Implementing Decisions in Java: If Statements, Blocks, and Comparison Operators

1. Chapter 6 - Decisions

2. Chapter Goals • To be able to implement decisions using if statements • To understand how to group statements into blocks • To learn how to compare integers, floating-point numbers, strings, and objects • To recognize the correct ordering of decisions in multiple branches • To program conditions using boolean operators and variables

3. Decisions • So far, our examples haven’t made any decisions • The same code gets executed no matter what • Assignment 1 • Path • BankAccount • This is called sequential execution • Is this how real applications work?

4. Decisions • The real world requires decisions • Is it time to feed the cat? Can a book be checked out? • Essential feature of nontrivial programs is to make decisions based on input • The answers to these decision require different actions • Introduces the need for control statements

5. If statements • Conditional statements are represented with if-then-else statements Scanner stdin = new Scanner(System.in); int testScore = stdin.nextInt(); if (testScore < 70) { System.out.println(“You are below the mean”); } else { System.out.println(“You are above the mean”); }

6. If Statements: Syntax if (<boolean expression>) { <then block> } else { <else block> }

7. Example • withdraw() method we implemented allowed user to withdraw as much money as they wanted • Is this realistic? • What decision should withdraw() make? • What should the decision be based on?

8. Example public void withdraw(double amount){ if (amount <= balance) balance -= amount; }

10. Example • Most banks also charge an overdraft penalty, how would we go about changing our example to implement this?

12. Statements • The decision we made only resulted in one instruction whichever way we took • Most decisions lead to a sequence of instructions (multiple statements). • In order to show that the entire sequence of instructions belong to the then-block (or else-block), we group them with curly braces{ } • A sequence of instructions surrounded by curly braces is called a block

13. Example if (amount <= balance) { System.out.println(“Legal Withdrawal”); balance = balance – amount; } else { balance = balance – OVERDRAFT_PENALTY; }

14. Statements • Block statement – a group of statements enclosed within curly braces • Methods • Classes • if statements, switch statements, loops • Simple statement • Basic statement such as balance = balance – amount; • Compound statement • Statements that have multiple paths of execution • If statements, loops

15. If Statements • If you have a single statement, { } not required if (testScore < 70) System.out.println(“You are below the mean”); else System.out.println(“You are above the mean”); • But convention to use them anyways • Helpful for adding temporary output to check if a branch is executed • Makes nested if-else statements easier to read

16. If Statements: Syntax if(condition)statement if (condition)statement1 elsestatement2 statementmust be a statement - simple, compound, or block

17. Style • Use indentation to indicate nesting levels public class BankAccount { | … | public void withdraw() | { | | if (amount <= balance) | | { | | | balance -= amount; | | } | } }

18. Die Example // Declare and create Die objects Die die1 = new Die(); Die die2 = new Die(); // Roll die objects die1.roll(); die2.roll(); // Save results in new variables int roll1 = die1.getTop(); int roll2 = die2.getTop();

19. Die Example // Test for doubles if ( roll1 == roll2 ) System.out.println( "Doubles" ); // rest of program

20. Die Example roll1 == roll2 true System.out.println( “doubles!” ); false ... continue with rest of program

21. Die Example // Test for doubles if ( roll1 == roll2 ) System.out.println( "Doubles" ); else System.out.println( "Sorry, no doubles" ); // rest of program

22. Die Example roll1 == roll2 true System.out.println( “doubles!” ); false System.out.println( “sorry ...” ); ... continue with rest of program

23. Selection Operator • Selection Operator condition ? value1 : value2 • If the condition is true, value1 is returned. • If the condition is false, value2 is returned. • Example: Absolute Value y = (x >= 0) ? x : -x;

24. Booleans • boolean is a primitive data type that holds one of two values • true • false • Any time a decision needs to be made, we make that decision using a boolean expression, which evaluates to a boolean value • Note: a mathematical expression returns a numerical value

25. Boolean Expressions • What operators are used for boolean expressions? < less than <= less than or equal to > greater than >= greater than or equal to == equal to != not equal to

26. Boolean Expressions • Boolean expressions are just like mathematical expressions, but they return true or false (not int, double, etc) • Examples: • 3 * 8 <= 5 * 5 • 6 % 4 > 0 • testScore < 80 • testScore * 2 > 350

27. If Statements • When the computer reaches an if statement, it first evaluates the boolean expression • If that expression is true, the <then block> is executed • Otherwise, the <else block> is executed • An if statement is called a branching statement because it branches to a block of code

28. Comparisons • Comparing whole numbers is trivial int a = 5; if (a == 5){ System.out.println(“Wow this is easy!”); } else { System.out.println(“Java really sucks”); }

