Why Java



Java Basics, Data Types, and Arithmetic

Variables and Computer Memory

Variables are used to store values. The operating system reserves locations in memory for each variable declared in a program. The value of a variable can change during program execution.

Constants

The value of a constant remains invariable during program execution. Constants are declared with the keyword final. By stylistic convention, constant names are all uppercase.

Literal Constant

final int WIDTH=20; //20 is a literal constant

myLbl.setText(“Hello World”); //Hello World is a literal constant

Symbolic Constant

Name of constant is used

m = WIDTH + 10; //WIDTH is a symbolic constant

Data types (by precision with maximum)

Integer types

byte 127

short 32767

int 2.1 billion (int is the default integer type)

long 9 x 1018

real types (decimal)

float 3.4E+38

double 1.8E+308 (double is the default real type)

char

a character in single quotes (‘)

char b = ‘$’;

Scientific Notation (double)

4.7E03 means 4.7 x 103

String (String Object)

Text enclosed in double quotes (“)

String name = “Sally”;

Variable declaration

int x;

int i,j,k;

Variable declare and initialize

int i=0;

double y = 8;

double x= 3*y;

Variable Assignment (= operator)

x = 21;

y = 4.7;

z = x + y; //x + y is evaluated and the result is stored in z

Arithmetic Operators

Add +

Subtract -

Multiplication *

Division /

Modulus % ( also called Remainder – usually used with integers)

When division is used with 2 integers the result is int. If one value is double, the result is double. For example if a = 12 and b = 5, x = 12.0 and y = 5.0:

a/ b is evaluated as 2

a / y is evaluated as 2.4

x / b is evaluated as 2.4

Arithmetic Operators - Rules of Precedence

(recall the phrase “Please Excuse My Dear Aunt Sally.”)

Sub expression ( )

Unary + or -

Multiplication/Division * / % (multiple * / % evaluate left to right)

Addition/Subtraction + - (multiple + - evaluate left to right)

Math Class

The Math class is built-in – no import statement is needed. Dot syntax is used for Math methods. The Math.pow method is used for exponentiation.

double x = Math.min(a,b); //minimum of a and b

area = Math.PI*r*r; //area of a circle with radius r

y = Math.pow(5.6,3),x,y); //the cube of 5.6

r = Math.random(); //random number in the range 0 - 1

z = Math.sqrt(405.33) //square root

Casting

Implicit Assignment Conversion uses Numerical Promotion

double x = 5; //stores 5.0 in x

Explicit - type cast operator

(double) x/3

Use casting to change the result of integer division to double. If a is 12 and b is 5:

(double) a / b is evaluated as 2.4. Without the cast, the result is 2.

Assignment Statement

sum = sum + x; // The new value of sum (the left had side or LHS) is the old value of sum (right hand side or RHS) plus x. The LHS of an assignment statement can list only one variable name. The RHS contains a variable name, value, or expression. The expression is evaluated and the result is stored in the variable (implicitly cast to the correct type).

Accumulator Expressions

Example: sum += X;

Operators are += -= /= *= %=

Increment Operators

Example: y++;

Operators are ++ --

Formating

Include the following import statement: import java.text.*;

For double (Inserts commas when needed)

NumberFromat myFormat = NumberFormat.getInstance();

myFormat.setMaximumFractionDigits(3);

myFormat.setMinimumFractionDigits(1);

String numStr = myFormat.format(num);

For Currency (Includes dollar sign and 2 decimal places)

NumberFromat myCurFormat = NumberFormat.getCurrencyInstance();

String currStr = myCurFormat.format(num);

For percents

NumberFormat pctFormat = NumberFormat.getPercentInstance();

String pctStr = pctFormat.format(num);

Exceptions (catching errors in numeric input)

try

{

//get data

int num = Integer.parseInt(ageTxt.getText());

}

catch(NumberFormatException err)

{

//handle the exception

showStatus(“Bad data”);

}

Wrapper Class Conversion Methods

String to Numeric

double a = Double.parseDouble(aTxt.getText());

int j = Integer.parseInt(jTxt.getText());

Numeric to String

String nStr =”” + num;

(There are also valueOf functions from Java 1 that are now deprecated).

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