CS 201 Lecture 4

1. CS 201 Lecture 4 John Hurley Cal State LA

2. If • If statements do just what you expect • test whether a condition is true and if so, execute some statements • syntax: if(test) { // statements to execute is condition is true } • Example: if(x < y){ System.out.println(“x < y”); }

3. If

4. If • If there is only one statement in the block, you can omit the brackets: if(x < y) System.out.println(“x < y”); • Note that there is no semicolon between the condition and the statements to execute!

5. If public class IfDemo{ public static void main(String[] args){ int firstInt = 0; int secondInt = 1; if(firstInt < secondInt) System.out.println(firstInt + " < " + secondInt); if(firstInt == secondInt) System.out.println(firstInt + " = " + secondInt); if(firstInt > secondInt) System.out.println(firstInt + " > " + secondInt); } }

6. If / else • If can be followed by else: if(condition) { // statements to execute if condition is true } else { // statements to execute if condition is false }

7. If / elseif / else • We can also use else if, with or without a final else: if(condition 1) { // statements to execute if condition 1 is true } else if(condition 2) { // statements to execute if condition 1 is false but condition 2 is true } // can use any number of else if blocks else { // statements to execute if condition none of the conditions are true }

8. Multiple Alternative if Statements

9. If / elseif / else public class IfElseDemo{ public static void main(String[] args){ int firstInt = 5; for(int secondInt = 0; secondInt < 10; secondInt++){ char compare; if(firstInt < secondInt) compare = '<'; else if(firstInt > secondInt) compare = '>'; else compare = '='; System.out.println(firstInt + " " + compare + " " + secondInt); } // end for } // end main() } // end class

10. If / else shorthand • Syntax: • condition?value if condition is true:value if condition is false; • System.out.println(a<b?"true!":"false!"); • This is equivalent to the following: • if(a < b) System.out.println(“true”); • else System.out.prinltn(“false”);

11. If / else shorthand public class ShorthandDemo{ public static void main(String[] args){ int firstInt = 5; for(int secondInt = 0; secondInt < 10; secondInt++){ String comp = null; int a = 0; a=(firstInt < secondInt)?1:0; System.out.println(a); System.out.println(firstInt < secondInt?"true!":"false!"); } // end for } // end main() } // end class

12. Logical Operators • AND: && • if(condition 1 && condition 2) {} • example: if(a < b && b < 1) c = 1; • example 2: if(a < b && a < 0) System.out.println(“a is negative and less than b”); • OR: || • if(condition 1 || condition 2) {} • example: if(a < b || b < 1) c = 1; • NOT: ! • !(a == b) is equivalent to (a != b) • Can also use with more complex conditions • !((a == b) && (a == 1)) • !(a || b) • !(a &&(b||c)) • Can string any number of conditions together, but be careful not to write code that is hard to understand

13. Multiple Conditions public class MultipleConditionsDemo{ public static void main(String[] args){ boolean trueCondition = true; boolean falseCondition = false; System.out.println("True is " + (trueCondition?"true":"false")); System.out.println("False is " + (falseCondition?"true":"false")); System.out.println("(True and False) is "+(trueCondition&& falseCondition?"true":"false")); System.out.println("(True or False) is "+(trueCondition||falseCondition?"true":"false")); } // end main() } // end class

14. Negations

15. Negations public class BangDemo{ public static void main(String[] args){ boolean a = true; boolean b = true; boolean c = true; System.out.println(!(a &&(b||c))); c = false; System.out.println(!(a &&(b||c))); b = false; c = true; System.out.println(!(a &&(b||c))); b = false; c = false; System.out.println(!(a &&(b||c)));

16. Negations a = false; b = true; c = true; System.out.println(!(a &&(b||c))); c = false; System.out.println(!(a &&(b||c))); b = false; c = true; System.out.println(!(a &&(b||c))); b = false; c = false; System.out.println(!(a &&(b||c))); } // end main() } // end class

17. Nested If Blocks • One if block can be inside another one • The inner if only runs if the condition in the outer one evaluates to true • Can be nested to any depth, but this quickly gets confusing • We will soon see nested loops, too!