29. Comparisons • Comparing real numbers is more difficult because we have to take into account roundoff errors. double r = Math.sqrt(2); double d = r * r – 2; if (d != 0){ System.out.println(“Your equation is incorrect, the result is “ + d); } Your equation is incorrect, the result is 4.440892098500626E-16

30. Comparisons • This is a problem, since logically the equation makes sense • Lesson: Floating point numbers may not be exact • Solution: Use range of values that are close enough • Should be close to 0

31. Comparisons • To avoid roundoff errors, don't use == or != to compare floating-point numbers • To compare floating-point numbers test whether they are close enough: |x - y| ≤ ε final double EPSILON = 1E-14;if (Math.abs(x - y) <= EPSILON)// x is approximately equal to y • ε is a small number such as 10-14

32. Comparisons • We can do comparisons with characters as well • They work according to ASCII values: char choice = 'Q'; if (choice < 'Z') System.out.println("valid"); else System.out.println("invalid"); • What is the output?

33. Side Note: Characters • Remember that we said a char is one letter, digit, or symbol that the computer thinks of as a number. • We can cast chars to int and vice versa • Examples: • (int) 'D' -> 68 • (char) 106 -> 'j' • (char) ('D' + 1) -> 'E'

34. Comparisons • Boolean operators can be used on objects, but do they mean the same thing? • They probably don’t do what you expect… • Primitive types vs. reference types (objects) • To test two Strings, use equals() method

35. String Comparisons • Don't use == for strings!if (input == "Y") // WRONG!!! • Use equals method:if (input.equals("Y")) string1.equals(string2); • == tests references, .equals() tests contents

36. :String 0 1 2 3 4 R o b b y s1 s2 String Comparisons String s1 = "Robby", s2; s2 = s1;

37. :String 0 1 2 3 4 R o b b y s1 s2 String Comparisons • What is the result of: s1 == s2 Why?

38. :String 0 1 2 3 4 R o b b y s1 s2 String Comparisons String s1 = "Robby", s2; s2 = "Robby";

39. :String 0 1 2 3 4 R o b b y str1 str2 String Comparisons • What is the result of: str1 == str2 Why?

40. :String :String 0 1 2 3 4 0 1 2 3 4 R o b b y R o b b y s1 s2 String Comparisons String s1 = "Robby", s2; s2 = new String( s1 );

41. :String :String 0 1 2 3 4 0 1 2 3 4 R R o o b b b b y y s1 s2 String Comparisons • What is the result of: s1 == s2 Why?

42. :String :String 0 1 2 3 4 0 1 2 3 4 R R o o b b b b y y s1 s2 String Comparisons • What is the result of: s1.equals(s2); Why?

43. String Comparisons • equalsIgnoreCase can be used to compare strings while ignoring case • Case insensitive test ("Y" or "y")if (input.equalsIgnoreCase("Y")) string1.equalsIgnoreCase(string2) // returns boolean • compareTo returns a number that tells which string comes before the other in the dictionary • 0 indicates that the strings are the same

44. String Comparisons string1.compareTo(string2) • Returns < 0 if string1 comes first • Returns 0 if they are equal • Returns > 1 if string2 comes first • Example: "car" comes before "cargo" • All uppercase letters come before lowercase: "Hello" comes before "car"

45. Comparisons • == tests for identity, equals() for identical content • Most classes have an equals() method defined Rectangle box1 = new Rectangle(5, 10, 20, 30); Rectangle box2 = box1; Rectangle box3 = new Rectangle(5, 10, 20, 30); • box1 == box3is false • box1.equals(box3)is true • box1 == box2 is true • Note: equals() must be defined for the class for this to work

47. Null • Reference variables store a reference (address) for an actual object • What do they store when they haven’t been set to refer to an object? • null is a Java reserved word to designate that no object has been set for that variable

48. Null • Can be used in tests: if (middleInitial == null)System.out.println(firstName + " " + lastName); elseSystem.out.println(firstName + " " + middleInitial + ". " + lastName); • Use ==, not equals, to test for null • null is not the same as the empty string ""

49. Multiple Alternatives if (score >= 90) System.out.println("Great!"); else if (score >= 80) System.out.println("Good"); else if (testScore >= 70) System.out.println("OK"); else if (testScore >= 60) System.out.println("Passing"); else if (testScore >= 50) System.out.println("Hmm..."); else if (testScore >= 0) System.out.println("Study!"); else System.out.println("Invalid Score");

50. Multiple alternatives • First condition that is true is the ONLY statement executed • Therefore, order matters: if (testScore >= 90) System.out.println(“Great!”); else if (testScore >= 80) System.out.println(“Good”); else if (testScore >= 70) System.out.println(“OK”);