software and programming 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

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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

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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

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Variables, methods and parameters

• classes contain data stored in the so-called variables and operations which can be invoked (they are called methods in Java)

• methods may have inputs (they are called parameters in Java) to get additional data needed to get them executed

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Structure of a method

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

Output’s type Inputs

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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(){ }; )

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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.

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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

initialised A 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

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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)

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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;

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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.

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Loop forfor(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 ( ),

– var is updated; – stop-condition is checked;

• if true, block {} is executed, then control returns to ( ), • if no, the program proceeds further on, after the block { }

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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

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Double loopclass AddTable {public static void main(String[ ] args){int sum; for (int ite1=1;ite1<3; ite1++) {//loop1for (int ite2=1;ite2<5; ite2++) {//loop2

sum=ite1+ite2;System.out.print(sum +" ");}//loop2System.out.println();}//loop1

} //method main ends} //class ends

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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

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Input/Output TextIO classTextIO.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.

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Input with Scanner classimport 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

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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”;

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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.

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Class MathMath.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 =aa…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 1int an= 6*aa; //a real number between 0 and 6int rand=(int) an; // whole number between 0 and 5int 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);

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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?

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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

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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]);

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User defined typeFollows 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”

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2D arraysSummary 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; }

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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

•

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• 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.

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• 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;}

•

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• 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(); }• }• }

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