18. Nested If Blocks if(condition 1) { // statements execute if condition 1 is true if (condition 2){ // statements execute if condition 1 and condition 2 are both true } else { // statements execute if condition 1 is true but condition 2 is false } } else{ // statements execute if condition 1 is false }

19. Nested If Blocks public class NestedIfDemo{ public static void main(String[] args){ boolean trueCondition = true; boolean falseCondition = false; if(trueCondition){ if(falseCondition) System.out.println("true and false"); else System.out.println("true and ~false"); } else{ if(falseCondition) System.out.println("~true and false"); else System.out.println("~true and ~false"); } } // end main() } // end class

20. Equality with Floating Point Types • Due to the limited precision of floating point types, it is unwise to test floats and doubles with == operator: public static void main(String[] args) { double d = 1.23456789; double e = d; e = e + 1; e = e - 1; System.out.println("d = " + d + "; e = " + e); } // end main()

21. Equality with Floating Point Types Instead of using equality test, use Math.abs as follows: • Math.abs(a-b) < tolerance; • For example, if(Math.abs(grade - 3.7) < .02) {}

22. Switch Chooses one statement or block to execute from among several options, based on the value of a variable switch (status) { case 0: compute taxes for single filers; break; case 1: compute taxes for married file jointly; break; case 2: compute taxes for married file separately; break; case 3: compute taxes for head of household; break; default: System.out.println("Errors: invalid status"); System.exit(0); }

23. switch Statement Flow Chart

24. The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses. The value1, ..., and valueN must have the same data type as the value of the switch-expression. The resulting statements in the case statement are executed when the value in the case statement matches the value of the switch-expression. Note that value1, ..., and valueN are constant expressions, meaning that they cannot contain variables in the expression, such as 1 + x. switch Statement Rules switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; … case valueN: statement(s)N; break; default: statement(s)-for-default; }

25. The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression. switch Statement Rules switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; … case valueN: statement(s)N; break; default: statement(s)-for-default; } The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed. The case statements are executed in sequential order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.

26. animation Trace switch statement Suppose ch is 'a': switch (ch) { case'a': System.out.println(ch); case'b': System.out.println(ch); case'c': System.out.println(ch); }

27. animation Trace switch statement ch is 'a': switch (ch) { case'a': System.out.println(ch); case'b': System.out.println(ch); case'c': System.out.println(ch); }

28. animation Trace switch statement Execute this line switch (ch) { case'a': System.out.println(ch); case'b': System.out.println(ch); case'c': System.out.println(ch); }

29. animation Trace switch statement Execute this line switch (ch) { case'a': System.out.println(ch); case'b': System.out.println(ch); case'c': System.out.println(ch); }

30. animation Trace switch statement Execute this line switch (ch) { case'a': System.out.println(ch); case'b': System.out.println(ch); case'c': System.out.println(ch); }

31. animation Trace switch statement Execute next statement switch (ch) { case'a': System.out.println(ch); case'b': System.out.println(ch); case'c': System.out.println(ch); } Next statement;

32. animation Trace switch statement Suppose ch is 'a': switch (ch) { case'a': System.out.println(ch); break; case'b': System.out.println(ch); break; case'c': System.out.println(ch); }

33. animation Trace switch statement ch is 'a': switch (ch) { case'a': System.out.println(ch); break; case'b': System.out.println(ch); break; case'c': System.out.println(ch); }

34. animation Trace switch statement Execute this line switch (ch) { case'a': System.out.println(ch); break; case'b': System.out.println(ch); break; case'c': System.out.println(ch); }

35. animation Trace switch statement Execute this line switch (ch) { case'a': System.out.println(ch); break; case'b': System.out.println(ch); break; case'c': System.out.println(ch); }

36. animation Trace switch statement Execute next statement switch (ch) { case'a': System.out.println(ch); break; case'b': System.out.println(ch); break; case'c': System.out.println(ch); } Next statement;

