1. Computer Numbers Integers (byte, short, int, long) –whole numbers –exact –Relatively limited in magnitude (~10 19 ) Floating Point (float, double) –fractional –often approximations (0.33333) –larger magnitude (~10 308 ) –Actually hold signed mantissa & exponent 6.023 x 10 23

2. List of Data Types Primitive Type Default Value boolean false char '\u0000' (null) byte (byte) 0 short (short) 0 int 0 long 0L float 0f double 0d void N/A Note: At times, the Java Language Specification refers to void as a primitive, though other parts (e.g., s. 14.7) say ‘void’ is not a primitive as in C/C++. One cannot cast to a void type in Java.

3. Data Type Ranges TypeSizeMinDefault booleanfalse Max 1false*true* char'\u0000' (null)16 byte(byte) 08-128127 short(short) 016-32,76832,767 int032 -2,147,483,6482,147,483,647 long0L64 -9,223,372,036,854,775,8089,223,372,036,854,775,807 float0.0F32 Approx ±3.4E+38 with 7 significant digits double0.0D64 Approx ±1.7E+308 with 15 significant digits void * Not truly min and max.

4. OK!Compiles Understanding Casting Last class, we covered some basics of casting. Sometimes an explicit cast is required, sometimes it’s not. int x = 10; float f1 = x; float f2 = 11.234f; int y = f2; Which one requires a cast to compile? Why?ERROR Explicit cast needed

5. Understanding Casting To understand when we need casting, let’s look closely at an example. Let’s work with these two types and identifiers: int x; char c; The real question is, How BIG are these data types in memory?

6. Recall We already discussed the sizes of various data types in Java. You are guaranteed to have four bytes in each and every int. We can think of an int as a container--much like a box. java int Nutritional Facts Serv. Size 1 int Amount per Serving Calories 0 % Daily Value Total Bytes 4 100% Cereal(tahıl) box

7. And recall... java char Nutritional Facts Serv. Size 1 char Amount per Serving Calories 0 % Daily Value Total Bytes 2 50% A char on the other hand is only 2 bytes. It’s smaller than an int. We might imagine it as a smaller container. tuna (ton balığı or orkinos) can

8. So... A char is 2 bytes, and an int is 4 bytes. So when we code the following: int x; char c; We get: x c Symbol Picture of Memory Each block is one byte 0000 0100 0001 0101 0110

9. Reality Check java char java int Will a can of tuna fit into a box of cereal? Yes, the can is smaller. Will a box of cereal fit into a can of tuna? Not neatly, the box is larger.

10. In a similar vein... Will a char fit into an int? Will an int fit into a char? x c Symbol Picture of Memory

11. Reality Check java char java int What if you wanted to fit HALF the box into the can. That would fit! For example, if we know the top half of the box is empty, we can throw it away!

12. Explicit Casting: Intentional Loss of Precision x c Symbol Picture of Memory int x = 45; char c = ‘A’; c = (char) x; // cast needed cast needed!

13. Testing Yourself What happens when we do this: int someInt = 2000; long longNumber; longNumber = (long) someInt; Is that cast legal? Is that cast required? Fill in the blank.

14. Another Example float f; int i, j; i = 9; j = 2; f = i / j; f = (float) i / j; Returns 4 because of integer truncation! f = (float) i / (float) j; If that is not what was intended, Three possible solutions f = i / (float)j;

15. Another example Sign bit 31 bits for data We also have to consider how Java preserves the sign of a value through casting. Consider the eight bytes used for an int. The highest bit is used for a sign (plus or minus, through a 0 or 1). int

16. Casting and Sign Preserves sign bit int byte Casting is not merely chopping one data type into another. The VM doesn’t merely throw away half of the information. The sign is preserved.

17. Another Example Suppose we had some very, very old data, where the date was expressed in a “int” (four byte) format, such as: 19291031 1929 10 (October) 31 This is very crude, but some old data source (e.g., tape archives) might have this format.

