Control flow statements in java web applications

Control flow statements
Rajith Karunarathne

Java Control Statements
Java Control statements control the order of execution in a java program, based
on data values and conditional logic.
There are three main categories of control flow statements;
 Selection statements: if, if-else and switch
 Loop statements: while, do-while and for
 Transfer statements: break, continue, return, try-catch-finally
We use control statements when we want to change the default sequential order of
execution

Selection statements
The If Statement
 The if statement executes a block of code only if the specified expression is true.
 If the value is false, then the if block is skipped and execution continues with the rest of the
program.
 You can either have a single statement or a block of code within an if statement.
 Note that the conditional expression must be a Boolean expression
The simple if statement syntax:
if (<conditional expression>)
<statement action>

Example
public class IfStatement {
public static void main(String[] args) {
int a = 10, b = 20;
if (a > b)
System.out.println("a is greater than b");
if (a < b)
System.out.println("b is greater than a");
}
}

Selection statements…
The If-else Statement
 If the statements in the if statement fails, the statements in the else block are executed.
 You can either have a single statement or a block of code within if-else blocks
The if-else statement syntax:
if (<conditional expression>)
<statement action>
else
<statement action>

Example
public class IfElseStatement {
int a = 10, b = 20;
if (a >b) {
System.out.println("a is greater than b");
} else {
System.out.println("b is greater than a");
}
}
}

Selection statements…
Switch Case Statement
 It is a multi-way branch with several choices
 It compares the value of the controlling expression to the values of each case label
 It includes a default label to use in cases where there are no matches
The Switch-Case statement syntax:
switch (<expression>) {
case label1: <statement1>; break;
…
case labeln: <statementn>; break;
default: <statement>;
}

Example
public class SwitchCaseStatement {
int status = -1;
switch (status) {
case 1: System.out.println(“You have entered number 1");
break;
case 2: System.out.println("You have entered number 2");
break;
case 3: System.out.println("You have entered number 3");
break;
default: System.out.println("You have entered something else");
}
}
}

Iteration statements
For Loops
 The for loop is a looping construct which can execute a set of instructions a specified number
of times.
 It’s a counter controlled loop.
The syntax of the loop is as follows:
for (<initialization>; <loop condition>; <increment expression>)
<loop body>

Example
public class ForLoop {
System.out.println("Printing Numbers from 1 to 10");
for (int count = 1; count <= 10; count++) {
System.out.println(count);
}
}
}

Iteration statements…
While Statement
 This is a looping construct control statement that executes a block of code while a condition
is true.
 You can either have a single statement or a block of code within the while loop.
 The loop will never be executed if the testing expression evaluates to false.
 The loop condition must be a boolean expression.
The while loop syntax:
while (<loop condition>)
<statements>

Example
public class WhileLoop {
int count = 1;
while (count <= 10) {
System.out.println(count++);
}
}
}

Iteration statements…
Do-while Loop Statement
 The do-while loop is similar to the while loop, except that the test is performed at the end of
the loop instead of at the beginning.
 This ensures that the loop will be executed at least once.
The do-while loop syntax:
do
<loop body>
while (<loop condition>);

Example
public class DoWhileLoop {
int count = 1;
do {
System.out.println(count++);
} while (count <= 10);
}
}

Transfer statements
Continue Statement
 A continue statement stops the iteration of a loop (while, do or for) and causes execution to
resume at the top of the nearest enclosing loop.
 You use a continue statement when you do not want to execute the remaining statements in the
loop.
 You can use the label in your continue statement (optional).
 It is usually only used when returning to the outermost loop in a series of nested loops.
The continue statement syntax:
continue; // the unlabeled form
continue <label>; // the labeled form

Example
public class ContinueExample {
System.out.println("Odd Numbers");
for (int i = 1; i <= 10; ++i) {
if (i % 2 == 0)
continue; // Rest of loop body skipped when i is even
System.out.println(i);
}
}
}

Transfer statements…
Break Statement
 The break statement transfers control out of the enclosing loop ( for, while, do or switch
statement).
 Use a break statement when you want to jump immediately to the statement following the
enclosing control structure.
 You can also provide a loop with a label, and then use the label in your break statement.
The break statement syntax:
break; // the unlabeled form
break <label>; // the labeled form

Example
public class BreakExample {
System.out.println("Numbers 1 - 10");
for (int i = 1; ; ++i) {
if (i == 11)
break; // Rest of loop body skipped when i is eleven
System.out.println(i);
}
}
}

Control flow statements in java web applications

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