CS 200 Branches

1. CS 200 Branches Jim Williams, PhD

2. This Week • Piazza: notes • Consulting Hours: Request Help Queue • Team Lab: take paper, draw pictures • Thursday: • P3 due • Pre-Exam 1: bring WISC ID, #2 Pencils • 6 questions, 15 minutes • Lecture: Using Objects, Branches

3. Primitive vs Reference Data Types Reference • Store a reference to another location in memory that contains value Integer num; num = new Integer(9); Primitive • Store value int i = 5; num i 5 9

4. Wrapper Classes • Wrapper classes contain helpful methods.

5. Data Type Conversions String weekNum = 3; String weekNum = "" + 3; String numStr = Integer.toString( 4); int num = Integer.parseInt( numStr);

6. Wrapper Classes • Boxing: Putting primitive into instance of wrapper class • Unboxing: retrieve primitive from instance • auto-boxing/unboxing: when compiler inserts code to box/unbox.

7. Primitive vs Reference Variables int num1; num1 = 5; Integer num2; num2 = new Integer(7); Integer num3 = 9; int num4 = num3; //use Java Visualizer, options Show String/Integer objects checkbox

8. What will show when d3 is printed? Double d1 = new Double(10); double d2 = d1; d1 = 14.1; Double d3 = d1; d1 = d2; System.out.println( d3);

9. String class • A reference (class), not a primitive data type. • Frequently used final String TITLE_PREFIX = "Welcome to Week "; System.out.println( TITLE_PREFIX + 4);

10. String String name2 = "Alex"; name2 = name2.toLowerCase(); String name3 = name2 + " Johnson"; System.out.print( name3); • instances of String are immutable (cannot change) • methods in String class that one might guess to change strings actually return new strings with changes.

11. Calling String methods String strA = "This is a string"; char aChar = strA.charAt( 3);

12. java.util.Random Random randGen; //Declare reference variable randGen = new Random(); //create instance // randGen.setSeed( 123); //set state int valueA = randGen.nextInt( 5); //get a value int valueB = randGen.nextInt( 5); int valueC = randGen.nextInt( 5);

13. What is the answer? What expression to get a value between 2 and 10, inclusive of both 2 and 10? Assume: Random rand = new Random(); int x;

14. Best statement? What is the best statement to get a value between 2 and 10, inclusive of both 2 and 10? Assume: Random rand = new Random(); int x; //x = ???

15. What are values of variables? String note = " C S\n2 \n33\n end\n"; Scanner input = new Scanner( note); String str1 = input.nextLine(); int num1 = input.nextInt(); String str2 = input.next(); String str3 = input.nextLine();

16. Today • preExam, lecture Branches • Sit in columns with space between (if possible)

17. Eclipse IDE

18. Relational and Equality Operators < <= > >= == != int n = 6; boolean result = n != 5;

19. Logical Operators && (AND) both sides true to be true, else false || (OR) either side true then true, else false ! (NOT) if true then false, and vice-versa Is !(a && b) equivalent to !a || !b ?

20. What is result? boolean result = flag = false || flag; Truth Table:

21. !(a && b) equivalent? !a || !b Truth Table:

22. Are these equivalent? boolean tired = true; if ( tired) { //take break tired = false; } if ( !tired) { //keep working } boolean tired = true; if ( tired) { //take break tired = false; } else { //keep working }

23. Floating Down a River http://www.thehiltonorlando.com/discover/pools-and-lazy-river/

24. Side Trip, maybe multiple side trips boolean tired = true; if ( tired) { System.out.println(“take break”); }

25. One side of the Island or the other boolean sunny = false; if ( sunny) { System.out.print(“sunny”); } else { System.out.print(“not sunny”); } System.out.println( “ and back together”);

26. Equivalent? char chr = //any valid char out = 'W'; if ( chr == 'A') { out = 'X'; } if ( chr == 'B') { out = 'Y'; } if ( chr != 'A' || chr != 'B') { out = 'Z'; } char chr = //any valid char out = 'W'; if ( chr == 'A') { out = 'X'; } else if ( chr == 'B') { out = 'Y'; } else { out = 'Z'; }

27. Switch: What is printed out? char choice = 'a'; switch (choice) { case 'a': System.out.print("a"); case 'b': System.out.print("b"); break; default: System.out.print("other"); break; }

28. What is the value of msg? boolean flag = true; String msg = "before if"; if ( flag) { msg = "" + flag; }

29. Comparing: == vs equals() • Primitive data types • use == for comparing primitive values • No equals() method for primitive data types • (but there is equals() method in wrapper classes) • Reference data types • use == for comparing references • use equals() for comparing instance/object contents • The meaning of equals() depends on the class it is defined in.

30. What is printed out? int i = 6; int j = 6; System.out.println( i == j); System.out.println( i.equals( j)); Integer k = new Integer( 7); Integer m = new Integer( 7); System.out.println( k == m); System.out.println( k.equals( m));

31. Values of a1 and a2? char ch1 = 'H'; String str = "Hello"; char ch2 = str.charAt(0); boolean a1 = ch1 == ch2; boolean a2 = ch1.equals(ch2);

32. String: == vs equals() String str1 = "hello"; String str2 = "hello"; String str3 = new String("hello"); String str4 = "hello"; System.out.println( str1 == "hello" ); System.out.println( str1 == str2); System.out.println( str1.equals("hello")); System.out.println( str3 == str1); System.out.println( str3.equals( str2));

33. Scanner Scanner input = new Scanner("1 \ntwo \n 2\n\n"); int a = input.nextInt(); if ( input.hasNextInt()) { int b = input.nextInt(); } else { input.nextLine(); } String line = input.nextLine(); System.out.println("#" + line + "#");

34. Scanner String note = "Hmm\na \na\n3\n\nline note."; Scanner input = new Scanner( note); int num = 0; String str1 = input.nextLine(); String str2 = input.next(); if ( input.hasNextInt()) { num = input.nextInt(); } String str4 = input.nextLine();

35. Debugging with Print statements See what is going on. Divide and conquer.