Control structures: if-else statements and switch statements  2000 Prentice Hall, Inc. All rights reserved. Modified for use with this course. Introduction to Computers and Programming in JAVA: V

 2003 Prentice Hall, Inc. All rights reserved (Modified). 2 Relational Operators (revisited) Not Equal to!= Equal to== Less than or equal to<= Greater than or equal to>= Less than< Greater than> MeaningOperator

 2003 Prentice Hall, Inc. All rights reserved (Modified). 3 Strings: example 1 public class string_example //String concatenation { public static void main(String[] args) { //String is a class String s1 = " The Cat "; String s2 = " in the Hat"; String s3 = s1 + s2;//concatenation example System.out.println(s3); }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 4 Strings -- example 2 import javax.swing.JOptionPane; //not in java.lang public class string_example2 //concatenate two strings read from input { public static void main(String[] args) { String s1 = JOptionPane.showInputDialog("type 1st input"); String s2 = JOptionPane.showInputDialog("type 2nd input"); String s3 = s2 + s1; System.out.println(s3); System.exit(0); //required for JOptionPane }

 2000 Prentice Hall, Inc. All rights reserved. (modified) 5 // less than five!! import javax.swing.JOptionPane; public class less_than_5{ public static void main(String args[]) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Enter a number from 1 to 10:", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); // Convert the string into an int value int num = Integer.parseInt(numString); // Display the result in a message dialog box if ( num < 5 ) System.out.println(" The number " + num + " is less than five."); else if (num == 5) System.out.println(" The number " + num + " is equal to five."); else System.out.println(" The number " + num + " is greater than five."); System.exit(0); }

 2000 Prentice Hall, Inc. All rights reserved. (modified) 6 Another example: writing a program in class about people’s ages: Program should ask user to input his or her age Program should determine and print the following based on the age: – A person is a teenager if their age is 13 through 19 –You are too old to be a teenager –You are too young to be a teenager

 2003 Prentice Hall, Inc. All rights reserved (Modified). 7 Nested if-else // ages import javax.swing.JOptionPane; public class age { public static void main(String args[]) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Enter your age:", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); int age = Integer.parseInt(numString); // Convert the string into an int value System.out.println(" Your age is " + age + "."); /* A person is a teenager if their age is 13 thu 19 */ if (age > 12) if (age <20) System.out.println("You are a teenager!\n"); else System.out.println("You are too old to be a teenager!\n"); else System.out.println("You are too young to be a teenager!\n"); System.exit(0); }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 8 Nested if-else // grades: example #2 import javax.swing.JOptionPane; public class grades_example2 { public static void main(String args[]) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Enter your grade (0-100):", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); // Convert the string into an int value int grade = Integer.parseInt(numString); System.out.println("Your grade is " + grade + "."); if (grade >= 90) System.out.println("You got an \"A\".\n"); else if (grade >= 80) System.out.println("You got a \"B\".\n"); else if (grade >= 70) System.out.println("You got a \"C\".\n"); else if (grade >= 60) System.out.println("You passed but you need a tutor.\n"); else System.out.println("You failed.\n"); System.exit(0); }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 9 switch Multiple-Selection Structure Used when testing a variable or expression for EQUALITY: ( >, =, <=) –tests separately for each of the constant integral values it may assume. Preferred over if else in situations: –where you are testing the same expressions for equality with many different values. Allows you to perform different actions for each test.

 2003 Prentice Hall, Inc. All rights reserved (Modified). 10 switch Multiple-Selection Structure switch (expression){ case value1: action(s); break; case value2: action(s); break; … default: actions(s); break; } keyword switch could use more than one case; if the same actions are required expression can be a variable or a more complicated expression actions within a single case do not need brackets the default case will be executed in the event that no other case is

 2003 Prentice Hall, Inc. All rights reserved (Modified). 11 switch Multiple-Selection FlowChart Case a action(s) True Case a break Case b action(s) break Case b Default action(s) Default false True

 2003 Prentice Hall, Inc. All rights reserved (Modified). 12 beware of “fall through” If you forget to use the break keyword between cases, unexpected things may happen. Once a case tests true, all the statements following that case, will be executed until the next break.

 2003 Prentice Hall, Inc. All rights reserved (Modified). 13 Nested if-else //dice_using_if import javax.swing.*; public class dice_using_if { public static void main( String args[] ) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Roll the die!! (1-6):", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); // Convert the string into an int value int roll = Integer.parseInt(numString); System.out.println(" You rolled a " + roll + "."); if (roll == 1 ) System.out.println("You rolled a one! Try again! \n"); else if (roll == 2 ) System.out.println("You rolled a two! Too bad! \n"); else if (roll == 3 ) System.out.println("You rolled a three! Better luck next time! \n"); else if (roll == 4 ) System.out.println("You rolled a four! Sorry! \n"); else if (roll == 5 ) System.out.println("You rolled a five! Way to go!!\n"); else if (roll == 6 ) System.out.println("You rolled a six! I guess you win...\n"); else System.out.println("This is some wierd die! \n"); System.exit( 0 ); // terminate application }}

