Java for Interfaces and Networks (DT3010, HT11)

Transcription

1 Java for Interfaces and Networks (DT3010, HT11) Introduction Federico Pecora School of Science and Technology Örebro University Federico Pecora Java for Interfaces and Networks Lecture 1 1 / 52

2 This course... Outline 1 This course... 2 What is Java? 3 Java applications 4 Syntax and Data types 5 Object Orientation in Java Federico Pecora Java for Interfaces and Networks Lecture 1 2 / 52

3 This course... Preliminary course plan Week 45 Week 46 Week 47 Introduction and Basics Classes, Interfaces and Packages Exceptions and Garbage Collection Applets Introduction to Swing Threads Networking Networking with Threads Container Classes Java Database Connectivity (JDBC) Week 48 Week 49 Week 50 Standard Methods Inner Classes JavaDoc Garbage Collection (cont) Swing (cont) Threads (cont) Mouse Events Timers Serialization Week 51-2 Self study Exam (week 2) Federico Pecora Java for Interfaces and Networks Lecture 1 3 / 52

4 This course... Course details Staff Labs Exam Federico Pecora (lectures) room T2222, tel Jonas Ullberg (teaching assistant) room T1219, tel project assignments these must be submitted you do not have to submit labs but attending and completing labs greatly facilitates assignments! written examination which involves designing a software application Federico Pecora Java for Interfaces and Networks Lecture 1 4 / 52

5 This course... Exam Details Exam will involve writing some code (mostly to complete an already given structure) 4 exercises, 30 points in total, 15 required to pass Points Swedish system 1: [16,22] = G, [23,30] = VG Points Swedish system 2: [15,19] = 3, [20,25] = 4, [26,30] = 5 Points ECTS: [15,17] = E, [18,20] = D, [21,23] = C, [24,27] = B, [28,30] = A Federico Pecora Java for Interfaces and Networks Lecture 1 5 / 52

6 What is Java? Outline 1 This course... 2 What is Java? 3 Java applications 4 Syntax and Data types 5 Object Orientation in Java Federico Pecora Java for Interfaces and Networks Lecture 1 6 / 52

7 What is Java? C, C++ and Java Let s compare a simple hello world program: hello.c 1 #include <stdio.h> 2 int main(int argc, char* argv[]) { 3 printf("hello, world!\n"); 4 return 0; 5 } /* main */ linux> gcc -Wall hello.c -o hello linux>./hello Hello, world! linux> Federico Pecora Java for Interfaces and Networks Lecture 1 7 / 52

8 What is Java? C, C++ and Java Let s compare a simple hello world program: hello.cpp 1 #include <iostream> 2 int main(int argc, char* argv[]) { 3 std::cout << "Hello, world!\n"; 4 } // main linux> g++ -Wall hello.cpp -o hello linux>./hello Hello, world! linux> Federico Pecora Java for Interfaces and Networks Lecture 1 7 / 52

9 What is Java? C, C++ and Java Let s compare a simple hello world program: Hello.java 1 public class Hello { 2 public static void main(string[] args) { 3 System.out.println("Hello, world!"); 4 } 5 } // Hello linux> javac Hello.java linux> java Hello Hello, world! linux> Federico Pecora Java for Interfaces and Networks Lecture 1 7 / 52

10 What is Java? C, C++ and Java Let s compare a simple hello world program: Hello.java 1 public class Hello { 2 public static void main(string[] args) { 3 System.out.println("Hello, world!"); 4 } 5 } // Hello linux> javac Hello.java linux> java Hello Hello, world! linux> javac (the compiler) does not produce an ordinary executable file! Federico Pecora Java for Interfaces and Networks Lecture 1 7 / 52

11 What is Java? Java files: *.java, *.class, *.jar Source code goes in <ClassName>.java javac compiles source into <ClassName>.class contains bytecode Class libraries can be located in filesystem as (.class files) or in ZIP or JAR files Federico Pecora Java for Interfaces and Networks Lecture 1 8 / 52

