Java Fundamentals

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First Program

public class FirstSample{ public static void main (String[] args) { System.out.println (“We will not use ‘Hello, World!;”); }}

• Everything in Java is a class …(or a method)• Java case sensitive• public – access modifier (supports Security Principle)• Keyword class – everything in Java a class (supports Regularity Principle)• Rules for class names

– start with letter, any combo of letters and digits; length unlimited (0-1-infinity principle).

– Can’t use a reserved word security principle  2

• File name for source must be same name as public class with .java AND case is important! 

• Javac – creates bytecodes in .class file. Run bytecode through interpreter

• Must have a main method and in Java Language Specification must be public.

• Textbook has a style – I LIKE that style!

• Every function is a method of some class supports Regularity Principle

• static void – static member functions do not operate on objects of that class – void indicates that method does not return a value (do not need an exit

code to the OS like C++ - unless you use threads and you will use threads)3

 • every statement ends in ; so statements can span several lines –

supports regularity principle

 • System.out is an object and println is a method (. Invokes a

method) …also a print method

 • Methods can use 0, 1 or more parameters or arguments – supports

regularity principle.

• Comments same as C++

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Data Types

• Java strongly typed– supports security principle (C++ is weakly typed) – 8 primitive types: int, short, long, byte (use int

unless…)• In Java range of values does not depend on machine

– supports portability– In C++ uses most efficient for each processor

• float, double – use double unless …• char – uses Unicode encoding scheme – 65,536 characters (2 bytes)

– escape sequences \t\n\r for white spaces• boolean – false and true (cannot convert to int)

– supports Security principle 5

Variables

• Every variable has a type

double salary;char answer;boolean done; int vacationDays;

• variable name must start with letter, followed by any number of letters and digits

– SUPPORTS 0-1-Infintity• can use Unicode letters/symbols• case sensitive• Some words keywords, some reserverd (can’t use reserve words, e.g. const,

goto)• FOLLOW CONVENTION Box box;

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Assgs/Inits

• SAME as C++

int vacationDays;vacationDays = 14;

• or

int vacationDays = 14;

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Constants

• used in one method

final double CM_PER_INCH = 2.54;• used in multiple methods

public static final double CM_PER_INCH = 2.54;

• use of final – supports defense in depth and labeling principles• different ways to declare – violates regularity principle• use of public – methods of other classes can use it.

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Operators

int n=5; int a= 2* n; x+=4; same as x = x+4; • syntactic sugar violates consistency• goal of portability of Java, but computations by different processors (and use

of registers) yields different results. • Initially, all machines required to truncate, and scientific community hated

it…so now do not require it) • If you require same results, use strictfp as a tag for the class.

public static strictfp void main (String args[]) 

n++; n--; a=2 * ++m; increments m before mult.

a=2*m++; after

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Relational and Boolean Ops

• same as C++

==, !=, < >, <=, >=, &&, !, || • compound expressions short circuit

– principle?

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Math functions and constants

double y = Math.sqrt(x); y = Math.pow(x,a);Math.PI and Math.E

• Math is a static class• sqrt a method – it does not operate on an object, hence if you look at

its declaration you’ll see it is a static method.• Can also use StrictMath class instead (slower, but more predictable

results)

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Conversions

• strongly typed FIG 3-1– can cast

int i = (int)x; truncates– can round

int I = (int)Math.round(x); • Table for hierarchy or use ( )’s

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Strings

• not built-in; but in a predefined class called String. • Each quoted string is an instance of the string class:

String greeting = “Hello”; • Exception to the rule to not use new new to create instance

violates Regularity principle

• + concat (ONLY overloaded operator in Java) no overloading supports Othogonality

String s = greeting.substring (0,4);int l = greeting.length( );char last = greeting.charAt (greeting.length ( ) – 1);

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Strings

• NO METHODS TO CHANGE a string – but can get around with reassignment

• String is a char* pointer basically.

• greeting = “Howdy”; – memory leak? No - automatic garbage collection – supports automation principle.

• Test strings CANNOT use == s.equals(t) s.equalsIgnoreCase(t);  • String toUpperCase( );

• See handount on String methods.

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Input

• Easy to output• More complex to input from ‘standard input device’

• Easy to supply a dialog box for keyboard input. Method is

JOptionPane.showInputDialog (promptString);

– Returns a String!!! – Will also show OK and Cancel buttons– Must use import javax.swing.*;– Must end program with System.exit(0); because showing

dialog box starts new thread

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Input Examples

String input = JOptionPane.showInputDialog (“What is your name?”);input = JOptionPane.showInputDialog (“Enter your age:”);int age = Integer.parseInt (input); input = JOptionPane.showInputDialog (“Enter your gpa:”);Double gpa = Double.parseDouble (input);

• Program 2 will use input from keyboard! Program 3 will have input from keyboard and file.

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Formatting output

System.out.print (x); prints the maximum non-zero digits for that type.

System.out.print (10000.0/3.0); 3333.3333333333335 Note: 10000/3 3333

Use NumberFormat class from java.text package NumberFormat formatter = NumberFormat.getNumberInstance (arg optional); getCurrencyInstance or getPercentInstanceformatter.setMaximumFractionDigits(4);String sdoubleNum = formatter.format (doubleNum);System.out.print (sdoubleNum);

• Can also SetMinimumFractionDigits17

Control Flow

• Conditional statements and loops support structure principle• No goto in Java supports structure principle• Labeled break statement to jump out of nested loops violates

structure principle

• Blocks define scope of variables - supports localized principle• Blocks can be nested• BUT cannot redefine a variable inside a NESTED block UNLIKE C+

+ - violates regularity, supports security, because lots of programmers make errors in C++

• EXCEPTION: for loop’s LCV can be redefined – violates regularity

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Control Flow

• Conditional statements if; if else if else; just like C++. NOTE: else paired with closest if.

 • Loops – two forms for indeterminate loops: while and do-

while just like C++

• For loop – just like C++ for loops support automation principle• CONVENTION in Java that three parts of for loop are init, test,

update the same counter variable (although like C++ you can show bad taste and do other things)

• SCOPE of LCV is for loop• CAREFUL: using real numbers in for loops i+= 0.1 for update

ROUNDOFF ERRORS19

Loops

• break in a loop• labeled break better!

read_data: while { break read_data; }

• continue transfer control to header of innermost loop

 • CONVENTION/ GOOD SWENG: don’t use break or continue!!!

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Switch

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• switch similar to C++

• cases support labeling principle• case labels must be integers!!! Can’t do any ordinal types, ie chars

• forget a break, falls through violates security and defense in depth principle• why do it: combine cases better way case 1, 2, 3: as in Pascal and Ada

BigNumbers

• BigInteger and BigDecimal from java.math package.

• BigInteger c = a.add(b);

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