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Flow control
statements
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page 1 of
5
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The flow control statements
allow you to conditionally execute statements, to repeatedly execute a block
of statements, or to just change the sequential flow of control.
if/else
statement
The if/else statement is used for
decision-making -- that is, it decides which course of action needs to be
taken. The syntax is:
if(boolean expression)
{
// do this if the expression is true
}
else
{
// do this if the expression is false
}
The
else part in the
if/else statement is optional. The
curly braces are optional if the body is limited to a single statement. (Note
that we cannot use numeric values to represent true and false as we do in
C/C++.) For instance:
if (x == 5) {} // compiles, executes body if x is equal to 5
if (x = 0) {} // does not compile, because expression is non-boolean
if (x = true) {} // compiles, but the body is always executed
In the case of nested
if/else statements, each
else clause belongs to the closest
preceding if statement that does not
have an else.
switch
statement
The switch statement is also used for
decision-making, based on an integer expression. The argument passed to the
switch and
case statements should be
int,
short,
char, or
byte. The argument passed to the
case statement should be a literal or a
final variable. If no case matches, the
default statement (which is optional)
is executed.
When a
break statement is encountered, the
control moves out of the switch
statement. If no break statement is
given, all the case statements are
executed until a break is encountered or the
switch statement ends. For
instance:
int x = 1;
switch(x)
{
case 1: System.out.println("one");
case 2: System.out.println("two");
}
// both one and two are printed
Notice the position of the
default statement. Though the
default statement is usually placed at
the end of all the case options, it is not mandatory as you can see in the
example below:
int i = 1;
switch(i)
{
default:
System.out.println("Default");
break;
case 0:
System.out.println("Zero");
break;
case 1:
System.out.println("One");
break;
}
Note that the control comes to
the default statement only if none of
the cases match. However, it is a good practice to have the
default statement at the end itself.
Loop statements
The three types of Java looping constructs are
while,
do-while, and
for.
while
loop
The syntax of the while loop is:
while(boolean expression) {}
The body of the
while loop is executed only if the
expression is true, so it may not be executed even once:
while(false){} // body never executed
while(1) {} // code will not compile, not a boolean
do-while
loop
The syntax of the do-while loop
is:
do { } while(boolean expression);
The body of the
do-while loop is executed at least once
because the test expression is evaluated only after executing the loop body.
Also, don't forget the ending semicolon after the
while expression.
for
loop
The syntax of the for loop is:
for(expr1; expr2; expr3)
{
// body
}
expr1
is for initialization, expr2 is the
conditional test, and expr3 is the
iteration expression. Any of these three sections can be omitted and the
syntax will still be legal:
for( ; ; ) {} // an endless loop
There can be more than one
initialization expression and more than one iteration expression, but only one
test expression.
break
and continue
The break statement is used to exit
from a loop or
switch statement, while the
continue statement is used to skip just
the current iteration and continue with the next.
In the case of nested loops,
the break statement exits from the
innermost loop only. If a break keyword
(see Java
keywords and identifiers for more information on keywords) is combined
with a label, a break statement will
exit out of the labeled loop:
outer: for (int i = 0; i < 10; i++)
{
while(y > 0)
{
break outer;
}
}
// breaks from the for loop if y > 0
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350 Mock Questions on SCJP 1.5 - JUST Rs.200 or 7 USD
Send us mail to sales@javabeat.net
more details
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