12 What is Java? Java Virtual Machine Java compiler does not compile source to machine instructions, rather it produces bytecode The Java Virtual Machine (JVM) interprets the bytecode Bytecode is minimal to facilitate transmission over a network Bytecode is totally portable a Java brogram compiled in Linux can run in Windows (etc.) Bytecode can be compiled into platform-specific code Federico Pecora Java for Interfaces and Networks Lecture 1 9 / 52

13 What is Java? Java Virtual Machine *.java *.class Java source code Java bytecode Java compiler JVM Bytecode compiler Machine code Federico Pecora Java for Interfaces and Networks Lecture 1 10 / 52

14 What is Java? Microsoft C# Hello.cs 1 World class Hello { 2 public static void Main() { 3 System.Console.WriteLine("Hello, world!"); 4 } 5 } Microsoft C# is very similar to Java One view is that C# (and the whole.net architecture), is as close as legally possible to Java... Federico Pecora Java for Interfaces and Networks Lecture 1 11 / 52

15 What is Java? C and C++ can be hazardous This program contains a serious flaw... helloname.c 1 #include <stdio.h> 2 int main(int argc, char* argv[]) { 3 printf("what s your name: "); 4 char* s; 5 gets(s); 6 printf("hello, %s!\n", s); 7 return 0; 8 } /* main */ linux>./helloname What s your name: Bob Hello, Bob! linux> Federico Pecora Java for Interfaces and Networks Lecture 1 12 / 52

16 What is Java? C and C++ can be hazardous This program contains a serious flaw... helloname.c 1 #include <stdio.h> 2 int main(int argc, char* argv[]) { 3 printf("what s your name: "); 4 char* s; 5 gets(s); 6 printf("hello, %s!\n", s); 7 return 0; 8 } /* main */ linux>./helloname What s your name: Bob with a very long name Hello, Bob with a very long name! Segmentation fault (core dumped) linux> Federico Pecora Java for Interfaces and Networks Lecture 1 12 / 52

17 What is Java? A simple program Read lengths of two sides of a rectangle Output area and perimeter Console text-based interface RectangleCalculator.java 1 import java.io.*; 2 class RectangleCalculator { 3 public static void main(string args[]) { 4 int a = 0, b = 0; String buf; 5 BufferedReader kbd_reader 6 = new BufferedReader( 7 new InputStreamReader(System.in)); 8 try { 9 System.out.print("Input side a: "); 10 buf = kbd_reader.readline(); 11 a = Integer.parseInt(buf); 12 //continues... Federico Pecora Java for Interfaces and Networks Lecture 1 13 / 52

18 What is Java? A simple program Read lengths of two sides of a rectangle Output area and perimeter Console text-based interface RectangleCalculator.java 1 //... continued 2 System.out.print("Input side b: "); 3 buf = kbd_reader.readline(); 4 b = Integer.parseInt(buf); 5 System.out.println("Area: " + a * b); 6 System.out.println("Perimeter: " + 2*a + 2*b); 7 } 8 catch (IOException e) { 9 System.out.println("Read error!"); 10 } 11 } // main 12 } // RectangleCalculator Federico Pecora Java for Interfaces and Networks Lecture 1 13 / 52

19 What is Java? A simple program This program contains a logical error... RectangleCalculator.java 1 //... 2 System.out.print("Input side a: "); 3 buf = kbd_reader.readline(); 4 a = Integer.parseInt(buf); 5 //... linux> javac RectangleCalculator.java linux> java RectangleCalculator Input side a: 5 Input side b: 4 Area: 20 Perimeter: 108 linux> Federico Pecora Java for Interfaces and Networks Lecture 1 14 / 52

20 What is Java? A simple program This program contains a logical error... RectangleCalculator.java 1 //... 2 System.out.print("Input side a: "); 3 buf = kbd_reader.readline(); 4 a = Integer.parseInt(buf); 5 //... linux> java RectangleCalculator Input side a: Kalle Anka Exception in thread "main" java.lang.numberformatexception: For input string: "Kalle Anka" at java.lang.numberformatexception.forinputstring( NumberFormatException.java:48) at java.lang.integer.parseint(integer.java:447) at java.lang.integer.parseint(integer.java:497) at RectangleCalculator.main(RectangleCalculator.java:15) linux> Federico Pecora Java for Interfaces and Networks Lecture 1 14 / 52

