Basic Java Programming

CSCI 392

Week Two

Stuff that is the same as C++

for loops and while loopsfor (int i=0; i<10; i++)

if elseif (count != 10)

increment, decrement, etcresult *= factorial--;

switch statementsswitch (answer) { case 'y': case 'Y': some stuff break; default:}

Java Statements Not in C++

Labeling blocks of codeouterloop: for (int i=0;…) for (int j=0; … if (something_bad) break outerloop;

Exception Handlingtry { value = x/y; } catch (ArithmeticException exceptvar) { System.out.println("Math Error"); }

Basic Data Types

Boolean boolean - 1 bit

Integers byte - 8 bits short - 16 bits int - 32 bits long - 64 bits

Characters char - 16 bits

Floats float double

Strings String still be careful when comparing

Boolean - So what?

This C++ feature is not legal in Java:

int factorial, result = 1;. . .while (factorial) result *= factorial--;

But neither is this common C++ error:

if (count = 10) // not legal Java

Variable Declaration Oddities

You must initialize a variable before it can be used, so most declarations include an initialization.

You can set a "constant's" value at run time.final int count = getCount();

Arrays require both a definition and declaration. int[] myarray;myarray = new int[25];

int[] myarray = new int[25];

char[] name = {'B', 'o', 'b'};

Type Checking

You can assign small sized variables into bigger variables, but big variables don't fit into small variables.

int X = 99;

long Y = 0;

Y = X; // legal

X = Y; // not legal

X = (int) Y; // legal

Comments

Just like C/* multi-line */

// single line

Except…/** document comment **/ not thrown away by compiler

Simple Output

Printing to "Standard Output"System.out.print ("Hello " + name );

System.out.println ();

Formatted OutputNumberFormat layout = NumberFormat.getNumberInstance();layout.setMaximumFractionDigits(4); double x = 12.3, y = 45.6;String outstr = layout.format(x/y);System.out.println("x/y = " + outstr);

Input - not so simple

import java.io.* ;

public class array_test_1 { public static void main(String[] args) throws Exception { InputStreamReader stdio = new InputStreamReader(System.in); BufferedReader instream = new BufferedReader (stdio); int count;

System.out.print ("Enter an integer: "); String indata = instream.readLine(); count = Integer.parseInt (indata); for (int i=0; i<count; i++) System.out.print (i + " "); System.out.println(); }}

improvement: catch the NumberFormatException