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Control structure part-2

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    • 1. Java I--Copyright © 2000 Tom Hunter
    • 2. Java I--Copyright © 2000 Tom Hunter Chapter 5 Control Structures: Part II
    • 3. Java I--Copyright © 2000 Tom Hunter • 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
    • 4. Java I--Copyright © 2000 Tom Hunter • 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
    • 5. Java I--Copyright © 2000 Tom Hunter • Counter-Controlled Repetition // WhileCounter.java import java.awt.Graphics; import javax.swing.JApplet; public class WhileCounter extends JApplet { public void paint( Graphics g ) { int counter; // 1.) count variable counter = 1; // 2.) starting value while( counter <= 10 ) // 4.) condition, final value { g.drawLine( 10, 10, 250, counter * 10 ); ++counter; // 3.) increment } } }
    • 6. Java I--Copyright © 2000 Tom Hunter • The for Loop • A common structure called a for loop is specially designed to manage counter-controlled looping. Counter-Controlled Repetition for( int x = 1; x < 10; x++ ) 1.) count variable, 2.) starting value 3.) Increment 4.) condition, final value
    • 7. Java I--Copyright © 2000 Tom Hunter // 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. Counter-Controlled Repetition
    • 8. Java I--Copyright © 2000 Tom Hunter Counter-Controlled Repetition • 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. )
    • 9. Java I--Copyright © 2000 Tom Hunter counter <= 10 ? int counter = 1; { } counter++ TRUE FALSE 1. 2. 3. body 4.
    • 10. Java I--Copyright © 2000 Tom Hunter • All Three Sections are Optional • Effects of Omitting Sections: condition Counter-Controlled Repetition 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++ )
    • 11. Java I--Copyright © 2000 Tom Hunter • Effects of Omitting Sections: initialization Counter-Controlled Repetition 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++ )
    • 12. Java I--Copyright © 2000 Tom Hunter • Effects of Omitting Sections: increment Counter-Controlled Repetition 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++; }
    • 13. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import 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 the JTextArea. JTextArea is one of many GUI classes that we use to put text onto a window
    • 14. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; 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. w
    • 15. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); System.exit( 0 ); } // end of main() } // end of class Interest 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.) w
    • 16. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 ); System.exit( 0 ); } // end of main() } // end of class Interest 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.
    • 17. Java I--Copyright © 2000 Tom Hunter • A JTextArea is a multi-line area that displays plain text. • This is the actual documentation for the JTextArea. You must become comfortable looking up and interpreting this information. 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
    • 18. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 ); output.append( “YeartAmount on depositn” ); System.exit( 0 ); } // end of main() } // end of class Interest 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.
    • 19. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 ); output.append( “YeartAmount on depositn” ); for( int year = 1; year <= 10; year++ ) { } // end of for System.exit( 0 ); } // end of main() } // end of class Interest
    • 20. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 ); output.append( “YeartAmount on depositn” ); 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 System.exit( 0 ); } // end of main() } // end of class Interest 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.
    • 21. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 ); output.append( “YeartAmount on depositn” ); 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); System.exit( 0 ); } // end of main() } // end of class Interest
    • 22. Java I--Copyright © 2000 Tom Hunter // Calculate Compound Interest import javax.swing.JOptionPane; import java.text.DecimalFormat; import javax.swing.JTextArea; public class Interest { public static void main( String args[] ) { double amount, principle = 1000.0, rate = 0.05; DecimalFormat twoDig = new DecimalFormat( “0.00” ); JTextArea output = new JTextArea( 11, 20 ); output.append( “YeartAmount on depositn” ); 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); System.exit( 0 ); } // end of main() } // end of class Interest
    • 23. Java I--Copyright © 2000 Tom Hunter Counter-Controlled Repetition
    • 24. Java I--Copyright © 2000 Tom Hunter 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.
    • 25. Java I--Copyright © 2000 Tom Hunter 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 is evaluated. If x contains a 1, then the case 1 branch is performed. Notice the ‘break;’ statement. This is required. Without it, every line after the match will be executed until it reaches a break; w
    • 26. Java I--Copyright © 2000 Tom Hunter Multiple-Selection Structure 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 will be used in the comparison.
    • 27. Java I--Copyright © 2000 Tom Hunter Multiple-Selection Structure w • 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.
    • 28. Java I--Copyright © 2000 Tom Hunter Multiple-Selection Structure w • 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.
    • 29. Java I--Copyright © 2000 Tom Hunter Multiple-Selection Structure • Question: if only integer values can appear in the switch( x ) statement, then how is it possible for a char to be the expression?
    • 30. Java I--Copyright © 2000 Tom Hunter 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
    • 31. Java I--Copyright © 2000 Tom Hunter 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.
    • 32. Java I--Copyright © 2000 Tom Hunter Statements break; and continue; • 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.
    • 33. Java I--Copyright © 2000 Tom Hunter Statements break; and continue; • 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”; }
    • 34. Java I--Copyright © 2000 Tom Hunter 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.
    • 35. Java I--Copyright © 2000 Tom Hunter 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.
    • 36. Java I--Copyright © 2000 Tom Hunter JScrollPane—a brief sidebar String JTextArea( ) JScrollPane( ) JOptionPane( ) JOptionPane( null, JScrollPane( JTextArea( String) ) )
    • 37. Java I--Copyright © 2000 Tom Hunter JScrollPane—a brief sidebar // ExploreJScrollPane.java import javax.swing.*; public class ExploreJScrollPane { public static void main( String args[] ) { String output = "TestnTestnTestn"; System.exit( 0 ); } }
    • 38. Java I--Copyright © 2000 Tom Hunter JScrollPane—a brief sidebar // ExploreJScrollPane.java import javax.swing.*; public class ExploreJScrollPane { public static void main( String args[] ) { String output = "TestnTestnTestn"; JTextArea outputArea = new JTextArea( 10, 10 ); outputArea.setText( output ); System.exit( 0 ); } }
    • 39. Java I--Copyright © 2000 Tom Hunter JScrollPane—a brief sidebar // ExploreJScrollPane.java import javax.swing.*; public class ExploreJScrollPane { public static void main( String args[] ) { String output = "TestnTestnTestn"; JTextArea outputArea = new JTextArea( 10, 10 ); outputArea.setText( output ); JScrollPane scroller = new JScrollPane( outputArea ); System.exit( 0 ); } }
    • 40. Java I--Copyright © 2000 Tom Hunter JScrollPane—a brief sidebar // ExploreJScrollPane.java import javax.swing.*; public class ExploreJScrollPane { public static void main( String args[] ) { String output = "TestnTestnTestn"; 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 ); } }
    • 41. Java I--Copyright © 2000 Tom Hunter JScrollPane—a brief sidebar
    • 42. Java I--Copyright © 2000 Tom Hunter 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
    • 43. Java I--Copyright © 2000 Tom Hunter 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.
    • 44. Java I--Copyright © 2000 Tom Hunter 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.
    • 45. Java I--Copyright © 2000 Tom Hunter Logical Operators • Logical NOT—single exclamation mark. ! if( !(age <= 65) ) • Negates the expression—not many opportunities to use.
    • 46. Java I--Copyright © 2000 Tom Hunter Logical Operators • Logical Boolean AND—one ampersand. & if( gender == ‘F’ & ++age >= 65 ) • A Logical Boolean AND [ & ] works exactly like a Logical AND [ && ] with one exception. • A Logical Boolean AND [ & ] will always check both halves of the equation, even if the first is false.
    • 47. Java I--Copyright © 2000 Tom Hunter 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.
    • 48. Java I--Copyright © 2000 Tom Hunter 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.