21 What is Java? A simple program... what would happen in C++? rectanglecalculator.cpp 1 //... 2 cout << "Input side a: "; 3 cin >> a; 4 cout << "Input side b: "; 5 cin >> b; 6 cout << "Area: " << a * b << ", " << 2*a + 2*b << "\n"; 7 //... linux> g++ rectanglecalculator.cpp -o rectanglecalculator linux>./rectanglecalculator Input side a: Kalle Anka Input side b: Area: 0, Perimeter: 0 linux> Federico Pecora Java for Interfaces and Networks Lecture 1 15 / 52

22 What is Java? A simple program Java provides exception handling capabilities, which are useful for detecting/handling run-time errors... but Java does not safeguard against all run-time contingencies! linux> java RectangleCalculator Input side a: Input side b: Area: Perimeter: linux> Federico Pecora Java for Interfaces and Networks Lecture 1 16 / 52

23 Java applications Outline 1 This course... 2 What is Java? 3 Java applications 4 Syntax and Data types 5 Object Orientation in Java Federico Pecora Java for Interfaces and Networks Lecture 1 17 / 52

24 Java applications The main method The main() method (as in C/C++) is invoked when you execute a class with the Java interpreter The runtime system starts by calling the class s main() method The controlling class of every Java application must contain a main() method having one of the following signatures public static void main(string[] args) public static void main(string args[]) Federico Pecora Java for Interfaces and Networks Lecture 1 18 / 52

25 Java applications The main method public static void main(string[] args) public static void main(string args[]) public: the method can be called by any object (we will discuss the keywords public, private, and protected in more detail) static: the method can be called without instantiating an object of the class (we will discuss class methods/variables) void: the method doesn t return any value args: an array of String objects, which contains command line arguments entered by the user Federico Pecora Java for Interfaces and Networks Lecture 1 19 / 52

26 Java applications The main method: a method of class Main Rectangle.java 1 public class Rectangle { 2 public Rectangle(int a, int b) { 3 this.a = a; 4 this.b = b; 5 } 6 public int perimeter() { 7 return 2 * this.a + 2 * this.b; 8 } 9 public int area() { 10 return a * b; 11 } 12 private int a = 0, b = 0; 13 } // Rectangle Federico Pecora Java for Interfaces and Networks Lecture 1 20 / 52

27 Java applications The main method: a method of class Main Main.java 1 import java.io.*; 2 public class Main { 3 public static void main(string args[]) { 4 BufferedReader kbd_reader; 5 //... 6 int a, b; 7 String buf; 8 try { 9 //... get a and b from user, then: 10 Rectangle rect = new Rectangle(a,b); 11 System.out.println("Area: " + rect.area()); 12 System.out.println("Perimeter: " + 13 rect.perimeter()); 14 } 15 catch (IOException e) { /*... */ } 16 } // main 17 } // Main Federico Pecora Java for Interfaces and Networks Lecture 1 20 / 52

28 Java applications The main method: a method of class Main linux> javac Rectangle.java linux> javac Main.java linux> java Main Input side a: 5 Input side b: 6 Area: 30 Perimeter: 22 linux> Federico Pecora Java for Interfaces and Networks Lecture 1 21 / 52

29 Java applications The main method: a method of class Rectangle Rectangle.java 1 public class Rectangle { 2 public Rectangle(int a, int b) { /*... */ } 3 public int perimeter() { /*... */ } 4 public int area() { /*... */ } 5 private int a = 0, b = 0; 6 public static void main(string[] args) { 7 try { 8 //... read input from user, then: 9 Rectangle rect = new Rectangle(a,b); 10 System.out.println("Area: " + rect.area()); 11 System.out.println("Perimeter: " + 12 rect.perimeter()); 13 } catch( /*... */ ) { /*... */ } 14 } 15 } // Rectangle Federico Pecora Java for Interfaces and Networks Lecture 1 22 / 52

