etc. • Furthermore, modifiers determining access rights can be used to determine which classes are allowed to access the data members in those instance variables. – public, private, and protected 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 21

Department of Computer Engineering Example public class My. Point } public double x; public double y; { • an object of the class My. Point can have two double values representing the x-coordinate and the y-coordinate of the point represented by that object. • The modifier public identifies that anyone can access the two instance variables using the dot operator. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 22

Department of Computer Engineering Example public class Test. My. Point 2 1 } 2 public static void main(String[] args) 3 { 4 My. Point p = new My. Point(); 5 My. Point q = new My. Point(); 6 p. x = 2; 7 p. y = 3; 8 q. x = 0. 5; 9 q. y = -0. 5; 10 System. out. println(“(”+p. x”+“, ”+p. y+“)”); 11 System. out. println (“(”+q. x”+“, ”+q. y+“)”); 12 } 13 } 14 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 23

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 24

Department of Computer Engineering Example • On line 5 and line 6, the variables named p and q are created. – Each of them is made to refer to a new My. Point object. My. Point p = new My. Point(); My. Point q = new My. Point(); • On line 7 and line 8, the instance variable x of the object referred to by p is set to 2 while y is set to 3. p. x = 2; p. y = 3; 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 25

Department of Computer Engineering Example • On line 9 and line 10, the instance variable x of the object referred to by q is set to 0. 5 while y is set to -0. 5. q. x = 0. 5; q. y = -0. 5; • The code on line 11 and line 12 print the output on the screen. – They use the values of x and y in both objects through p. x, p. y, q. x, and q. y. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 26

Department of Computer Engineering Example • if we change the class definition of My. Point to: public class My. Point } private double x; private double y; { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 27

Department of Computer Engineering Example • Compiling Test. My. Point 2. java again will lead to compilation errors 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 28

Department of Computer Engineering Example • The modifier private makes instance variables private to the class they are declared. – That means the instance variables can be used or accessed by that class or in the class definition of that class only. • Errors occur whenever the private instance variables x and y of any instances of My. Point are accessed directly by other classes. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 29

Department of Computer Engineering Example • In this case, the class trying to access those variables is Test. My. Point 2. • The modifier private allows the creator of the class to hide data members from the outside world. • Doing this is crucial to the data encapsulation concept in Object-Oriented Programming (OOP). (not intend to elaborate on OOP concepts in this course. ) 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 30

Department of Computer Engineering Protected • the modifier protected. – Protected elements cannot be access by any classes other than the class they are declared and their subclasses. (will be discussed in the next chapter. ) – The default access level for Java is protected. – That means if no access level is specified, it is, by default, protected. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 31

Department of Computer Engineering Data members • Data members of a class can be objects of other classes. – both standard and user-defined. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 32

Department of Computer Engineering Example • let’s suppose we would like to create a class representing polygons – each of which has its associated text label. • We might decide that its data members include – an array of My. Point objects for storing the location of every vertex of the polygon. – a String object representing the label. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 33

Department of Computer Engineering Example public class My. Labelled. Polygon } private My. Point [] vertices; private String label; //. . . other elements are omitted. . . } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 34

Department of Computer Engineering Static and Non-static Data Members • Data members can be either static or nonstatic. • Non-static data members – are attributes of instances of the class – each instance of the class has its own copy of nonstatic data members – Data members are non-static by default. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 35

Department of Computer Engineering Static Data Members • static data members – are attributes of the class itself. – static data members are shared among every instances of the class. – To make a data member static, we use the modifier static in front of the declaration of variables storing the data members. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 36

Department of Computer Engineering Static Data Members • we will not call variables storing static data members instance variables since the variables are not the attributes of any specific instances but they are shared among every instances. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 37

Department of Computer Engineering Example public class L 11 A { public static int i; public int j; } 1 2 3 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 4 5 38

Department of Computer Engineering Example public class Static. Data. Member. Demo { public static void main(String[] args) { L 11 A x = new L 11 A(); L 11 A y = new L 11 A(); L 11 A z = new L 11 A(); x. j = 5; 8 y. j = 10; z. j = 15; 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 1 2 3 4 5 6 7 9 10 39

Department of Computer Engineering Example System. out. println("x. j = "+x. j); System. out. println("y. j = "+y. j); System. out. println("z. j = "+z. j); x. i = 0; 14 y. i++ ; z. i + =3; System. out. println("x. i = "+x. i); System. out. println("y. i = "+y. i); System. out. println("z. i = "+z. i); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 11 12 13 15 16 17 18 19 20 21 40