37. Command Line Input • There are several ways to get input from a command line • In production, you will usually write programs that use GUIs, not command line I/O • In school most programming classes focus on functionality, not user interface, so you need to know how to use command line I/O

38. Command Line Input • The simplest command line input class is Scanner • import java.util.Scanner; • Scanner has a variety of methods to get input of different data types

39. Scanner Input Methods • We describe methods using in this format: • Class.method() • If there are any parameters, their type goes inside the parentheses • You have already seen • System.out.println(String) • You will often replace the class name with the name of an instance of the class: Scanner input = new Scanner(System.in); … stuff deleted… name = input.next(); • In the example above, input is an instance of Scanner. • We set up a Scanner and called it input!

40. Scanner Input Methods • Scanner.next() reads the next parsable String • Scanner.nextLine() reads up to the next line break and puts the result in a String • Scanner.nextDouble() reads the next parseable string and tries to convert it to a Double • double d = Scanner.nextDouble(); • There are equivalent methods for nextInteger(), nextBoolean(), etc.

41. Scanner.next Example package demos; import java.util.Scanner; public class InputDemo { public static void main(String[] args) { Scanner input = new Scanner(System.in); String name = null; System.out.println("Guess my name:"); name = input.next(); if (name.equals("Rumpelstiltskin")) System.out.println("Correct! Continue spinning straw into gold!"); else System.out.println("Wrong! I will now eat your children!"); } }

42. Scanner.nextDouble Example import java.util.Scanner; public class ReadDouble { public static void main(String[] args) { Scanner input = new Scanner(System.in); double myDouble = 0.0; do { System.out.print("Input a double:"); myDouble = input.nextDouble(); System.out.println("\nYou entered: " + myDouble); } while (stuff != 0.0); } }

43. Switch public static void main(String[] args) { Scanner sc = new Scanner(System.in); char ageCat = 'y'; while (ageCat != 'q') { System.out .println("Welcome to John's bar. Please enter your age category: \ny for under 21, m for 21-29, o for 30 and older. Enter q to quit."); String input = sc.next(); ageCat = (char) input.charAt(0); switch (ageCat) { case 'y': System.out.println("Enjoy your Shirley Temple, Junior"); break; case 'm': System.out.println("You'd better stick to beer"); break; case 'o': System.out.println("There's whiskey in the jar."); break; case 'q': break; default: System.out.println("Invalid input"); } // end switch } // end while } // end main()

44. Eclipse IDE • IDE = Integrated Development Environment • An IDE provides services that make it easier for you to program • Editor with syntax checking, automatic formatting, etc • One-step compile and run • Debugging • Organization

45. Eclipse IDE • The IDE most often used for Java programming is called Eclipse. Eclipse is the standard IDE at CSULA. • Eclipse supports many different programming languages with available plug-ins, but it is mostly used for Java • Eclipse is open-source; you can get it at www.eclipse.org • Get the “Eclipse IDE for Java Developers” • Others often used with Java include NetBeans, JBuilder, many others

46. Eclipse IDE • IDE = Integrated Development Environment • An IDE provides services that make it easier for you to program • Editor with syntax checking, automatic formatting, etc • One-step compile and run • Debugging • Organization

47. Eclipse IDE • The most widely used IDE at CSULA is Eclipse. • Eclipse supports many different programming languages with available plug-ins, but it is mostly used for Java • Eclipse is open-source; you can get it at www.eclipse.org • Get the “Eclipse IDE for Java Developers” • Others often used with Java include NetBeans, JBuilder, many others

48. Eclipse IDE • The current general-release version of Eclipse predates the most recent update to the JDK. If you don’t already have Eclipse working on your home computer, you may deal with this in one of two ways: • Get the JDK 7 instead of 8, OR • Get the JDK 8 and then follow the easy instructions at https://wiki.eclipse.org/JDT/Eclipse_Java_8_Support_For_Kepler

49. Eclipse IDE • Create a new project in Eclipse and write a class

50. Eclipse: Create a Project