30 Java applications The main method: a method of class Rectangle linux> javac Rectangle.java linux> java Rectangle Input side a: 5 Input side b: 6 Area: 30 Perimeter: 22 linux> Federico Pecora Java for Interfaces and Networks Lecture 1 23 / 52

31 Java applications A simple program Applications run through a Java Virtual Machine (JVM) Applets run in a web browser (through a plugin which provides a JVM) Hello.java 1 import java.applet.*; 2 import java.awt.*; 3 public class Hello extends Applet { 4 public void paint(graphics gr) { 5 gr.drawstring("hello, world!", 100, 100); 6 } //paint 7 } //Hello NB: this will not run with java command, as there is no main() method Federico Pecora Java for Interfaces and Networks Lecture 1 24 / 52

32 Java applications A simple program Applications run through a Java Virtual Machine (JVM) Applets run in a web browser (through a plugin which provides a JVM) hello.html 1 <html> 2 <head> 3 <title>an applet example</title> 4 </head> 5 <body> 6 <applet code="hello.class" width="200" height="200"/> 7 </body> 8 </html> Federico Pecora Java for Interfaces and Networks Lecture 1 24 / 52

33 Java applications A simple program Applications run through a Java Virtual Machine (JVM) Applets run in a web browser (through a plugin which provides a JVM) Federico Pecora Java for Interfaces and Networks Lecture 1 24 / 52

34 Syntax and Data types Outline 1 This course... 2 What is Java? 3 Java applications 4 Syntax and Data types 5 Object Orientation in Java Federico Pecora Java for Interfaces and Networks Lecture 1 25 / 52

35 Syntax and Data types Basic syntax Syntax in Java is much like in C++ Assignment is done with =, comparison with ==... although you cannot override these two operators in Java! Arithmetic operators: +, -, *, /, ++, -, +=, /=,... Control flow: if, while, do...while, for, switch, break, continue. Comments 1 // Extends over the rest of the line 2 3 /* A comment over 4 several 5 lines */ 6 Federico Pecora Java for Interfaces and Networks Lecture 1 26 / 52

36 Syntax and Data types Data types Class (a struct with methods ) e.g., String, a class for working with strings e.g., Rectangle, a class for introducing Java to students in this course Primitive types byte, short int, long for integers char for characters float, double for floating point boolean for logical values (can be true or false) Primitive types also have wrapper classes which provide convenience methods associated to the primitive type e.g., Integer for int provides method parseint(string) for reading an integer from a String Federico Pecora Java for Interfaces and Networks Lecture 1 27 / 52

37 Syntax and Data types Primitive data types Type Size Range boolean 1 bit false or true char 16 bits Unicode characters byte 8 bits short 16 bits int 32 bits long 64 bits float 32 bits E E-45 double 64 bits E E-324 Federico Pecora Java for Interfaces and Networks Lecture 1 28 / 52

38 Syntax and Data types Variable declaration Can be done anywhere within a block ({... }) a block defines the scope of a variable Objects are declared with new e.g., Rectangle rect = new Rectangle(2,3); As in C/C++ you can do int i; or int i = 5; NB: in Java it is a good idea to maintain a variable s scope as small as possible Federico Pecora Java for Interfaces and Networks Lecture 1 29 / 52

39 Syntax and Data types Arrays Java has several different "container" types, which can contain other items, such as lists, sets and mappings None of these types work like in C/C++! Built-in, fixed-size arrays: String[] fruits = ("Apples", "Pears", "Oranges"); For dynamic sets of objects: Vector, ArrayList, LinkedList Federico Pecora Java for Interfaces and Networks Lecture 1 30 / 52

