Control statements

Chapter 4
Control Structures

http://www.java2all.com

Structured Programming
“ There is No goto in Java ”
• Structured programming: the building blocks
• There are 3 different kinds of operations in a program:
perform a sequence of actions,
perform a selection between alternative actions, or
perform a repetition or iteration of the same action.

Sequence, Selection, Iteration


• Sequence: one thing after another

task1

task2

task3


• Selection: making choices

YES
? taskA

Structured NO
programming,
only one entrance, taskB
only one exit.


• Repetition, Part I: doing the same thing again until
there’s a reason to stop.

TRUE
expression taskA
?
FALSE

Do while: maybe won’t ever do taskA even once.
“A while loop repeats as long as a condition is true.”


• Repetition, Part II: doing the same thing again until
there’s a reason to stop.

taskA

TRUE
?

FALSE
Do until: will always do taskA at least once.
“A Do Until loop repeats as long as a condition is false.”


Procedural Structured Programming
• Begin at the top, move to the bottom.
• Each program unit has only one entrance and only
one exit.


Selection in Java
Object Oriented Programming
• Within a method, procedural code.
• Simple ‘if’ with or without brackets.

if( expression )
statement;

if( expression )
{
statement;
}

Selection in Java

• Simple ‘if’ with or without brackets.

if( expression )
statement;

if( expression )
{
statement; • Within brackets, a “block.”
}

Selection in Java
• Simple ‘if’ / ‘else’ without brackets.
if( expression )
statement;
else
statement;

• Without brackets, limit of only one statement per branch.


w
Selection in Java
• Simple ‘if’ / ‘else’ with brackets.

if( expression )
{
statement;
statement;
}
else
{
statement;
statement;
} http://www.java2all.com

Selection in Java
• Compound ‘if’ / ‘else if’ / ‘else if’ / ‘else’.
if( expression )
{
statement;
}
else if( expression )
{
statement;
}
else
{
statement;
}

If Statement Syntax
• Decision Making
• The if exactly mirrors C/C++, and it has three variants:
1.) if( expression )
statement;
statement;
else
statement;
statement;
statement;
else
statement;


If Statement Syntax
• Simple If
• The “expression” must be something that uses the
comparison operators and resolves to either true or false.
if( expression )
if( expression )
{ statement;
statement1;
statement2;
}
• The statement is executed if the expression is true.
• Only one statement can be made conditional without
brackets. If you wish to conditionally execute more than
one statement, you use brackets to create a block.

If Statement Syntax
• Simple if/else
• If the “expression” is true, the if branch executes, if not,
the else branch executes.

if( expression )
statement;
else
statement;


If Statement Syntax to label the
Don’t bother
closing brackets unless
• Simple if/else you have a really long if.
• If the “expression” is true,Still if branch executes, if not,
the you should always line
the else branch executes. brackets.
up your
if( expression )
if( expression )
{
{
statement1;
statement1;
statement2;
statement2;
} } //end of if else
{
else{
statement3;
statement3;
statement4;
statement4; }
} // end of else


If Statement Syntax
• Compact if/else if/ else
• To prevent your nested ‘if’s from marching across
the page, you can use this nested if. You can go on nesting
them as long as you like, and the last one is just an else.

if( expression )
statement;
statement;
else
statement;


Multiple-Selection Structure
(switch selection statement)
• Once you start nesting many ‘if’s, it becomes a nuisance.
• Java—like C and C++ before it—provides the switch
structure, which provides multiple selections.
• Unfortunately—in contrast to Visual Basic’s Select
Case and even COBOL’s Evaluate—you cannot use
any of type of argument in the switch statement other
than an integer.


int x = 0;
switch( x ) • The integer expression x is
{
case 1:
evaluated. If x contains a 1,
do stuff; then the case 1 branch is
break;
case 2: performed. Notice the
do stuff; ‘break;’ statement.
break;
case 55: This is required. Without it,
do stuff; every line after the match
break;
case 102: will be executed until it
case 299:
do stuff okay for both;
reaches a break;
break;
default:
if nothing else do this stuff;
break;
} w

• The expression within the switch( expression )
section must evaluate to an integer.
• Actually, the expression can evaluate to any of these
types (all numeric but long):

byte
short
int
char

but they will be reduced to an integer and that value
w
will be used in the comparison.


• The expression after each case statement can only be a

constant integral expression

—or any combination of character constants and integer
constants that evaluate to a constant integer value.


• The default: is optional.
• If you omit the default choice, then it is possible
for none of your choices to find a match and that
nothing will be executed.

• If you omit the break; then the code for every
choice after that—except the default!—will be
executed.


