SOFTWARE AND PROGRAMMING 1

1. SOFTWARE AND PROGRAMMING 1 Revision Lecture 11/5/2011: Review of concepts involved in lectures and exam Review of questions in previous exam papers (they are posted on SP1 site, for example, http://www.dcs.bbk.ac.uk/~mirkin/sp109/ - see in menu on the left) PLEASE BRING YOUR QUESTIONS on the exam questions !!! Will try to address them all

2. Fundamental concepts • How a Java program works • Class; object; instance • Method; constructor • Parameter • Variable (local, static, instance) • Expression • Control structures: looping, branching • Input/output • Data type, including that user-defined • Array, 2D array

3. Objects and classes • Classes: program templates • represent all objects of a kind (example: “student”) • Object = instance • A template copy to represent a specific element of the class (“an individual student”) • Instances are created with the so-called constructors, explicitly in JDK or somewhat easier in BlueJ

4. Variables, methods and parameters • classescontain data stored in the so-called variables and operations which can be invoked (they are called methodsin Java) • methods may have inputs (they are called parameters in Java) to get additional data needed to get them executed

5. Structure of a method Output’s type Inputs modifiers return-type name ( parameter-list ) { statements; return variable/expression; //if return type is not void } Modifiers: • static -       method/variable that belongs to class as whole and is shared by all • public -    method/variable that is accessible from anywhere • private -    method/variable that is accessible from only within the class

6. Constructor • Constructor is a special method that • Has the same name as the class • No return type (nor “return”) • Is called with modifier “new” • to reserve a memory space for an object of class type (an instance) • List of parameters, as well as the block, can be user defined • The constructor with no parameters is available by default (like Const(){ }; )

7. Assigning values • Values are stored into fields (and other variables) via assignment statements: • variable = expression; • price = ticketCost; • The value on the right is assigned to a variable on the left. • A variable stores a single value, so any previous value is lost.

8. Variable • It provides for multiple uses of the same program • A variable is a name for a location in memory that can hold data. • Variables are declared and/or initialisedA variable declaration includes the following: • A data type that identifies the type of data that is stored in the variable • An identifier that is the variable’s name • An optional assigned initial value

9. Scope of a variable:        - The range of statements that can access the variable. It stretches from the declaration point to the end of the block containing the declaration Q: WHAT is BLOCK ? (part within curly braces{…} ) Q: WHAT is DECLARATION? (type name ; 3-part command)

10. Type casting       The unifying type can be overridden by explicitly stating a type cast: • Place the desired type result in parentheses followed by the variable or constant to be cast •      (int) 6.0+7=13 • Example: • int bankbalance=931786; float weeklybudget = (float) bankbalance /4;

11. Static and instance variables Static variable: belongs to its class, and it is shared by all class instances, with the same value Instance variable: a class variable without the “static” modifier, is shared by all class instances, but its values can differ in different instances Local variable: is created within a method or instance in a { } block. Its scope is limited within the block.

12. Loop for for(int var=1;var<=st;var++) % no ‘;’ here!!! {do operation depending on var} var++ is var=var+1 , not var=var+2; • Two types of parentheses: (loop specified) and {body to do} • The expression in () consists of three items in this order: • initialising the counting variable once, • variable update, and • stop-condition • First, • var is intialised, • stop-condition is tested; • if true, block {} is executed, • if no, the program proceeds further on, after the block { } • control returns to ( ) • After control returns to ( ), • varis updated; • stop-condition is checked; • if true, block {} is executed, then control returns to ( ), • if no, the program proceeds further on, after the block { }

13. Loop while for(init;test;update){ statements } All three in the parentheses refer to a counter that is initialised, updated and tested over reaching the pre-specified threshold Structure of while loop, less rigid – init, test and update are not necessarily based on counter: init; while(test){ statements; update } Similar elements: ( ), { }, initialisation, test condition (not necessarily involving the counter!), and update

14. Double loop class AddTable { public static void main(String[ ] args){ int sum; for (int ite1=1;ite1<3; ite1++) {//loop1 for (int ite2=1;ite2<5; ite2++) {//loop2 sum=ite1+ite2; System.out.print(sum +" ");}//loop2 System.out.println();}//loop1 } //method main ends } //class ends

15. Java branching structure : if( ) … else if( ) … else if( ) … else if(BooleanExpr1) Statement1; (e.g. Tax=2;) else if(BooleanExpr2) Statement2; (e.g. Tax=18;) else Statement3; (e.g. Tax=7;) • Note: No (Boolean Expression) at else

16. Input/Output TextIO class TextIO.java, added to the directory that contains your class, eases input of data from the keyboard To input an integer:   int UsInput = TextIO.getInt(); Computer will wait for the user to type in an integer value to UsInput.