40 Syntax and Data types Arrays Built-in arrays can store both primitive types and objects Usually instantiated with new Example 1 int[] intarray = new int[10]; 2 int intarray[] = new int[10]; 3 String[] fruits = { "Apples", "Pears", "Oranges" }; 4 int intarray[10]; //Compilation error! 5 6 //Iterating over an array (method 1) 7 for (int i = 0; i < intarray.length; i++) { 8 System.out.println("intarray[" + i + "] = " 9 + intarray[i]); 10 } //Iterating over an array (method 2) 13 for (String fruit : fruits) { 14 System.out.println("fruits contains: " + fruit); 15 } Federico Pecora Java for Interfaces and Networks Lecture 1 31 / 52

41 Syntax and Data types More differences between Java and C/C++ Garbage collection and how objects are handled No pointers (or, better, everything is a pointer ) Parameters always transferred by value Exception handling Standard library (very large) Classes and files (no.h files) Environment-independence (e.g., window management in C++ is very different in Windows, Linux and OS/X) Federico Pecora Java for Interfaces and Networks Lecture 1 32 / 52

42 Object Orientation in Java Outline 1 This course... 2 What is Java? 3 Java applications 4 Syntax and Data types 5 Object Orientation in Java Federico Pecora Java for Interfaces and Networks Lecture 1 33 / 52

43 Object Orientation in Java Classes and inheritance An animal has a name, a weight, and it can eat Different types of animals exist: fish and birds Fish Bird an animal that can swim has a maximum depth at which it can swim an animal that can fly has a maximum altitude at which it can fly flutters its wings when it eats Federico Pecora Java for Interfaces and Networks Lecture 1 34 / 52

44 Object Orientation in Java Animal class An animal has a name and eats Animal.java 1 class Animal { 2 protected String name; 3 private int weight; 4 public Animal(String n, int w) { 5 name = n; 6 weight = w; 7 } 8 public void eat(string food) { 9 System.out.println(name + " eats some " + food + "."); 10 weight += 0.5; 11 } 12 } //class Animal Federico Pecora Java for Interfaces and Networks Lecture 1 35 / 52

45 Object Orientation in Java Fish class An fish can swim at a maximum depth Fish.java 1 class Fish extends Animal { 2 private int maxdepth; 3 public Fish(String n, int w, int d) { 4 super(n, w); 5 maxdepth = d; 6 } 7 public void swim() { 8 System.out.println(name + " swims."); 9 } 10 } //class Fish Federico Pecora Java for Interfaces and Networks Lecture 1 36 / 52

46 Object Orientation in Java Bird class An bird can fly at a maximum altitude and flutters when eating Bird.java 1 class Bird extends Animal { 2 private int maxaltitude; 3 public Bird(String n, int w, int a) { 4 super(n, w); 5 maxaltitude = a; 6 } 7 public void fly() { 8 System.out.println(name + " flies."); 9 } 10 public void eat(string food) { 11 System.out.println(name + " flutters its wings."); 12 super.eat(food); 13 } 14 } //class Bird Federico Pecora Java for Interfaces and Networks Lecture 1 37 / 52

47 Object Orientation in Java The Zoo test class Let s put all animals together in a test class Zoo.java 1 class Zoo { 2 public static void main(string[] args) { 3 Bird tweety = new Bird("Tweety", 3, 1000); 4 tweety.eat("corn"); 5 tweety.fly(); 6 7 Fish b = new Fish("Bubbles", 13, 100); 8 b.eat("fish food"); 9 b.swim(); Animal w = new Fish("Willy", 4000, 1000); 12 w.eat("tourists"); 13 // w.swim(); -- Doesn t work. See below. 14 } 15 } //class Zoo Federico Pecora Java for Interfaces and Networks Lecture 1 38 / 52

48 Object Orientation in Java The Zoo test class Let s put all animals together in a test class linux> javac *.java linux> java Zoo Tweety flutters its wings. Tweety eats some corn. Tweety flies. Bubbles eats some fish food. Bubbles swims. Willy eats some tourists. linux> Federico Pecora Java for Interfaces and Networks Lecture 1 38 / 52

