General Features of Java Programming Language

Variables and Data Types

Operators

Expressions

Control Flow Statements

The Basic Demo Program

public class BasicsDemo {

public static void main(String[] args) {

int sum = 0;

for (int current = 1; current <= 10; current++) {

sum += current; }

System.out.println("Sum = " + sum);

}

}

The Count Class

import java.io.*;public class Count { public static void countChars(Reader in) throws IOException { int count = 0; while (in.read() != -1) count++; System.out.println("Counted " + count + " chars."); }}

Go Back: 6, 8, 17

Running the countChars

• import java.io.*;public class Count {// ... countChars method omitted ...public static void main(String[] args) throws Exception{if (args.length >= 1) countChars(new FileReader(args[0]));else System.err.println("Usage: Count filename"); }}

Variables and Data Types

Variables: Entities that act or are acted upon

Two Variables in Count: count and in

Variable Declaration

• Variable Declaration:– Type of the variable– Name of the variable

• The location of the variable declaration determines its scope.

Data Type

• Java is static-typed: All variables must have a type and have to be declared before use.

• A variable's data type determines its value and operation

• Two categories of data types in Java– primitive data type: byte, short, int, long, float....– reference data type: class, interface, array

count is ?

in is ?

Primitive

Reference

Primitive and Reference Data Type

int x;x = 5;

x: 5

Point p1, p2;p1 = new Point();p2 = p1;

p1:

p2:

x: 0

y: 0Primitive Data

TypeReference Data Type

Primitive Data Types

• byte 8-bit• short 16-bit• int 32-bit• long 64-bit• float 32-bit floating point• double 64-bit floating point• char 16-bit Unicode• boolean true/false

Variable Names

• Java refers to a variable's value by its name.• General Rule

– Must be Legal Java identifier– Must not be a keyword or a boolean literal– Must not be the same name as another variable in the

same scope

• Convention:– Variable names begin with a lowercase letter

• isEmpty, isVisible, count, in– Class names begin with an uppercase letter

• Count

Reserved Words(Keywords)

abstract default if private throwboolean do implements protected throwsbreak double import public transientbyte else instanceof return trycase extends int short voidcatch final interface static volatilechar finally long super whileclass float native switchconst* for new synchronizedcontinue goto* package this

Don't worry about what all these words mean or do, but be aware that you cannot use them for other purposes like variable names.

Don't worry about what all these words mean or do, but be aware that you cannot use them for other purposes like variable names.

Variable Scope

• The block of code within which the variable is accessible and determines when the variable is created and destroyed.

• The location of the variable declaration within your program establishes its scope

• Variable Scope: – Member variable – Local variable – Method parameter – Exception-handler parameter

Variable Scope

Variable Initialization

• Local variables and member variables – can be initialized with an assignment statement when

they're declared.– The data type of both sides of the assignment

statement must match.• int count = 0;

• Method parameters and exception-handler parameters– cannot be initialized in the same way as local/member

vars– The value for a parameter is set by the caller.

Final Variables

• The value of a final variable cannot change after it has been initialized.

• You can view final variables as constants.• Declaration and Initialization

– final int aFinalVar = 0;– final int blankfinal;

. . . blankfinal = 0;

Literals (I)

• To represent the primitive types• Integer

– Decimal Value– Hexadecimal Value: 0x... (0x1f = 31)– Octal Value: 0... (076=62)

• Floating Point– 3.1415– 6.1D2 (64-bit Double; Default)– 3.4F3 (32-bit Float)

Literals (II)

• Characters– ' ..... ‘ e.g. ‘a’– '\t', '\n' (Escape Sequence)

• Strings– ".......“ e.g. “Hello World!”– String Class (Not based on a primitive data type)

Operators

Operators perform some function on operands.

An operator also returns a value.

Operators (I)

• Arithmetic Operators– Binary: +, -, *, /, %– Unary: +, -, op++, ++op, op--, --op

• Relational Operators– >, >=, <, <=, ==, != (return true and false)

• Conditional Operators– &&(AND), ||(OR), !(NOT), &(AND), |(OR)– expression ? op1 : op2

• Bitwise Operators– >>, <<, >>>, &, |,^,~

(i>5) ? j=1 : j=2

if (i>5) j=1;else j=2;

Operators (II)

• Assignment Operators– =– += – -=, *=, /=, %=, &=, !=, ^=, <<=, >>=, >>>=

op1 += op2

s += 2

op1 = op1 + op2

s = s + 2

Expressions

Perform the work of a Java ProgramPerform the computation

Return the result of the computation

Expression

• An expression is a series of variables, operators, and method calls that evaluates to a single value. – count ++– in.read() != -1

• Precedence– Precedence Table– Use (.....)

• Equal precedence – Assignment: Right to Left (a = b =c)– Other Binary Operators: Left to Right

Expressions and operators• An expression is a program fragment that evaluates

to a single value– double d = v + 9 * getSalary() % PI;

• Arithmetic operators– additive +, -, ++, --

– multiplicative *, / % (mod operator)

• Relational operators– equality == (NB)– inequality !=– greater than and less than >, >=, <, <=

Control Flow Statement

If Statements

• if (boolean) {/* ... */ }else if (boolean) {/* ... */ }else {/* ... */ }

• The expression in the test must return a boolean value– Zero('') can't be used to mean false, or non-zero("...")

to mean true

Statement Block

Example

if (income < 20000){ System.out.println (“poor”);}else if (income < 40000){ System.out.println (“not so poor”);}else if (income < 60000){ System.out.println (“rich”);}else { System.out.println (“ very rich”);}

Loops

• while (boolean expression) {/* ... */}

• do {/* ... */} while (boolean expression)

• for (expression; booleanExpression; expression) {/* ... */}

Example

// count from 1 to 10int i = 1;while (i<=10) {

System.out.println (i);i= i+ 1;

}

Example

// count from 1 to 10int i = 1;do {

System.out.println (i)i= i+ 1;

} while (i< 10);

The for statement

• The for statement has the following syntax:

for (initialisation; condition; increment) statement;

Reserved word

The initialisation partis executed once beforethe loop begins

The statement isexecuted until thecondition becomes false

The increment part is executedat the end of each iteration

Example

// count from 1 to 10for (int i = 0; i < 10; i++) System.out.println (i);

Secret! A for Loopcan always be converted to

a while loop

i = 0;

while (i < 10)

{

System.out.println(i);

i++;i++;

}

for(i = 0; i < 10; i++i++)

{

System.out.println(i);

}

This will help you understand the sequence of operations of a for loop

Switch

• switch (expression) {case Constant1:/* ... */break;case Constant2:/* ... */break;....default:/* ... */break;}

Multiple Selections via switch Note the "optional" default case at the end of the switch statement. It is technically optional It is technically optional onlyonly in terms of syntax. in terms of syntax.

switch (number) { case 1:

System.out.println ("One");break;

case 2:System.out.println ("Two");break;

case 3:System.out.println ("Three");break;

default: System.out.println("Not 1, 2, or 3"); break; // Needed???

} // switch

For safety and good programming practice, always include a 'default' case.

This mightwork without

the defaultcase, but would be

poortechnique

How many days?if (month == 4 || month == 6 ||

month == 9 || month == 11)

numdays = 30;

else if (month == 2)

{

numdays = 28;

if (leap)

numdays = 29;

}

else

numdays = 31;

Switchswitch (month)

{

case 4:

case 6:

case 9:

case 11:

numdays = 30;

break;

case 2:

numdays = 28;

if(leap)

numdays = 29;

break;

default: /* Good idea? */

numdays = 31;

}