CMPS4143 Contemporary Programming Languages
Java Fundamentals
Concentrate on writing applications…
public class FirstSample
{
public static void main (String[] args)
{
System.out.println (“We will not use ‘Hello, World!;”);
}
}
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
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)
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++
Data Types
Java strongly typed – supports security principle (C++ is weakly typed) See fig 3-1
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
Variables
double salary;
char answer;
boolean done;
int vacationDays;
-every variable has a type
-variable name must start with letter, followed by
and number of letters and digits SUPPORTS 0-1-Infintity
- can use Unicode letters/symbols
- case sensitive
- can’t use reserve words
- FOLLOW CONVENTION Box box; or better Box aBox;
Assgs/Inits
int vacationDays;
vacationDays = 14;
Constants
- used in one method
- used in multiple methods
int vacationDays = 14;
SAME as C++
final double CM_PER_INCH = 2.54;
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.
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
Relational and Boolean Ops
==, !=, < >, <=, >=, &&, !, ||
same as C++
compound expressions short circuit – principle?
Math functions and constants
double y = Math.sqrt(x);
y = Math.pow(x,a);
Math.PI and Math.E
Math is a 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 Strict Math class instead (slower, but more predictable results)
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
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 not using 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);
NO METHODS TO CHANGE a string – but can get around with reassg.
String is a char* pointer basically.
greeting = “Howdy”;
memory leak? No automatic garbage collection – supports
automation principle.
Test strings = s.equals(t) s.equalsIgnoreCase(t);
CANNOT use ==
String toUpperCase( );
CMPS4143 Contemporary Programming Languages
Java Fundamentals cont.
Reading 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
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.
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 getPercentInstance
formatter.setMaximumFractionDigits(4);
String sdoubleNum = formatter.format (doubleNum);
System.out.print (sdoubleNum);
Can also set min integer digits???
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
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 in for loop
CAREFUL: using real numbers in for loops i+= 0.1 for update ROUNDOFF ERRORS
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
break in a loop
labeled break better!
read_data:
while
{
break read_data;
}
continue transfer control to header of innermost loop
CONVENTION: don’t use break or continue!!!
BigNumbers
BigInteger and BigDecimal from java.math package.
BigInteger c = a.add(b);