49 Object Orientation in Java Inheritance and method overriding The Bird class has its own method called eat(), which overrrides the eat() method in Animal Overriding you have a method in a subclass with the same signature of a method in the superclass If you want to call the superclass method despite the existence of an overriding method, use the super keyword (e.g., super.eat()) This is done in the definition of Bird.eat() Federico Pecora Java for Interfaces and Networks Lecture 1 39 / 52

50 Object Orientation in Java Inheritance and method overriding Notice that w is defined as an object of type Animal w.swim() will give a compilation error because Animal has no method swim() Variable w contains Willy (a Fish), which has a swim() method Therefore it should be possible to call it when the program is running but the compiler must be sure that whatever is in w will always have a swim() method One way to promise the compiler that this will always be true is to use a cast: ((Fish)w).swim() Federico Pecora Java for Interfaces and Networks Lecture 1 40 / 52

51 Object Orientation in Java Polymorphism The world consists of figures, and a Figure can be either a Rectangle or a Circle A Figure has a center, as indicated by the coordinates x_center and y_center One can calculate the area and perimeter of a Figure, as well as its maximum x and y coordinates Note that how to calculate the area and perimeter depends entirely on the type of figure! Federico Pecora Java for Interfaces and Networks Lecture 1 41 / 52

52 Object Orientation in Java Figure class A Figure must implement area and perimeter methods Figure.java 1 public abstract class Figure { 2 public Figure(int x, int y) { 3 this.x_center = x; 4 this.y_center = y; 5 } //Figure 6 public abstract int max_x(); 7 public abstract int max_y(); 8 public abstract int area(); 9 public abstract int perimeter(); 10 protected int x_center, y_center; 11 } //class Figure Federico Pecora Java for Interfaces and Networks Lecture 1 42 / 52

53 Object Orientation in Java Rectangle class A Rectangle is a kind of Figure, defined by side and height attributes Rectangle.java 1 public class Rectangle extends Figure { 2 public Rectangle(int x, int y, int s, int h) { 3 super(x, y); 4 this.side = s; 5 this.height = h; 6 } //Rectangle 7 public int max_x() { return super.x_center+this.side/2; } 8 public int max_y() { return super.x_center+this.height/2; } 9 public int area() { return this.side*this.height; } 10 public int perimeter() { 11 return 2*this.side+2*this.height; 12 } 13 private int side, height; 14 } //class Rectangle Federico Pecora Java for Interfaces and Networks Lecture 1 43 / 52

54 Object Orientation in Java Rectangle class A Circle is a kind of Figure, defined by a radius Circle.java 1 public class Circle extends Figure { 2 public Circle(int x, int y, int radius) { 3 super( x, y ); 4 this.radius = radius; 5 } //Circle 6 public int max_x() { return super.x_center+this.radius; } 7 public int max_y() { return super.y_center+this.radius; } 8 public int area() { 9 return (int)(math.pi*this.radius*this.radius); 10 } //area 11 public int perimeter() { 12 return (int)(2*math.pi*this.radius); 13 } //perimeter 14 public int diameter() {return this.radius+this.radius; } 15 private int radius; 16 } //class Circle Federico Pecora Java for Interfaces and Networks Lecture 1 44 / 52

55 Object Orientation in Java The FigureMain test class Let s put all figures together in a test class FigureMain.java 1 public class FigureMain { 2 public static void main(string args[]) { 3 Figure[] figures = new Figure[4]; 4 /* x y side height */ 5 figures[0] = new Rectangle(10, 10, 10, 10); 6 /* x y radius */ 7 figures[1] = new Circle(10, 10, 10); 8 /* x y side height */ 9 figures[2] = new Rectangle(20, 20, 20, 20); 10 /* x y radius */ 11 figures[3] = new Circle(20, 20, 20); for (int index = 0; index < figures.length; index++) { 14 //continues... Federico Pecora Java for Interfaces and Networks Lecture 1 45 / 52