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 41

Department of Computer Engineering Example L 11 A x = new L 11 A(); L 11 A y = new L 11 A(); L 11 A z = new L 11 A(); x. j = 5; 8 y. j = 10; z. j = 15; 9 10 5 6 7 three instances of L 11 A are created and referred to by x, y andz. • the values of 5, 10, and 15 are assigned to the instance variables j belonging to the objects referred to by x, y, and z, respectively. • These objects do not share the value of j. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 42

Department of Computer Engineering Example x. i = 0; 14 y. i++; z. i += 3; 15 16 • the variable i is shared by the three objects. • The statement x. i = 0 assign 0 to i. • y. i and z. i are also 0 since they refer to the same thing. • i can be modified via any objects. • Therefore, we can see that the resulting value of i, shared by x, y and z, is 4. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 43

Department of Computer Engineering Methods • Methods describe behaviors of objects of the class. • In Chapter 5, we also mentioned that there were two types of methods: – static (class) methods and non-static (instance) methods. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 44

Department of Computer Engineering Static and Non-Static Method • non-static method – it can be invoked by other classes via the instance name of an object of that class using the dot operator. – A (public) method defined in a class definition is by default non-static • static methods – To make a method static, the keyword static is put in the method header. – This way, the method can be invoked using the dot operator with the name of the class. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 45

Department of Computer Engineering Methods )public|private|protected) (static) return. Type method. Name(argument. List}( method. Body { • An access level modifier – It determines whether which classes can make use of this method. – The access levels specified by public, private, and protected – If this modifier is not specified, the default access level is protected. • The keyword static – makes the method static, or a class method. – If omitted, the method is considered as non-static, or an instance method. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 46

Department of Computer Engineering Methods • The other parts of the method definition are the same as what we discussed in Chapter 8. • We can define as many methods as we would like in the class definition. • If the definition contains a public method named main(), the class can be executed. • In other words, the class is in fact a Java program. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 47

Department of Computer Engineering Accessor , Mutator Methods • Typically, in OOP, data members in a class are defined as private to prevent users of the class accessing the data members directly. • the creator of the class usually provides public methods for reading or changing some data members. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 48

Department of Computer Engineering Accessor , Mutator Methods • accessor methods – The methods provided for other classes to read the values of data members. • mutator methods – The methods provided for changing the values of data members. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 49

Department of Computer Engineering to. String() • to. String() – Whenever an object of a class needs to be converted to its String representation, Java automatically calls a specific method called to. String. () – In order to provide a meaningful String representation of the class we create, it is sensible to provide the method named exactly as to. String() that returns the String representation we want. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 50

Department of Computer Engineering Example public class My. Point { //data members private double x; private double y; //accessor methods public double get. X(){ return x; } public double get. Y(){ return y; } 1 2 3 4 5 6 7 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 8 9 10 11 12 13 14 51

Department of Computer Engineering Example //mutator methods public void set. X(double x){ this. x = x; } public void set. Y(double y){ this. y = y; } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 15 16 17 18 19 20 21 22 52

Department of Computer Engineering Example //other methods public void set. Location(double x, double y){ 24 this. x = x; this. y = y; } public double distance. To(My. Point p){ 28 double diff. XSquare = Math. pow((p. get. X()-x), 2); double diff. YSquare = Math. pow ((p. get. Y()-y), 2); return Math. sqrt(diff. XSquare+diff. YSquare); } public String to. String(){ 33 return "("+x+", "+y+")"; } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 23 25 26 27 29 30 31 32 34 35 36 53

Department of Computer Engineering Example • The methods get. X() and get. Y() declared on line 8 and line 11 – allows other classes to read the values of the private variables x and y, respectively. – These are accessor methods. • The methods set. X() and set. Y() declared on line 16 and line 19 – allows other classes to set the values of the private variables x and y. – These are mutator methods. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 54

Department of Computer Engineering Example • the usage of this. – this is a reference used for referring to the current instance of the class. – On line 17 and line 20, this. x and this. y refer to the instance variables x and y of the current instance • i. e. the instance from which the methods are invoked. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 55

Department of Computer Engineering Example public class Test. My. Point 3 { public static void main(String[] args) { My. Point p = new My. Point(); My. Point q = new My. Point(); p. set. X(6. 0); p. set. Y(5. 0); q. set. Location(p. get. X(), p. get. Y()); System. out. println("q="+q); p. set. Location(10. 0, 2. 0); System. out. print("Distance from "+p+" to "); System. out. println(q+" is "+p. distance. To(q)); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 56

