java i lecture_5


Chapter 5

Control Structures: Part II


• 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.

Counter-Controlled Repetition


• 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.



•

Counter-Controlled Repetition// WhileCounter.javaimport java.awt.Graphics;import javax.swing.JApplet;

public class WhileCounter extends JApplet{ public void paint( Graphics g ) {

int counter; // 1.) count variablecounter = 1; // 2.) starting value

while( counter <= 10 ) // 4.) condition, final value{

g.drawLine( 10, 10, 250, counter * 10 ); ++counter; // 3.) increment

} }}


• 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.javaimport 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. )


counter <= 10?

int counter = 1;

{ }

counter++

TRUE

FALSE

1. 2.

3. body

4.


• All Three Sections are Optional

• Effects of Omitting Sections: condition


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

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

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


• Effects of Omitting Sections: initialization


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

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

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


• Effects of Omitting Sections: increment


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

• You can omit the increment of the variable if you are doing so within the body of the loop.

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

other stuff

x++;}


// Calculate Compound Interestimport javax.swing.JOptionPane;import java.text.DecimalFormat;import javax.swing.JTextArea;

public class Interest{

} // end of class Interest

Introducing two new class objects:DecimalFormat

and theJTextArea.

JTextArea is one of many GUI classes that we use to put text onto a window



public class Interest{ public static void main( String args[] ) {

double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig =

System.exit( 0 ); } // end of main()} // end of class Interest

This is the common style of creating objects.ObjectType instanceName.

After this statement executes, twoDig is a complete example, or instance, of the

DecimalFormat class.Actually, since it hasn’t been initialized,

twoDig is still only a reference.

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double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” );


Every class has a default method—a Constructor—whose only purpose is to initialize a fresh instantiation of the class.The new keyword fires the default “Constructor” method. The Constructor always has the exact same name as the class. ( Chapter 6 covers this in greater detail.)

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double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 );


Once again, we have the same pattern. The name of a class—JTextArea—followed by the name of an instantiation of that class—output.Next, we fire off the default Constructor using the new keyword. In this case, we are making a JTextArea 11 columns wide by 20 columns tall.


• A JTextArea is a multi-line area that displays plain text.• This is the actualdocumentation for theJTextArea. You must become comfortablelooking up andinterpreting thisinformation.Here, you should notice that there are several Constructors.The top one—which you notice takes no arguments—is the “default” Constructor.

JTextArea—a brief sidebar





output.append( “Year\tAmount on deposit\n” );


Since output is an object of type JTextArea, it possesses all the methods of that object. Method append says to add the String data to

the JTextArea in the order that we wish it to appear within the JTextArea.

Notice, we can either pass it a String object (a variable that contains a String object), or we can just pass it a String.





output.append( “Year\tAmount on deposit\n” ); for( int year = 1; year <= 10; year++ ) { } // end of for






output.append( “Year\tAmount on deposit\n” ); for( int year = 1; year <= 10; year++ ) { amount = principle * Math.pow( 1.0 + rate, year ); output.append( year + “\t” + twoDig.format(amount) + “\n”); } // end of for


Since Java has no exponentiation operator, we calling on the Math library’s method

(pow for power).Notice, it expects as arguments two

doubles. These will be used in this way:

ab or (1.0 + rate)year

The return value is a double.





output.append( “Year\tAmount on deposit\n” ); for( int year = 1; year <= 10; year++ ) { amount = principle * Math.pow( 1.0 + rate, year ); output.append( year + “\t” + twoDig.format(amount) + “\n”); } // end of for

JOptionPane.showMessageDialog( null, output, “Compound Interest”, JOptionPane.INFORMATION_MESSAGE);



Multiple-Selection Structure• 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 )

{case 1:

do stuff;break;

case 2:do stuff;break;

case 55:do stuff;break;

case 102:case 299:

do stuff okay for both;break;

default:if nothing else do this stuff;break;

}

Multiple-Selection Structure

• The integer expression x isevaluated. If x contains a 1,then the case 1 branch isperformed. Notice the‘break;’ statement. This is required. Without it,every line after the matchwill be executed until it

reaches a break;

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• 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):

byteshortint char

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



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• 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.



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• 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?


Statements break; and continue;

• Both of these statements alter the flow of control.

• The break statement can be executed in a:

whiledo/whileforswitch

• 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”;}


JScrollPane—a brief sidebar

• We are all familiar with the scrollable pane—it means the amount of text available is larger than the screen.

• Java provides the JScrollPane, which is an API that is capable of scrolling text in this manner.


JScrollPane—a brief sidebar • Bizarre construction, because it uses composition—where several smaller tools are combined to form a larger tool.

• A String object feeds into a JTextArea, which feeds into a JScrollPane object.


JScrollPane—a brief sidebar String

JTextArea( )

JScrollPane( )

JOptionPane( )

JOptionPane( null, JScrollPane( JTextArea( String) ) )


JScrollPane—a brief sidebar // ExploreJScrollPane.javaimport javax.swing.*;

public class ExploreJScrollPane{

public static void main( String args[] ){

String output = "Test\nTest\nTest\n";

System.exit( 0 );}

}





String output = "Test\nTest\nTest\n";JTextArea outputArea = new JTextArea( 10, 10 );outputArea.setText( output );

System.exit( 0 );}

}





String output = "Test\nTest\nTest\n";JTextArea outputArea = new JTextArea( 10, 10 );outputArea.setText( output );JScrollPane scroller = new JScrollPane( outputArea );

System.exit( 0 );}

}





String output = "Test\nTest\nTest\n";JTextArea outputArea = new JTextArea( 10, 10 );outputArea.setText( output );JScrollPane scroller = new JScrollPane( outputArea );

JOptionPane.showMessageDialog( null, scroller, "Test JScrollPane",JOptionPane.INFORMATION_MESSAGE );

System.exit( 0 );}

}


JScrollPane—a brief sidebar


Logical Operators• So far, all of the conditions we have tested were simple.• It is possible to construct complex conditions using the Java Logical Operators, which—again—were inherited form C/C++.

&& Logical AND

|| Logical OR

! Logical NOT


Logical Operators• Logical AND—two ampersands together.

&&

if( gender == ‘F’ && age >= 65 )

• Condition is true only if both halves are true.• Java will short-circuit the process—skipping the 2nd half of the expression—if the first half is false.


Logical Operators• Logical OR—two “pipes” together. (Shift of key underneath backspace.)

|| (no space between)

if( gender == ‘F’ || age >= 65 )

• Entire condition is true if either half is true.


Logical Operators• Logical NOT—single exclamation mark.

!

if( !(age <= 65) )

• Negates the expression—not many opportunities to use.


Logical Operators• Logical Boolean AND—one ampersand.

&

if( gender == ‘F’ & ++age >= 65 )

• A Logical Boolean AND [ & ] works exactly likea Logical AND [ && ] with one exception.

• A Logical Boolean AND [ & ] will always check both halves of the equation, even if the first is false.


Logical Operators• Logical inclusive Boolean OR—one pipe.

|

if( gender == ‘F’ | age >= 65 )

• Again, this works just like the Logical OR, but you are guaranteed that both sides of the expression will always be executed.• If either half is true, the entire ‘if’ is true.


Logical Operators• Logical exclusive Boolean OR.

^ ( shift 6 )

if( gender == ‘F’ ^ age >= 65 )

• This Logical exclusive Boolean OR is true only if one side is true and the other false. • If both sides are true, the entire expression is false.

java i lecture_5

Technology

tom hunterchapter

int counter

valuewhile counter

times youwant

public void paint graphics

graphicsimport javax

control structures

number of repetitions2