18. Explicit Cast needed public class CutData { public static void main(String[] args) { int iDate = 20001225; // this is how it was read in byte byDay = (byte) (iDate % 100); System.out.println ("The day was " + byDay); iDate = iDate / 100; byte byMonth = (byte) (iDate % 100); System.out.println ("The month was " + byMonth); iDate = iDate / 100; short sYear = (short) iDate; System.out.print ("The year was = " + sYear); } } // CutData

19. public class PackDate { public static void main(String[] args) { byte month = 12; byte day = 25; short year = 2000; int date = year * 10000 + month * 100 + day; System.out.println ("The date = " + date); } } // PackDate When working in the opposite direction (accumulating bytes and shorts into an int), no casting is needed, because we do not lose precision or information.

20. Given: int start = 10; float temp = 5.5f; temp = (int) temp * start; What does temp now hold? Casting: Test Your Knowledge Given: char c = ‘A’; int x; c = x; Legal? Quick Review

21. Test Your Knowledge Here’s the problem: int iVar = 10; float fVar = 23.26f; // gives compile-time error iVar = iVar * fVar Which solution works best? iVar = (int) iVar * fVariVar = (int) (iVar * fVar)iVar = iVar * (int) fVar 1 2 3 iVar = (int) ((float) iVar * fVar) 4 232 Same Compile Error 232 230 Quick Review

22. Shorthand Operators counter = counter + 1; OR counter++; counter = counter - 1; OR counter--; counter = counter + 2; OR counter+=2; counter = counter * 5; OR counter*=5; Last two examples : perform operation first (eg. counter+2) then performs the assignment. Quick Review Understand this, but please avoid these types of expressions.

23. The Short End of Shorthand Be careful to avoid ‘clever’ expressions with shorthand notations. For example, we can declare a variable called “_”. (Yes, and underline character alone is a valid name.) int _ = 1; We can then use a combination of operators to make Morse code: _ -= --_ - _--; Yes, this is valid; however, it’s completely unreadable. (Your replacement will not appreciate this style!)

24. Documentation & Comments Three ways to do it: // comments out everything until the end of the line /* comments out everything until the */ (There are no nested comments as in C++.) /** * This is syntax for Javadoc comments (similar to second style * of commenting, but allows for HTML formatting features. */ For CS1302, use Javadoc comments Quick Review Let’s work a javadoc example

25. Javadoc: Simple Example import java.util.*; /** * HelloComments.java * * Created: Wed Jan 12 18:17:29 2000 * * @author David Dagon * @version 98 beta */ public class HelloComments { /** * A VERY simple variable. */ public int x = 10; Javadoc comments must appear above the item they modify. In the case of class comments, place them BELOW the import statements. Recall: HTML is OK!

26. Short, one-liner, followed by period Simple Example (cont’d) Just give the variable name; no dashes, etc. /** * This comment is reproduced twice, because * of the comma delimited declaration of ints. * The solution would be to use separate lines * for each declaration. */ public int y = 4, z = 11; /** * The Main method. This method is the starting * point of this program, when this class is run. * * @param args The arguments from the command line. */ public static void main(String[] args) { } } // HelloComments

27. @author @param @return @exception @deprecated // jdk 1.1 @since // jdk 1.1 @serial // jdk 1.2 @see @version You may include HTML tags (but avoid structuring tags, like, etc.) Javadoc comments should immediately preceed the declaration of the class, field or method. The first sentence should be a summary. Use the special javadoc tags--@. When '@' tags are used, the parsing continues until the doc compiler encounters the next '@' tag. Javadoc (Cont’d) You might refer back to these notes once you start writing your own javadoc comments.

28. Questions?

29. Constants Java: –public final static = ; –public final static int MIN_PASSING = 60; –public final static float PI = 3.14159f; Details on “why this syntax” to come soon... Quick Review

30. Printing to Screen Java: –System.out.println( ); –System.out.println( ); // prints blank line –System.out.println(5); // prints 5 –System.out.println(“Hello World”); // prints Hello World –“println” vs. “print” in Java: println includes “carriage return” at end, hence the next print or println starts new line print – not carriage return included. causes next print or println to begin on same line Quick Review

31. Method Madness

32. Caution Before we can talk about methods in Java, we need to look at programming languages in general, from a very high level. Don’t be alarmed by some of the syntax on the following slides. The big picture is what’s important, not the coding details....

