Java Coding David Davenport Computer Eng. Dept., Bilkent University Ankara - Turkey. Syntax for Variables & Constants Input, Output and Assignment a complete Java program data representations

IMPORTANT… Students… This presentation is designed to be used in class as part of a guided discovery sequence. It is not self- explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn! Instructors… You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it. thank you, David. David

From problem to program… The story so far... Java Source Code Java bytecode Machine code Problem Algorithm Data/Memory requirements

Need Java Syntax for… Algorithm (in pseudo-code) Sequence, Decision & Repetition, of Data flow operations Input, Output & Assignment Data/Memory requirements Meaningfully named memory locations Restriction on data (data types) Variables or Constants & initial value Plus comments & methods!

Comments & White space Comments Syntax: // any text on remainder of current line /* any text across multiple lines */ /** any text across multiple lines; a Javadoc comment! */ Examples: Java ignores line endings, blanks lines & white space! Layout program code for ease of reading! Use to automatically generating client API from source code. // Author: David. // Date: Oct /* This program blah, blah, blah */

Identifiers User-defined names Used for variables, constants, methods, etc. Any sequence of letters, digits and the underscore character only. First character may not be a digit! Upper and lower case are considered different (i.e. case sensitive!) Cannot use Java reserved words i.e. words such as while, for, class, if, etc. CS101 rule: Names must be meaningful!

Data Types For now, use only the following… Primitive int (for numeric integer, e.g. 5, -27, 0, 510…) double (for numeric real, e.g. 5.75, 3.0, -2.6…) char (for any character, e.g. A, a, B, b, 3, ?, &, … ) boolean (for true / false only) Non-primitive String (for any sequence of zero or more characters e.g. “CS101”, “A”, “Well done!”, … )

Declaring Variables Syntax: Type Any Java type Name (identifier) Convention: first letter of embedded words capital, except first! Examples: int age; double area; double initialSpeed; char letterGrade; char lettergrade; boolean exists; CAUTION Java is case sensitive! type name;

Declaring Constants Syntax: Type Any Java type Name (identifier) Convention: all capital letters (& underscore!) Value (literal, variable, constant, expression) Examples: final int SPEEDOFLIGHT = 300; final double PI = 3.142; final String COMPANY = “Bilkent”; final char LETTER_GRADE = ‘A’; Literal values String use “…” char use ‘.’ final type name = value;

Output (1) Syntax: where output is Literal value eg. “The area is ”, ‘?’, 12.5, … Named variable or constant eg. area, userName, TAX_RATE, … Expression eg. 2 * PI * radius, “The area is ” + area System.out.println( output ); Value is output exactly as is! Value in named memory location is output Resulting value of expression is output Note use of + for string concatenation

Output (2) Use To output the value & leave text cursor on current line. System.out.print( output ); System.out.println( “Welcome to CS101”); System.out.println( “The tax rate is ” + TAXRATE + ‘%’); System.out.println( “Goodbye.”); System.out.println( “Welcome to CS101”); System.out.print( “The tax rate is ”); System.out.print( TAXRATE); System.out.println( ‘%’); System.out.println( “Goodbye.”); System.out.println(); Output blank line!

Outline Java Program The CS101 console template… import java.util.Scanner; /** 1.00, date */ public class ClassName { public static void main( String[] args) { Scanner scan = new Scanner( System.in); // constants // variables // program code } ClassName.java In Java program = class Classname Convention: first letters capitalised Filename & classname MUST be the same.

Demo… Creating, compiling, running & debugging a Java program, using the console & a text editor an IDE (Integrated Development Environment) JRE ~Java Runtime Environment (the Java interpreter, java.exe, and libraries!) JDK ~Java Development Kit (the Java compiler, javac.exe, and other tools)

A COMPLETE EXAMPLE… Syntax for Input and assignment statements, and the complete Circle Computer program.

Input Syntax: Examples StringVariable = scan.next(); intVariable = scan.nextInt(); doubleVariable = scan.nextDouble(); userName = scan.next(); age = scan.nextInt(); salary = scan.nextDouble(); str = scan.nextLine(); Standard from Java5.0 on Invalid input may give run-time error! Program must include: import java.util.Scanner; Scanner scan = new Scanner( System.in); Variables must be declared before use

Assignment Syntax: where expression is operand or operand operator operand & Operand is Literal value Named Variable or constant Result of method call Expression (can use brackets to disambiguate) ! Operator is +, -, *, /, % (modulus, remainder after integer division) resultVariable = expression; Result of expression must be of suitable type to put into resultVariable! … is assigned the result of …

Assignment Examples What is the result of this? Evaluation rules Bracketed sub-expressions first Operator precedence ( * / % before + - ) Left to right total = 0;x = y; sum = firstNumber + secondNumber; netPay = grossPay * ( 1 – TAX_RATE/100); count = count + 1; c = Math.sqrt( a * a + b * b ); / 3 – 1 … is assigned the result of …

Outline Java Program The CS101 console template… import java.util.Scanner; /** 1.00, date */ public class ClassName { public static void main( String[] args) { Scanner scan = new Scanner( System.in); // constants // variables // program code } ClassName.java In Java program = class Classname Convention: first letters capitalised Filename & classname MUST be the same.