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 57

Department of Computer Engineering Example • On line 7 and 8 p. set. X(6. 0); p. set. Y(5. 0); 7 8 – On line 7, set. X() is invoked from p, This set the value of x belonging to the My. Point object referred to by p to the value input to the method. – the value of y belonging to the My. Point object referred to by p is set to 5. 0 on line 8. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 58

Department of Computer Engineering Example • On line 9 q. set. Location(p. get. X(), p. get. Y()); 9 – p. get. X() and p. get. Y() return the value of the instance variables x and y belonging to the My. Point object referred to by p. – These values are used as input parameters to set. Location() invoked from q. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 59

Department of Computer Engineering Example • Whenever the String representation of a My. Point object is needed , – for example in argument lists of print() and println() on line 10, line 12, and line 13, – to. String() of that object is invoked. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 60

Department of Computer Engineering Static Method • Static methods are also useful when we would like to build a class providing useful functionalities to be used by other classes or programs. – such as the standard Math class. • Such a class is not commonly instantiated, or in other words, it is not common to create an object of such a class. • Therefore, the functionalities are provided through its public static methods. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 61

Department of Computer Engineering Example public class My. Int. Array. Util { public static int[] create. Random. Elements(int n, int min, int max){ int[] a = new int[n]; for(int i=0; i<n; i++){ a[i] = (int)Math. round(Math. random()*(max-min)+min); } return a; } public static void show. Elements(int[]a) 10 { System. out. print(“[”+a[0]); for(int i=1; i<a. length; i++){ System. out. print(“, ”+a[i]); } System. out. print(“]n”); } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 62

Department of Computer Engineering Example public static int [] remove. At(int [] a, int n){ if(n<0||n>a. length-1) return a; int[] b = new int[a. length-1]; for(int i=0; i<n; i++){ b[i]=a[i]; } for(int i=n+1; i<a. length; i++){ 24 b[i-1] = a[i]; } return b; } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 18 19 20 21 22 23 25 26 27 28 63

Department of Computer Engineering Example public static int [] insert. At(int [] a, int n, int k){ if(n<0 || n>a. length) return a; int [] b = new int[a. length+1]; for(int i=0; i<n; i++){ b[i] = a[i]; } b[n] = k; for(int i=n; i<a. length; i++){ b[i+1] = a[i]; } return b; } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 29 30 31 32 33 34 35 36 37 38 39 40 41 64

Department of Computer Engineering Example • The class My. Int. Array. Util created here contains four public static methods. • The first one defined on line 3 creates an int array of length n whose elements are integer randomly chosen from min to max, inclusively. public static int [] create. Random. Elements(int n, int min, int max){ 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 3 65

Department of Computer Engineering Example • The method defined on line 10 prints all elements of the input array on screen. public static void show. Elements(int [] a) 10 • The method defined on line 18 removes the element at a specified position. public static int [] remove. At(int [] a, int n){ 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 18 66

Department of Computer Engineering Example • Defined on line 29, the method inserts a given value to a specified position of the input array. public static int [] insert. At(int [] a, int n, int k){ 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 29 67

Department of Computer Engineering Example a = My. Int. Array. Util. insert. At(a, a. length, 1); My. Int. Array. Util. show. Elements(a); System. out. print("remove at 2: tt"); 15 16 17 a = My. Int. Array. Util. remove. At(a, 2); My. Int. Array. Util. show. Elements(a); System. out. print("remove at 0: tt"); 18 19 20 a = My. Int. Array. Util. remove. At(a, 0); My. Int. Array. Util. show. Elements(a); System. out. print("remove the last: t"); 21 22 23 a = My. Int. Array. Util. remove. At(a, a. length-1); My. Int. Array. Util. show. Elements(a); 24 25 26 27 } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 69

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 70

Department of Computer Engineering Constructors • Constructors are special methods invoked whenever an object of the class is created. • Constructors are defined in the same fashion as defining methods. • However, – constructors must have the same name as the class name – there must not be any return types specified at the header of the constructors – they have to be public. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 71

Department of Computer Engineering Constructors • Constructors are usually for initializing or setting instance variables in that class. • An example of a no-argument (no input) constructor for My. Point. public My. Point(){ x = 1. 0; y = 1. 0; } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 72

Department of Computer Engineering Example public class My. Point } //data members private double x; private double y; //constuctors public My. Point}() x = 1. 0; y = 1. 0; { ………… //Here, details are omitted………… public String to. String}() return "("+x+", "+y; "("+ { { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 73