 2003 Prentice Hall, Inc. All rights reserved (Modified). 14 Switch statements // dice_using_switch import javax.swing.*; public class dice_using_switch { public static void main( String args[] ) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Roll the die!! (1-6):", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); // Convert the string into an int value int roll = Integer.parseInt(numString); System.out.println(" You rolled a " + roll + "."); switch ( roll ) { case 1: System.out.println("You rolled a one! Try again! \n"); break; case 2: System.out.println("You rolled a two! Too bad! \n"); break; case 3: System.out.println("You rolled a three! Better luck next time! \n"); break; case 4: System.out.println("You rolled a four! Sorry! \n"); break; case 5: System.out.println("You rolled a five! Way to go!!\n"); break; case 6: System.out.println("You rolled a six! I guess you win...\n"); break; default: System.out.println("This is some wierd die! \n"); } // end switch System.exit( 0 ); // terminate application } // end main }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 15 Switch statements // switch example #2 import javax.swing.*; public class switch_example2 { public static void main( String args[] ) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Pick a number from 1-6:", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); int pick = Integer.parseInt(numString); // Convert the string into an int value System.out.println(" You picked a " + pick + "."); switch (pick) { case 1: System.out.println("You entered 1!\n"); break; case 2: case 3: System.out.println("You entered a 2 or a 3!\n"); break; case 4: case 5: case 6: System.out.println("You entered a 4, 5, or a 6!\n"); break; default: System.out.println("You did not enter an 1,2, 3, 4, 5 or 6!\n"); break; } /* end switch */ System.exit( 0 ); } }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 16 Nested if-else // grades: example #1 import javax.swing.JOptionPane; public class grades_example1 { public static void main(String args[]) { // Prompt the user to enter a number: String numString = JOptionPane.showInputDialog(null, "Enter your grade (0-100):", "Input Window Demo", JOptionPane.QUESTION_MESSAGE); // Convert the string into an int value int grade = Integer.parseInt(numString); System.out.println(" Your grade is " + grade + "."); if (grade > 60) if (grade > 70) System.out.println("You passed.\n"); else System.out.println("You passed but you need a tutor.\n"); else System.out.println("You failed.\n"); System.exit(0); } }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 17 The boolean Type and Operators boolean lightsOn = true; boolean lightsOn = false; boolean b = (1 > 2); && (and) (1 < x) && (x < 100) || (or) (1 < x) || (x < 100) ! (not)!(1<x)

 2003 Prentice Hall, Inc. All rights reserved (Modified). 18 Comparison Operators Operator Name < less than <= less than or equal to > greater than >= greater than or equal to == equal to != not equal to

 2003 Prentice Hall, Inc. All rights reserved (Modified). 19 Boolean Operators Operator Name ! not && and || or

 2003 Prentice Hall, Inc. All rights reserved (Modified). 20 Truth Table for Operator ! Operand!Operand true false false true

 2003 Prentice Hall, Inc. All rights reserved (Modified). 21 Truth Table for Operator && Operand1Operand2 Operand1 && Operand2 falsefalsefalse falsetruefalse truefalsefalse truetruetrue

 2003 Prentice Hall, Inc. All rights reserved (Modified). 22 Truth Table for Operator || Operand1Operand2Operand1 || Operand2 falsefalsefalse falsetruetrue truefalsetrue truetruetrue

 2003 Prentice Hall, Inc. All rights reserved (Modified). 23 Increment and Decrement Operators

 2003 Prentice Hall, Inc. All rights reserved (Modified). 24 Increment and Decrement Operators PostDecrement Operator (x--): –use the current value of x in the expression. Then, decrease by 1. PreDecrement Operator (--x): –Decrease x by 1. Then, use the new value of x in the expression.

 2003 Prentice Hall, Inc. All rights reserved (Modified). 25 Increment and Decrement Operators, cont.

 2003 Prentice Hall, Inc. All rights reserved (Modified). 26 Increment and Decrement Operators, cont. F Using increment and decrement operators makes expressions short F but it also makes them complex and difficult to read. F Avoid using these operators in expressions that modify multiple variables, or the same variable for multiple times such as this: int k = ++i + I;

 2003 Prentice Hall, Inc. All rights reserved (Modified). 27 What's the output of this program? public class shortcut_operators { public static void main(String[] args) { // declare variables int x = 10; int y = 5; int z = 3; System.out.println("x = "+x+", y = "+y+", z = "+z+ "\n"); x++;; y += x; z *= x; System.out.println("Now x = "+x+", y = "+y+", z = "+z+ "\n"); x--; y *= x; z %= x; System.out.println("And now x = "+x+", y = "+y+", z = "+z+ "\n"); System.exit(0); }

 2003 Prentice Hall, Inc. All rights reserved (Modified). 28 What's the output of this program? public class shortcut_operators { public static void main(String[] args) { // declare variables int x = 10; int y = 5; int z = 3; System.out.println("x = "+x+", y = "+y+", z = "+z+ "\n"); x++;; y += x; z *= x; System.out.println("Now x = "+x+", y = "+y+", z = "+z+ "\n"); x--; y *= x; z %= x; System.out.println("And now x = "+x+", y = "+y+", z = "+z+ "\n"); System.exit(0); } x = 10, y = 5, z = 3 Now x = 11, y = 16, z = 33 And now x = 10, y = 160, z = 3 Press any key to continue...