17. Input with Scanner class import java.util.* class PrintDot{ int num=0; public static void main(String[ ] args){ Scanner scap = new Scanner(System.in); System.out.println(“How many dots to print? “); num=scap.nextInt(); for (ik=0; ik<num; ik++) System.out.print(‘.’); System.out.println(); } //end of main } //end of class

18. Strings(1) Declaring a String Object String variable •      An object of the class String –    The class String is defined in java.lang.String and is automatically imported into every program      Create a String object by using the keyword new and the String constructor method •      String aGreeting = new String(“Hello”); or String aGreeting = “Hello”;

19. Strings(2) Comparing String Values •     Strings are never actually changed; instead new Strings are created and String variables hold the new addresses; A part of the Java system called the garbage collector will discard the unused strings •     It is impossible to make a simple comparison of Strings; thus a number of methods are: –   equals() method   if s1 and s2 are declared and initialised as String: s1.equals(s2) true if s1 and s2 are exactly the same sequences of characters NEVER s1==s2 !!! This is wrong, == applies to numbers only.

20. Class Math Math.pi  =3.14…, the ratio of the circumference to its diameter Math.abs(a) a if a >= 0, or -a if a < 0 Math.log(a) the natural logarithm (base e) of a Math.sqrt(a) square root of a Math.pow(a,b) ab , if b is integer then ab =aa…a (b times) Math.random() pseudorandom number: double within interval [0.0, 1.0) (zero included, unity not) How to use it to generate a random integer between 1 and 6 (inclusive), to mimic casting a dice? double aa=Math.random(); //aa, a real number between 0 and 1 int an= 6*aa; //a real number between 0 and 6 int rand=(int) an; // whole number between 0 and 5 int randw=rand+1;// whole number betw. 1 and 6 Casting in Java: converting from higher number types to lower types int randw= (int) (6*Math.random()+1); How to generate a random integer between 10 and 20 inclusive? Answer: int rdt= (int) (11*Math.random()+10);

21. Math.random() pseudorandom number: double within interval [0.0, 1.0) (zero included, unity not) How to use it to generate a random integer between 1 and 6 (inclusive), to imitate casting a dice?

22. Array (1) Array is an indexed list of elements of the same type A string array nam[ ]: contains both entries and index. String nam[] ={“John”,“Paul”,“George”,“Ringo”};Index   is        0          1          2         3 Length (the number of entries) is  4 An integer array age[ ]: int age[ ]= {23, 32, 19, 30, 25, 25, 23, 30};Index  is    0  1    2  3   4   5   6  7 Length is  8

23. Array (2) bmi[ ]=new double[5]; for (int I = 0; I < 5; I + +) bmi[I]=weight[I] / (height[I]height[I]); If we do not know the length of student arrays or it is variable: bmi[ ]=new double[height.length]; for (int I = 0; I < height.length; I + +) bmi[I]=weight[I] / (height[I]height[I]);

24. User defined type Follows classes: - Oblong and OblongTester, sections 6.3 and 7.2, Charatan and Kans, “Java in two semesters” - Counter and CounterTest, sections 6.3-6.6, Pohl and McDowell, “Java by dissection” - Employee and TwoEmployee, p. 96-112, Ch.3, Farrell, “Java programming”

25. 2D arrays Summary sales: •     int sum =0; •     for (int shop = 0; shop < 4; shop ++) •     for(int day = 0; day < 7; day ++) •     sum = sum + sales[shop][day]; As a method:  public int sum( int[][] a) { •  int total = 0; • for (int row = 0; row < a.length; row ++) •  for( int col = 0; col < a[0].length; col ++) •  total = total + a [row][col]; •                     return total;               }

26. Array of user-defined type(1) • The setting: We have a number of applicants for whom we have separate, but matching, lists of names and id’s organised as arrays. • We would like to develop a new type for an Applicant to hold all individual data of the applicant, in this case just id and name, but it can have as many attributes as it takes. • With this new type, we would like to organise a list of applicants as an array of this type

27. Array of user-defined type(2) • To develop an array of applicants, we need • A named class, say Appl, with variables declared to hold all individual applicant data (A); • A constructor in this class that would take values from the arrays holding id and name information (B); • Representation of the id and name arrays (C); • Generation of an instance of array of new type, Appl (D); • Feeding the Id’s and names into the Appl array (E); • We can show that this does work by printing out data of all the entries in the Appl array.

28. Array of user-defined type(3) • class Appl{ • public int ids; • public String nms; • public Appl(int iden, String nnn){ • ids=iden; • nms=nnn;} • static int[] iden(){ • int ii[]={12, 15, 22, 18, 11}; • return ii;} • static String[] namen(){ • String jj[]={"Aa", "Bb", "Cc", "Dd", "Ee"}; • return jj;}

29. Array of user-defined type(4) • public static void main(String[] args){ • int id[]=iden(); • String name[]=namen(); • Appl[] many=new Appl[id.length]; • for(int i=0;i<id.length;i++) • many[i]=new Appl(id[i],name[i]); • //many – array of Appl type objects. Check: • for(int i=0;i<name.length;i++){ • System.out.println(i +"th applicant data:"); • System.out.println("Id: "+many[i].ids); • System.out.println("Name: "+many[i].nms); • System.out.println(); } • } • }