33. Programming Paradigms Procedural Programming ‘Imperative’ assignment used to create state, and procedures manipulate state. E.g., C, Assembly, Pascal int y; int x = 3; y = manipulateData(x); Functional Programming Functions (procedures that do not depend on outside data) are used to provided data. E.g., Lisp. (defun check-member (input-item input-list) (cond ((null input-list) nil) ((equal input-item (first input-list)) T) (T (check-member input-item (rest input-list)))))

34. Object-Oriented Programming All data exists in objects; interaction occurs only between objects. Even numbers are objects that know how to add themselves. E.g., SmallTalk: | array | array := Array new: 5. rect := 0@0 corner: 8@9. 1 to: array size do: [ :item | rect origin: item@item. array at: item put: rect copy ]. Where does Java fit? Object-oriented, but not 100% OO, since it contains primitives, and tolerates some (hopefully small) degree of procedural programming. Programming Paradigms There are other paradigms, but these three help explain where Java comes from

35. Java Methods There exists in Java a single construct, the method, for both procedures and functions: when a procedure is called for, specify the return type “void” before method name public void printHelloWorld( ) { System.out.println(“Hello World!”); } // printHelloWorld Note: All methods must have parentheses for parameters... even if no parameters! Note the comment

36. Single construct (method) for both procedures and functions: when a function is called for, specify the appropriate return type before method name public float average (float num1, float num2, float num3) { float answer; answer = (num1 + num2 + num3)/ 3; return (answer); } // of average Java Methods

37. Writing Methods: A Larger Look A Java requirement: --Each method belongs to an object (or class). --It must be unambiguous which object or class that is when a method called. --To run an application program, there must be a class whose name is that of the program and that class must have a method called main: a class method, not an instance method visible to all nothing returned Method name for command line parameters public static void main (String[ ] argv)

38. class A { public static void main(... } class B { public static void main(... } class C { public static void main(... } Thus, each class may have it’s own main method. You pick the one you wish to run when invoking the JVM. This fact becomes critical when we learn to write debug test mains.

39. Method Signatures “The signature of a method consists of the name of the method and the number and types of formal parameters to the method. A class may not declare two methods with the same signature, or a compile time error occurs.” --Java Language Specification s.8.4.2 Method overloading occurs where identically named methods have subtle variations in the method parameters. public int getCube(int num){ return num*num*num; } public int getCube(float num){ return (int)(num*num*num); } public int getCube(double num){ return (int) (num*num*num); } Java lets you overload instead.

40. Methods: Common Mistakes public float average (float num1, float num2, float num3); { float answer; answer = (num1 + num2 + num3)/ 3; return (answer); } // average Note ending semicolon -- could be viewed as ‘abstract’ method (more on abstract methods later) -- results in unhelpful error message about abstract methods (more on this later) -- EASY mistake to make!

41. Where do Methods Go? Just like the main method we saw last time, any method we create must appear in classes, as either “class” or “instance” members. More on creating classes and objects shortly... For now, just know that methods belong in a class. i.e. They are defined inside a class.

42. OO Programming Note Already, we’ve covered one of the most important aspects of Object Oriented (OO) programming: Data and methods belong together in a class. Right now, it’s sufficient that you merely know that variables and methods belong in classes. Later, we’ll see how this enables us to encapsulate state (the variables) with behavior (the methods). So, remember this day. You’ve started to learn THE cornerstone of OO programming.