Department of Computer Engineering Example • Once My. Point is defined this way, let’s observe the result of the following program. public class Test. My. Point 4 { public static void main(String[] args) { My. Point p = new My. Point(); System. out. println(p); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 74

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 75

Department of Computer Engineering Example • we can see that the values of x and y belonging to the My. Point object referred to by p are both 1. 0. • The values are set in the constructor when it is called due to the creation of a new My. Point instance. • It should be obvious now that operations that should be performed once an instance of the class is created can be put in a constructor. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 76

Department of Computer Engineering Constructors • Constructors can be overloaded just like methods. – A class can have multiple constructors with different input arguments. • Which constructor to be called when an instance of the class is created depends on the input arguments of the new statement. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 77

Department of Computer Engineering Example public class My. Point } //data members private double x; private double y; //constructors public My. Point}() x = 1. 0; y = 1. 0; System. out. println("My. Point() is called. "); } public My. Point(double x, double y){ this. x = x; this. y = y; System. out. println("My. Point(double, double) is called. "); } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 78

Department of Computer Engineering public My. Point(My. Point p}( x = p. get. X; () y = p. get. Y; () System. out. println("My. Point(My. Point) is called; (". { ………… //Here, details are omitted……………… public String to. String}() return "("+x+", "+y; "("+ { { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 79

Department of Computer Engineering Example • The first constructor, My. Point(), does not take any input arguments. public My. Point}() – it is called via the statement new Mypoint. () – Such a constructor is usually called a no-argument constructor. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 80

Department of Computer Engineering Example • The second constructor public My. Point(double x, double y}( – My. Point(double x, double y) is a constructor that takes two double values as its input. – It is called via the statement new Mypoint(a, b , ( • where a and b are any double values. – This constructor initializes the instance variables to the input values. – Such a constructor that requires the values of the instance variables as its input is usually referred to as a detailed Constructor. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 81

Department of Computer Engineering Example • The last constructor public My. Point(My. Point p}( – This constructor is invoked as a response to the statement new Mypoint(c( • where c is an instance of My. Point. – the value of x is set to the value of x from the instance of My. Point supplied as the input to the constructor – the value of y is set to the value of y from the same instance. – Such a constructor that copies all attributes from the input instance is usually referred to as a copy constructor. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 82

Department of Computer Engineering Example public class Test. My. Point 5 } public static void main(String[] args( } My. Point p = new My. Point; () System. out. println("p-->"+p; ( My. Point q = new My. Point(2. 0, 5. 0; ( System. out. println("q-->"+q; ( My. Point r = new My. Point(q; ( System. out. println("r-->"+r; ( { { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 83

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 84

Department of Computer Engineering Constructors • When there is no constructor provided, Java automatically adds a default no-argument constructor, inside which all variables in the class are initialized with default values based on their data types. – zero for numeric data type, false for boolean, and null for non-primitive types • However, if there is at least one constructor defined in the class, the default noargument will not be added automatically. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 85

Department of Computer Engineering Example • The following code runs fine since the compiler automatically adds a default noargument constructor 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 86

Department of Computer Engineering Example public class L 11 C { private int a; public int get. A(){ return a; } } public class Test. L 11 C { public static void main(String[] args) { L 11 C x = new L 11 C(); System. out. println(x. get. A()); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 87

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 88

Department of Computer Engineering Example • However, the following code leads to compilation error since the compiler cannot find any constructors for new L 11 C(). • The compiler does not add a default noargument constructor automatically since a constructor has already been defined. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 89

Department of Computer Engineering Example public class L 11 D } private int a; public L 11 D(int a){ this. a = a; } public int get. A(){ return a; } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 90

Department of Computer Engineering Example public class Test. L 11 D { public static void main(String[] args) { L 11 D x = new L 11 D(); System. out. println(x. get. A()); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 91

Department of Computer Engineering Example 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 92

Department of Computer Engineering Calling a Constructor from Other Constructors • A constructor can be invoked within another constructor using this([argument list([ – where [argument list] is the list of arguments corresponding to the argument list of the constructor to be called. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 93

Department of Computer Engineering Calling a Constructor from Other Constructors • Given a detailed constructor, other constructors can be implemented by purely calling the detailed constructor. • If the invocation of a constructor via this() statement is used, the statement must be the first statement in the constructor. – Otherwise, it will lead to a compilation error. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 94