• Question: if only integer values can appear in the
switch( x ) statement, then how is it possible for
a char to be the expression?


Counter-Controlled Repetition
• Used when you know in advance how many times you
want the loop to be executed.
4 Requirements:
1. Variable to count the number of repetitions
2. Starting value of counter
3. Amount in increment the counter each loop
4. The condition that decides when to stop looping.


• The for Loop
• A common structure called a for loop is specially
designed to manage counter-controlled looping.

for( int x = 1; x < 10; x++ )

1.) count variable,
2.) starting value
3.) Increment

4.) condition, final value


// ForCounter.java
import java.awt.Graphics;
import javax.swing.JApplet;

public class ForCounter extends JApplet
{
public void paint( Graphics g )
{ 1. 2. 4. 3.
for( int counter=1 ; counter <= 10 ; counter++ )
{
g.drawLine( 10, 10, 250, counter * 10 );
}
}
}
1.) count variable
2.) starting value
3.) increment
4.) condition, final value

• When appropriate, the for is quick and easy.


• The for loop is a do-while.

• It tests the condition before it
executes the loop for the first time.

( • Note: since the variable int counter was
declared within the for , it vanishes after the
for is finished. )


1. 2.
int counter = 1;

TRUE
3. counter <= 10
?
{ body }

FALSE counter++

4.


• All Three Sections are Optional
• Effects of Omitting Sections: condition

for( int int x = x < 10; x++ )
for( x = 1; 1;; x++ )

• If you omit the condition, Java
assumes the statement is true, and
you have an infinite loop.


• Effects of Omitting Sections: initialization

int x = 1;
for( int x = 1; x++ )
for(; x < 10; x < 10; x++ )

• You can omit the initialization if
you have initialized the control
variable someplace else.


• Effects of Omitting Sections: increment

for( int x x = 1; x < 10;)
for( int = 1; x < 10; x++ )
{
other stuff
x++;
}
• You can omit the increment of
the variable if you are doing so
within the body of the loop.


• Can Use the while Loop
• Although the while is usually used when we don’t
know how many times we’re going to loop, it works just
fine.
• Still must supply the 4 Requirements.


Repetition: while Part I

while—“the Do While”
• The Test is First

while( expression )
statement;


Repetition: while Part I
w

while—“the Do While”
• The Test is First

The while { “Do While” } is used when you
can’t predict exactly how many times your
loop will be executed.

while( expression )
{ The while may not be executed even once.
statement;
It executes the loop while the expression is
statement; still true.
}

// WhileTest.java
// Since "c" is already false when it reaches the
// test, the loop never executes.

public class DoWhile
{
public static void main( String args[] )
{
boolean c = false

while( c )
{
System.out.println( ”Execute DoWhile while c is
true" );
}

System.exit( 0 );
}
}


Repetition: while Part w
II
while—“the Do Until”
• The Test is Last

do
{
statement;
statement;
}
while( expression );


Repetition: while Part w
II
while—“the Do Until”
• The Test is Last

do
{This do/while {“Do Until”} is also used when
you statement;
can’t predict exactly how many times
statement; be executed.
your loop will
}
It executes at least once.
while( expression ); becomes
It executes Until the expression
false.


// DoUntil.java
// Even though "c" begins the loop false,
// it still executes at least once.

public class DoUntil
{
public static void main( String args[] )
{
boolean c = false;

do
{
System.out.println( ”Execute DoUntil at least once " );
}
while( c );

System.exit( 0 );
}
}


Statements
break; and continue;
• Both of these statements alter the flow of control.

• The break statement can be executed in a:

while
do/while
for
switch

• break causes the immediate exit from the structure

Statements break; and continue;
• After a break exits the “structure”—whatever that
is—execution resumes with the first statement
following the structure.

• If you have nested structures—be they a while,
do/while/ for or switch—the break will only exit the
innermost nesting.

• break will not exit you out of all nests. To do that,
you need another break*

* There is a variant of the break called a labeled break
—but this is similar to a goto and is frowned upon.


• The continue statement, when used in a while or
do/while or a for, skips the remaining code in the
structure and returns up to the condition.

• If the condition permits it, the next iteration of the
loop is permitted to continue.

• So, the continue is a “temporary break.”

• The continue is only used in iterative structures,
such as the while, do/while and for.


• The “Labeled” continue and break statements send
execution to the label to continue execution.
• Note: using the labeled break and continue is bad code.
Avoid using them!
stop:

for(row = 1; row <= 10; row++)
{
for(col=1; col <=5; col++)
{
if( row == 5)
{
break stop; // jump to stop block
}
output += “* “;
}
output += “n”;
}

Control statements

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