Java has very flexible
three looping mechanisms. You can use one of the following three loops:
· while Loop
· do...while Loop
· for Loop
As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.
The while Loop:
A while loop is a
control structure that allows you to repeat a task a certain number of times.
Syntax:
The syntax of a while
loop is:
while(Boolean_expression)
{
//Statements
}
When executing, if the boolean_expression result is true, then the actions inside the loop
will be executed. This will continue as long as the expression result is true.
Here, key point of the while loop is that the loop might not ever run. When the expression is
tested and the result is false, the loop body will be skipped and the first
statement after the while loop will be executed.
Example:
public class Test {
public static void main(String args[]) {
int
x = 10;
while(
x < 20 ) {
System.out.print("value of x :
" + x );
x++;
System.out.print("\n");
}
}
}
This would produce the
following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
The do...while Loop:
A do...while loop is
similar to a while loop, except that a do...while loop is guaranteed to execute
at least one time.
Syntax:
The syntax of a
do...while loop is:
do
{
//Statements
}while(Boolean_expression);
Notice that the Boolean
expression appears at the end of the loop, so the statements in the loop
execute once before the Boolean is tested.
If the Boolean
expression is true, the flow of control jumps back up to do, and the statements
in the loop execute again. This process repeats until the Boolean expression is
false.
Example:
public class Test {
public static void main(String args[]){
int
x = 10;
do{
System.out.print("value of x :
" + x );
x++;
System.out.print("\n");
}while( x < 20 );
}
}
This would produce the
following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
The for Loop:
A for loop is a
repetition control structure that allows you to efficiently write a loop that
needs to execute a specific number of times.
A for loop is useful
when you know how many times a task is to be repeated.
Syntax:
The syntax of a for loop
is:
for(initialization; Boolean_expression; update)
{
//Statements
}
Here is the flow of
control in a for loop:
· The initialization step is executed first, and
only once. This step allows you to declare and initialize any loop control
variables. You are not required to put a statement here, as long as a semicolon
appears.
· Next, the Boolean expression is evaluated. If it
is true, the body of the loop is executed. If it is false, the body of the loop
does not execute and flow of control jumps to the next statement past the for
loop.
· After the body of the for loop executes, the
flow of control jumps back up to the update statement. This statement allows
you to update any loop control variables. This statement can be left blank, as
long as a semicolon appears after the Boolean expression.
· The Boolean expression is now evaluated again.
If it is true, the loop executes and the process repeats itself (body of loop,
then update step, then Boolean expression). After the Boolean expression is
false, the for loop terminates.
Example:
public class Test {
public static void main(String args[]) {
for(int x = 10; x < 20; x = x+1) {
System.out.print("value of x :
" + x );
System.out.print("\n");
}
}
}
This would produce the
following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
Enhanced for loop in
Java:
As of Java 5, the
enhanced for loop was introduced. This is mainly used for Arrays.
Syntax:
The syntax of enhanced
for loop is:
for(declaration : expression)
{
//Statements
}
· Declaration: The newly declared block variable, which is of a type compatible
with the elements of the array you are accessing. The variable will be
available within the for block and its value would be the same as the current
array element.
· Expression: This evaluates to the array you need to loop through. The
expression can be an array variable or method call that returns an array.
Example:
public class Test {
public static void main(String args[]){
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ){
System.out.print( x );
System.out.print(",");
}
System.out.print("\n");
String [] names ={"James", "Larry", "Tom", "Lacy"};
for( String name : names ) {
System.out.print( name );
System.out.print(",");
}
}
}
This would produce the
following result:
10,20,30,40,50,
James,Larry,Tom,Lacy,
The break Keyword:
The break keyword is used to stop the entire loop. The break keyword must be
used inside any loop or a switch statement.
The break keyword will
stop the execution of the innermost loop and start executing the next line of
code after the block.
Syntax:
The syntax of a break is
a single statement inside any loop:
break;
Example:
public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
break;
}
System.out.print( x );
System.out.print("\n");
}
}
}
This would produce the
following result:
10
20
The continue Keyword:
The continue keyword can be used in any of the loop control structures. It
causes the loop to immediately jump to the next iteration of the loop.
· In a for loop, the continue keyword causes flow
of control to immediately jump to the update statement.
· In a while loop or do/while loop, flow of
control immediately jumps to the Boolean expression.
Syntax:
The syntax of a continue
is a single statement inside any loop:
continue;
Example:
public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
continue;
}
System.out.print( x );
System.out.print("\n");
}
}
}
This would produce the
following result:
10
20
40
50
No comments:
Post a Comment