Web Programming with Java - Java Basics docx

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Web Programming with Java
Java Basics
Huynh Huu Viet
Email:
Department of Information Systems
2008 © Department of Information Systems - University of Information Technology
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Outline
Primitive Data Types and Operations
Control Structures
 Selection Statements
 Loop Statements
Methods
Arrays
Q&A
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Identifiers
 Identifiers:
 Names for the things you will refer to in your
programs
 Used for programming entities as variables,
constants, methods, classes, and packages
 Rules for naming identifiers:
 A sequence of characters that consists of letters,
digits, underscores (_), and dollar signs ($).
 Cannot start with a digit.
 Cannot be a reserved word.
 Cannot be true, false, or null.
 An identifier can be of any length.

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Variables & Constants
 Variables: used to store data in a program
datatype variableName;
datatype variable1, variable2, , variablen;
 Constants
 Represents permanent data that never changes
final datatype CONSTANTNAME = VALUE;
 What is benefits of using constants ???
• Don't have to repeatedly type the same value;
• Can be changed in a single location if necessary;
• Makes the program easy to read.
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Data Types and Operations (1)
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Data Types and Operations (2)
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Numeric Type Conversions
 Conversions rules
 If one of the operands is double, the
other is converted into double.
 Otherwise, if one of the operands is float,
the other is converted into float.
 Otherwise, if one of the operands is long,
the other is converted into long.
 Otherwise, both operands are converted

into int.
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Operand Evaluation Order
The rule of evaluating an expression:
 Rule 1: Evaluate whatever sub-expressions
you can possibly evaluate from left to right.
 Rule 2: The operators are applied according
to their precedence
 Rule 3: The associativity rule applies for two
operators next to each other with the same
precedence
You can use parentheses to force an
evaluation order
Does parentheses slow down the
execution of the expression???
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Operator Precedence (1)
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Operator Precedence (2)
Example:
 int a = 0;
 int x = a + (++a);
 Îx=?
 int a = 0;
 int x = ++a + a;
 Îx=?
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boolean Data Type & Operations
Java also provides
 & and | operators (unconditional)
 A little bit different from && and || operators
(conditional)
(x != 0) & (100 / x) ???
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Programming Errors (1)
Syntax errors (compilation errors)
Runtime Errors
public class ShowSyntaxErrors {
public static void main(String[] args) {
i = 30;
System.out.println(i + 4);
}
}
public class ShowRuntimeErrors {
public static void main(String[] args)
{
int i = 1 / 0;
}
}
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Programming Errors (2)
Logic Errors
public class ShowLogicErrors {
public static void main(String[] args) {

// Add number1 to number2
int number1 = 3;
int number2 = 3;
number2 += number1 + number2;
System.out.println("number2 is " + number2);
}
}
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Control Structures
Selection Statements
 if Statements
 switch Statements
 Conditional Expressions
Loops
 while Loop
 do-while Loop
 for Loop
 break and continue
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Outline
Primitive Data Types and Operations
Control Structures
 Selection Statements
 Loop Statements
Methods
Arrays
Q&A
2008 © Department of Information Systems - University of Information Technology

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if Statements
Simple if Statements
if else Statements
if (booleanExpression) {
statement(s);
}
if (booleanExpression) {
stat(s)-for-the-true-case;
}
else {
stat(s)-for-the-false-case;
}
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if Statements - notes
Common mistake
else matching
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switch Statements
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2;
break;
…
case valueN: statement(s)N;
break;
default: statement(s)-for-default;

}
 switch-expression must yield a value of char, byte, short, or int
 The value1, . . ., and valueN must have the same data type as the value
of the switch-expression and be constant expressions
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Conditional Expressions
Syntax:
• booleanExpression ? expression1 : expression2;
Example
Why we need this operator???
y = (x > 0) ? 1 : -1;
if (x > 0) y = 1
else y = -1;
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Control Structures
Selection Statements
 if Statements
 switch Statements
 Conditional Expressions
Loops
 while Loop
 do-while Loop
 for Loop
 break and continue
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while Loop
while (loop-continuation-condition) {

// Loop body
Statement(s);
}
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do-while Loop
do {
// Loop body;
Statement(s);
} while (loop-continuation-condition);
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for Loop
for (initial-action; loop-continuation-condition; action-after-each-iteration) {
// Loop body;
Statement(s);
}
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Loops
Which Loop to Use?
Common errors:
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break and continue
break
 Immediately ends the innermost loop that
contains it.
 Generally used with an if statement.
continue

 Only ends the current iteration.
 Generally used with an if statement.