Department of Computer Engineering Calling a Constructor from Other Constructors //constructors public My. Point}() this(1. 0, 1. 0); } public My. Point(double x, double y}( this. x = x; this. y = y; { public My. Point(My. Point p}( this(p. get. X(), p. get. Y; (() { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 95

Department of Computer Engineering Example 1 public class L 11 B { private int a, b, c; public L 11 B(){ this(1, 2, 3); System. out. println("Inside L 11 B()"); } public L 11 B(int a, int b, int c){ this. a = a; this. b = b; this. c = c; System. out. println("Inside L 11 B(int, int)"); } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 96

Department of Computer Engineering Example 1 public L 11 B(double a, double b, double c){ this((int)Math. round(a), (int)Math. round(b), (int)Math. round (c)); System. out. println("Inside L 11 B(double, double)"); } public L 11 B(L 11 B x){ this(x. a, x. b, x. c); System. out. println("Inside L 11 B(L 11 B)"); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 97

Department of Computer Engineering Example 1 • Now observe the program listed below public class Test. L 11 B 1 { 2 public static void main(String[] args) 3 { 4 System. out. println("n. Executing: L 11 B x = new L 11 B(); "); 5 L 11 B x = new L 11 B(); 6 System. out. println("n. Executing: L 11 B y = new L 11 B(1. 0, 1. 0); "); 7 L 11 B y = new L 11 B(1. 0, 1. 0); "); 8 System. out. println("n. Executing: L 11 B z = new L 11 B()); "); 9 L 11 B z = new L 11 B()); 10 } 11 } 12 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 98

Department of Computer Engineering Example 1 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 99

Department of Computer Engineering Example 2 • A complex number is of the form a+jb – where a and b are real numbers – j is a quantity representing. • We would like to define a new class for complex numbers. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 100

Department of Computer Engineering Example 2 • Complex numbers are added, subtracted, and multiplied by formally applying the associative, commutative and distributive laws of algebra, together with the equation j 2. = 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 101

Department of Computer Engineering Example 2 • The reciprocal or multiplicative inverse of a complex number can be written as: 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 102

Department of Computer Engineering Example 2 • Division between two complex numbers is defined as: • Complex conjugate of a complex number a+jb is a-jb, while the magnitude of a+jb is calculated by 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 103

Department of Computer Engineering Example 2 public class Complex } //attributes: (re) + j(im( private double re; private double im; //constructors public Complex}() this(0, 0); } public Complex(double r, double i){ re = r; im = i; } public Complex(Complex z){ this(z. get. Re(), z. get. Im()); } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 104

Department of Computer Engineering Example 2 //accessor methods public double get. Re}() return re; { public double get. Im}() return im; { //mutator methods public void set. Re(double r}( re = r; { public void set. Im(double i}( im = i; { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 105

Department of Computer Engineering Example 2 //other methods public Complex adds(Complex z}( return new Complex(re+z. get. Re(), im+z. get. Im; (() { public Complex subtracts(Complex z}( return new Complex(re-z. get. Re(), im-z. get. Im; (() { public Complex multiplies(Complex z}( double r = re*z. get. Re()-im*z. get. Im; () double i = im*z. get. Re()+re*z. get. Im; () return new Complex(r, i; ( { public Complex divides(Complex z}( return this. multiplies(z. mult. Inverse; (() { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 106

Department of Computer Engineering Example 2 public Complex mult. Inverse}() double den = Math. pow(this. magnitude(), 2; ( return new Complex(re/den, -im/den; ( { public Complex conjugate}() return new Complex(re, -im; ( { public double magnitude}() return Math. sqrt(re*re+im*im; ( { public String to. String}() if(im>=0) return re+"+j"+im; else return re+"-j"+(-im); } } 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 107

Department of Computer Engineering Example 2 • The following program shows the class Complex in action. 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 108

Department of Computer Engineering Example 2 public class Test. Complex } public static void main(String[] args( } Complex p = new Complex(1, 1; ( Complex q = new Complex(3, 4; ( System. out. println("p="+p+", q="+q; ( System. out. println("p+q="+p. adds(q; (( System. out. println("p-q="+p. subtracts(q; (( System. out. println("p*q="+p. multiplies(q; (( System. out. println("p/q="+p. divides(q; (( System. out. println("conjugate of p="+p. conjugate; (() System. out. println("magnitude of q="+q. magnitude; (() { { 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 109

Department of Computer Engineering Example 2 2140101 Computer Programming for International Engineers INTERNATIONAL SCHOOL OF ENGINEERING CHULALONGKORN UNIVERSITY 110