Java

1. Java • Java (1995) developed inSun Microsystems byJames Gosling, Bill Joy andGuy Steele • Now owned by Oracle • Influenced by C++ • Implements classes, encapsulation, simple inheritance, polymorphism, interfaces, garbage collection

2. The java Model • When compiling a Java Program an intermediate code is generated (defined by Sun) which is named bytecode

3. The Java Model • This bytecode is portable to different platforms

4. Types of java Files • All files define either a class or an interface • Class: • Defines and implements a type of object • Variables (representation) • Static variables: common for all objects of the class • Dynamic variables: one for each object • Procedures (functions) • Statics (a special one: main) • And dynamics (are performed on an object) • Interface • Defines the signature of a procedure

5. A basic file to run a java program public class MyClass { static public void main(String[] args) { here come the instructions that will be performed when the resulting class file is interpreted } } • Program must be written in a file named MyClass.java (MyClass is the class name, chosen by the programmer, starts with uppercase by convention • When compiled generates MyClass.class, which is interpreted by the JVM (java command) • If something is changed in the program then it should be compiled again to reflect the changes

6. Problem Write a program to let the computer compute the percentage of men and women according to the following dialogue with a person (user):  Number of men 19 user Number of women 12 user % of men = 61.29032258064516 % of women = 38.70967741935484

7. import java.util.Scanner; • public class Problem1 { • public static void main(String[] args) { • Scanner sc = new Scanner(System.in); • System.out.println("Number of men "); • int m = sc.nextInt(); • System.out.println("Number of women "); • int w = sc.nextInt(); • double p = 100.0 * m / (w + m); • System.out.print("% of men = "); • System.out.println(p); • System.out.print("% of women = "); • System.out.println(100 - p); • sc.close(); • } • }

9. Evaluation of mathematical expressions • 1. Unary operators ( +3, -4 ) • 2. “multiplicative” operators ( *, / ) • 3. “additive” operators( +, - ) • Notes • In case of operators of same priority the evaluation is done from left to right. • Ex.: 100.0*m/(w+m) is equivalent to (100.0*m)/(w+m) • Parenthesis are used to • Modify the evaluation order Ex.: 100.0*m/(w+m) • Confirm the evaluation order Ex.: (100.0*m)/(w+m) • Type of the result • Common type Ex: 1 / 2 is 0 (integer), 1.0 / 2.0 is 0.5 (real) • real if operands are of different types. Ex: 1.0/2 is 0.5 • Note. the result of m/(w+m)*100.0? Why?

10. System.out.println(p); Semantic: writes the value (or expression) of p and “skips” to the next line System.out.print(p); Semantic: writes the value (or expression) of p and “stays” in the same line (what is written after this appears on the same line) Syntax: System.out.println(arithmetic expression); Examples: variable (p) constant (100) expression (a*b,(a/b))

11. Example 2 Program that reads an integer number which contains a date in the format yyyymmdd and prints it as dd/mm/yyyy import java.util.*; //uses Scanner public class ChangeDateFormat{ static public void main(String[] args) { Scanner U = new Scanner(System.in); System.out.print("date as yyyymmdd : "); int n = U.nextInt(); int y=n/10000, d=n%100, m=n%10000/100; System.out.print("date as dd/mm/aaaa : "); System.out.print(d+"/"+m+"/"+y); } }

12. Types, Values and Variables • A java variable can store 2 types of values: • Primitives: variable corresponds to a mnemonic name of a place in memory where the value is stored • References: variable corresponds to a name of the place in memory that contains the address where the data is stored • Primitive variables and references are strongly "typed"

13. Primitive Data

14. Type conversion (cast) • Java is strongly typed • Checks type compatibility during compilation • Allows casting among types

15. Implicit conversion • When a numeric value of smaller range is assigned to a variable of a wider range byte short int long float double smaller wider • Example inti = 1000; double d = i;

16. Explicit cast (primitive types) • May cause loss of precision double d = 20.5; long l = (long) d; System.out.println(l); • May cause loss of digits long l = 1000000; short s; s = (short) l; System.out.println(s); 20 16960

18. Solution 1 • Scanner U = new Scanner(System.in); • System.out.print("side of the square?"); • double x = U.nextDouble(); • double r = x / 2; • System.out.print("area="); • System.out.println(x * x - 3.1416 * r * r); • System.out.print("perímeter="); • System.out.println(4 * x + 2 * 3.1416 * r); • U.close(); • Solution 2 • ... • ... • System.out.println("area=" + (x * x - Math.PI * r * r)); • System.out.println("perímeter=" + (4 * x + 2 * Math.PI * r));

19. Solution 3 (with some useful new things) • double x = U.nextDouble(), r = x / 2; • System.out.println("area=" + (Math.pow(x, 2) - Math.PI * Math.pow(r, 2))); • System.out.println("perimeter=" + (4 * x + 2 * Math.PI * r)); • double x=…,r=…; • Equivalent to: double x=…; double r=…; • Math.PI • Pre-defined value of л (3.14159265358979) • real constant (cannot be changed) • Math.pow(r,2) • computes and returns r2 • Math.pow: predefined function • syntax: Math.pow(x,y), x and y: expressions • semantic: xy, result is a real value (Math.pow(2,2) is 4.0)

20. Proposed Problem1 . Write a program to let the computer compute the velocity of a moving object according to the following dialogue with a person (user): Meters ? 50 user Seconds ? 10user Velocity = 18.0 km/hour

21. Proposed Problem2 . Write a program to let the computer compute the % of Armenians in Armenia, Artsakh and the Diaspora according to the following dialogue with a person (user): In Armenia ? 2800000user In Artsakh? 150000user In Diaspora ? 8000000user % in Armenia = X% % in Artsakh = Y% % in Diaspora = Z%

22. Proposed Problem3 . Write a program to let the computer read a number of two digits and print the number with the digits inverted (if user gives 78 the computer writes 87, if user writes 90 the computer writes 09): Two digits number ? 79user Number with inverted digits = 97