56 Object Orientation in Java The FigureMain test class Let s put all figures together in a test class FigureMain.java 1 //... continued 2 if (figures[index] instanceof Circle) { 3 System.out.println("Circle:"); 4 System.out.println(" Diameter: " + 5 ((Circle)figures[index]).diameter() ); 6 } 7 else { System.out.println("Rectangle:"); } 8 System.out.println(" Highest x: " + 9 figures[index].max_x()); 10 System.out.println(" Highest y: " + 11 figures[index].max_y()); 12 System.out.println(" Area: " + 13 figures[index].area() ); 14 System.out.println(" Perimeter: " + 15 figures[index].perimeter()); 16 } //for 17 } //main 18 } //class FigureMain Federico Pecora Java for Interfaces and Networks Lecture 1 45 / 52

57 Object Orientation in Java The FigureMain test class Let s put all figures together in a test class linux> java FigureMain Rectangle: Highest x: 15 Highest y: 15 Area: 100 Perimeter: 40 Circle: Diameter: 20 Highest x: 20 Highest y: 20 Area: 314 Perimeter: 62 Rectangle: Highest x: 30 Highest y: 30 Area: 400 Perimeter: 80 Circle: Diameter: 40 Highest x: 40 Highest y: 40 Area: 1256 Perimeter: 125 linux> Federico Pecora Java for Interfaces and Networks Lecture 1 45 / 52

58 Object Orientation in Java What is a Java class? A class represents a type of objects which have attributes and can perform operations A collection of instance methods (in C++, member functions) the operations performed by objects of this type A collection of instance variables (in C++, member variables) the attributes that define the type of objects A collection of class variables (in C++, static-declared member variables) attributes that are the same for all objects of this type A collection of static methods Federico Pecora Java for Interfaces and Networks Lecture 1 46 / 52

59 Object Orientation in Java What is a Java object? An object is an instance of a class Circle circle; value is null by default circle = new Circle( 10, 10, 10 ); The variable circle is a reference to an object of type Circle Circle circle2; circle2 = circle; Now circle and circle2 both refer to the same object C++ s delete() does not exist in Java, which has automatic garbage collection Federico Pecora Java for Interfaces and Networks Lecture 1 47 / 52

60 Object Orientation in Java General class definition syntax <modifier> class <name> [ extends <baseclassname> ] [ implements <interfacename> ] {... <variable definitions>... <method definitions>... } Federico Pecora Java for Interfaces and Networks Lecture 1 48 / 52

61 Object Orientation in Java Accessing variables/methods inside an object radius variable in Circle is accessible only in the class methods: private int radius; To make it accessible outside we declare it public: private int radius; Now we can access radius from, e.g., the main method in FigureMain: Circle circle = new Circle( 10, 10, 10 ); circle.radius = 27; much like structs in C The same goes for methods: we can define private methods for use only inside the class Federico Pecora Java for Interfaces and Networks Lecture 1 49 / 52

62 Object Orientation in Java Overriding and Overloading Overriding: a method in a derived class has the same signature as a method in the superclass used to give a specific definition to a method in the derived class Bird.eat(String) overrides Animal.eat(String) Federico Pecora Java for Interfaces and Networks Lecture 1 50 / 52

63 Object Orientation in Java Overriding and Overloading Overloading: a method which has the same name but different signature of another method in the same class used to give provide different functionality depending on the specified parameters public void DrawShape(int x1, int y1, int x2, int y2) { /* draw a rectangle */ } public void DrawShape(int x1, int y1) { /* draw a line */ } Federico Pecora Java for Interfaces and Networks Lecture 1 50 / 52

64 Object Orientation in Java Constructors One or more methods with the same name of the class No return type! If no constructors are defined, a default constructor with no arguments that does nothing is provided by default Multiple constructors overloading public Figure (int x, int y) { /*... */ } public Figure () { /*... */ } Federico Pecora Java for Interfaces and Networks Lecture 1 51 / 52

65 Object Orientation in Java Introduction Thank you! Federico Pecora Java for Interfaces and Networks Lecture 1 52 / 52