A Complete Example (1) Problem – find area & circumference… Algorithm Data requirements 1. Print welcome message 2. Ask for & get radius from user 3. Compute area as pi.radius.radius 4. Compute circumference as 2.pi.radius 5. Report area, circumference & radius Lradius - int Larea, circumference - double PI – double, constant = 3.142

A Complete Example (2) import java.util.Scanner; /** 1.00, 2005/10/07 */ public class AreaCircum { public static void main( String[] args) { // constants // variables // 1. Print welcome message // 2. Ask for & get radius from user // 3. Compute area as pi.radius.radius // 4. Compute circumference as 2.pi.radius // 5. Report area, circumference & radius } AreaCircum.java

A Complete Example (3) import java.util.Scanner; /** * AreaCircum - computes area & circum of circle given radius * David 1.00, 2005/10/07 */ public class AreaCircum { public static void main( String[] args) { // constants final double PI = 3.142; // variables intradius; doublearea; doublecircumference; Header has been edited to include program description & author name AreaCircum.java

A Complete Example (3) Scanner scan = new Scanner( System.in); // 1. Print welcome message System.out.println( "Welcome to area circumference finder."); // 2. Ask for & get radius from user System.out.print( "Please enter the radius: "); radius = scan.nextInt(); // 3. Compute area as pi.radius.radius area = PI * radius * radius; // 4. Compute circumference as 2.pi.radius circumference = 2 * PI * radius; // 5. Report area, circumference & radius System.out.print( "The area of a circle of radius "); System.out.print( radius); System.out.print( " is "); System.out.println( area); System.out.print( "and its circumference is "); System.out.print( circumference); System.out.println(); } } // end of class AreaCircum Template line required for Keyboard input. Steps 2 & 5 expanded as per original algorithm.

Compile & Run… Um… why the weird circumference value?

Testing… It compiled & ran, but…is it correct? How can you tell? Enter input & check results it outputs (e.g. radius 5  area & circumference 31.42) are these actually the right answers? Really need more input/output sets what input values should we use? & how many do we need? Thinking about testing during design can help produce better programs!

DATA & DATA TYPES In more depth…

Communication…

Communicating… a)One man multiple messages b)Many men multiple messages combine state for single message?

Communicating… Can use anything… Abstractions Up / down On / Off X / O True / False 1 / 0

Data Types For now, use only the following… Primitive int (for numeric integer, e.g. 5, -27, 0, 510…) double (for numeric real, e.g. 5.75, 3.0, -2.6…) char (for any character, e.g. A, a, B, b, 3, ?, &, … ) boolean (for true / false only) Non-primitive String (for any sequence of zero or more characters e.g. “CS101”, “A”, “Well done!”, … )

Numeric representations Number bases Base 2 - binary digits 0 & 1 2 n values 0  (2 n – 1)

Characters… Coding Size…? ASCII 128 bit, English only! UNICODE 16 bit All languages! 0000 — ‘a’ 0001 — ‘b’ 0010 — ‘c’ 0011 — ‘d’ 0100 — ‘+’ 0101 — ‘-’ 0110 — ‘*’ 0111 — ‘/’ 1000 — ‘0’ 1001 — ‘1’ 1010 — ‘2’ 1011 — ‘3’ 1100 — ‘ ’ 1101 — ‘x’ 1110 — ‘y’ 1111 — ‘z’ ‘A’.. ‘Z’  26 ‘a’.. ‘z’  26 ‘0’.. ‘9’  10 punc.  ?? = = = 256 Standard Codes

Data Types Primitive byte, short, int, long (numeric integer) float, double (numeric real) char - any character, e.g. A, a, B, b, 3, ?, &, … (Java uses ISO Unicode standard, 16 bit/char) boolean - true / false Non-primitive String - any sequence of zero or more characters enum – an ordered set of user-defined values anything & everything else! (we will come to these shortly)

Type byte short int long float double Storage 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits Max Value ,767 2,147,483,647 9 x significant digits 15 significant digits Primitive Numeric Types integer real Min Value ,768 -2,147,483, x ±3.4 x 10 ±38 ±1.7 x 10 ±308

Data Types Primitive byte, short, int, long (numeric integer) float, double (numeric real) char - any character, e.g. A, a, B, b, 3, ?, &, … (Java uses ISO Unicode standard, 16 bit/char) boolean - true / false Non-primitive String - any sequence of zero or more characters enum – an ordered set of user-defined values anything & everything else! (we will come to these shortly)

Misc… Why so many numeric types? Error in reals? Typecasting int into double, but not double into int! Overflow/underflow Division by zero Why not use String for everything?