43. Questions?

44. Conditionals

45. Decision Statements Java: if (condition) single statement; else single statement; or: if (condition) { statements; } else { statements; }

46. Examples Java: What happens here? int testGrade = 65; boolean passing = true; if (testGrade < 60) passing = false; System.out.println(“Is “ + testGrade + “passing? “ +passing);

47. Boolean and Relational Operators Boolean: Java: AND && OR || NOT ! Relational: equal to == not equal to != less than < less than or equal to <= greater than > greater than or equal to >= Note: Assignment =

48. Example if (enrolled && passing) { // etc. } Java also supports short-circuiting, where only part of a boolean will be evaluated, as necessary: if (enrolled(studentNum) || getPassing(studentNum)) { // etc. } If first condition is true, it stops evaluation

49. A bit o’ code public static void main(String args[]) { int quiz1 = 42; int quiz2 = 99; if(isPassing(quiz1) && isPassing(quiz2)) System.out.println(“Passed both quizzes”); } public static boolean isPassing(int testGrade) { boolean passing = true; if (testGrade < 60) passing = false; System.out.println(“Is “ + testGrade + “ passing? “ + passing); return passing; }

50. Final Answer? Is 42 passing? false Is 99 passing? true a b

51. Multiple Selections via switch Use if construct for one selection. Use if/else construct for double selection. Use switch construct for multiple selection. (e.g., situations appropriate for if-elseif-elseif-else) Note: Useful when making a selection among multiple values of the same variable. Not useful when selecting among values of different variables.

52. Switch switch ( ) { case : // whatever code you want break; // optional case : // whatever code you want break; // optional default: // whatever code you want break; // optional }

53. Multiple Selections via switch Note the “optional” default case at the end of the switch statement. It is optional only in terms of syntax. switch (iNumber) { case 1: System.out.println (“One”); break; case 2: System.out.println (“Two”); break; case 3: System.out.println (“Three”); break; default: System.out.println(“Not 1, 2, or 3”); break; // Needed??? } // switch In practice you should always include a ‘default’ case statement. This would work without the default, but would be poor technique

54. How many days? if(month == 4 || month == 6 || month == 9 || month == 11) numdays = 30; else if(month == 2) { numdays = 28; if(leap) numdays = 29; } else numdays = 31;

55. Switch switch (month) { case 4: case 6: case 9: case 11: numdays = 30; break; case 2: numdays = 28; if(leap) numdays = 29; break; default: /* Good idea? */ numdays = 31; }

56. The switch statement can only be used with the following types: int, char, short & byte (You can cast floats, doubles, etc.) The case values must all be of the same type. The case values must all be FINAL constants. Multiple Selections via switch--Notes

57. Multiple Selections via switch switch (grade) { case ‘A’: case ‘a’: countOfAGrades++; break; case ‘B’: case ‘b’: countOfBGrades++; break; case ‘C’: case ‘c’: countOfCGrades++; break; case ‘D’: case ‘d’: countOfDGrades++; break; case ‘F’: case ‘f’: countOfFGrades++; break; default: System.out.println(“Invalid grade”); break; } (assume these variables exist and have value) if (grade==‘A’ || grade==‘a’) countOfAGrades++; else if (grade==‘B’ || grade==‘b’) countOfBGrades++; else if (grade==‘C’ || grade==‘c’) countOfCGrades++; else if (grade==‘D’ || grade==‘d’) countOfDGrades++; else if (grade==‘F’ || grade==‘f’) countOfFGrades++; else System.out.println (“Invalid grade”); same

58. Before we go too far... We often take liberties with good coding practice just to fit material onto a slide. Your coding style should reflect clearly and unambiguously what the code is supposed to do. Keep in mind your two audiences –The machine –Other programmers

59. If Style 1 if( ) { // code if true } else { // code if false }

60. If Style 2 if( ) { // code if true } // comment else { // code if false } // comment

61. Conditional Assignment boolean b; int count; b = checkCompletion(); count = (b) ? 3 : 1; Must resolve to boolean If true...... if false ? : Note: This is not any faster. It’s less readable. It exists only for ‘recovering C hackers’

62. A Semi Partial Conditional Summary Control structures –Two kinds of conditional (summary) if {... } else {... } –branch on boolean expression switch (...) { case...; break; default:... } –branch on constant value –don’t forget to break !!

63. Questions?