Understanding Java: Object-Oriented Programming and Key Concepts

1. JAVA (something like C)

2. Object Oriented Programming • Process orientated – code acting on data • Object oriented – data controls access to code • Encapsulation – java has a ‘class’. The data is the state and the methods (code) is the behavior. You control the methods and access to the data • Inheritance – where one class acquires the properties of another class • Polymorphism – one interface multiple methods

3. JAVA editors • did a ‘search’ for JAVA editors on the NET and obtained... • JCreator – www.jcreator.com • Download the free one • editPlus - I have used this one • http://www.editplus.com/ • shareware - pay after 30 days • WingSoft • http://www.wingsoft.com/wingeditor.shtml

4. comments • A single line comment • int row; // keeps track of the row • multiple line comments • /* this is one line of the comment • this is the 2nd line • this is the third line */

5. Documentation comments • acts like multiple line comments • /** this is one line of the comment • this is the 2nd line • this is the third line */ • note the initial /** The javadoc program • will collect these blocks of comments as documentation for the program • javadoc filename.java

6. special symbols • { } indicates a block of code • ( ) parameters to a method • [ ] indicates an array • ; terminates a statement • , a seperator • . used to separate package names • Note table of reserved words on page 39

7. Identifier names • Start with a letter • Contains • letters (upper or lower case) • numbers • _ (the underscore) • $ • Can be any length • No imbedded spaces or special characters • Do not start an identifier with a number

8. Examples/Naming Conventions • numberOfRows • num_rows • row8 • Naming Conventions • classes start with a capitol letter • HelloWorld ShoppingCart • methods & identifiers with a small letter • println() getChar() • constants are all caps PIE

9. System.out.println( ) • System.out.println (….) sends a String to the monitor • System.out.println(“Hi Mom”); • The + ‘concatenates’ two variables • int aVal = 17; • System.out.println(“a = “ + aVal); • System.out.println(“a=“ + aV + “ b=“ + bV)

10. Primitive data types • JAVA has 8 primitive data type • Name size |-------range-------| • byte 8-bits -128 ~ 127 • short 16-bits -32k ~ +32k • int 32-bits -2**31 ~ +2**31 • long 64-bits -2**63 ~ + 2**63

11. Primitive data types (cont.) Name size |-------range-------| • float 32-bits +- 3.4 X 10**38 • double 64-bits +- 1.8 X 10**308 • char 16-bits 0 ~ 65,535 • boolean 1-bit true or false • (not a 0 or a 1)

12. Examples of primatives • int x; • int x, y=7, zz, rows=8; • boolean isRunning = false; • double dd; • float f = 10.3; (error - defaults to a double) • float fx = 22.4f; note the ‘f’ • byte b1, b2 = 44, cx; • short ss=5;

13. The char • char uses a 16-bit Unicode character set • The Unicode set allows 65,536 different characters. (about half are assigned) • char ch = ‘a’; • char ch1, ch2 = ‘7’, ch3 = ‘X’; • char delta = ‘\u0394’ delta symbol • char copyrt = ‘\u00AE’ copy write symbol • escape seq. – page 32 (EX \t is tab)

14. the String class • String is a class, not a primitive data type • is a sequence of characters • not an array of char terminated by a null • immutable – cannot be changed once created • String ss; • String s1 = “hi mom”; • String s1, su=“XXX”, s3=“SDE”;

15. Java DOS commands • Javac A:\>javac test.java • compiler • uses a text file named ????.java • creates a ????.class file of byte code • java A:\>java test • the interpreter (runs the class code) • runs an application • appletviewer A:\>appletviewer test.html • runs an applet

16. Minimum JAVA program • class Skeleton • { • public static void main(String args[]) • { • } // end main • } // end class

17. arithmetic • Expression – series of variables (methods) that evaluate to a single variable • = assign (evaluate right, assign to right) • + add x = x + 7; • - subtract x = 8 – cx; • * multiply pay = rate * time; • / divide tax = too_much / 1.0; • % remainder (mod) • row = x % 4; • row is the remainder of x / 4 • ans = 33 % 6; ans is 3

18. Consider the primitives • byte 8 bits • short 16 bits • int 32 bits • long 64 bits • float 32 bits • double 64 bits • char 16 bits

19. Some examples • Define and initialize all the primitives • Add the following to skeleton and compile • short = byte; • byte = short; • double = float • float = double; • int = boolean; • int = short * byte; • int = short * float; • double = int * double; • float = short * double; • int = char; chat = int;

20. Conversion and casting • Automatic – • types are compatible • destination is larger than the source • Casting (target type) value • byte b = (byte) someValue; • byte b = intExample; error • byte b = (byte) intExample; OK • byte b = 50; • byte x = b/2; Error • Byte x = (byte)(b/2); ok

21. Shortcuts in arithmetic • Shortcuts • x++ x = x + 1; • x-- x = x – 1; • x+=7; x = x + 7; • x*=3; x = x * 3; • x/=2; y = y / 2; • q-=5; q = q – 5;

22. ternary expression • expression01 ? expression02 : expression03 • Expression01 must evaluate to a boolean • If expression01 is true, • expression 02 is evaluated, else • expression03 is evaluated

23. Example of ? : • if (x > y) • max = x; • else • max = y; • is the same as • max = x>y ? x : y;

24. ‘pre’ & ‘post’ • Difference between x++; and ++x; • Example • int x = 5; • System.out.println(x + “ “ + x++); • int x = 5; • System.out.println(x + “ “ + ++x);

25. Operator precedence • Do work within ( ) • Then, left to right…. • ++ -- • * / % • + - • = • What’s the answer to the following.. • n = 1 - 2 * 3 - 4 + 5;

26. if statement • if(expression) // expression is evaluated • block of code // execute if true • else • block of code // execute if false • Note – a block of code can contain a nested if statement

27. examples • if (x > 6) • System.out.println(“x is bigger); • else • System.out.println(“x is less than 6); • if (x>8) • {} • else • System.out.println(“x is less than 8”);

28. logical operators • & evaluate both, both must be true • | evaluate both, either must be true • && both must be true. If 1st is false, do not evaluate the 2nd • || one must be true. If 1st is true, do not evaluate the 2nd • ~ tilda - not • Shift and bit-wise operators – you are on your own (page 80)

29. && and & • if ((a>b) & (c<=7)) • System.out.println(“both true”); • else • System.out.println(“one not true”); • if ((a>b) && (c<=7)) • System.out.println(“both true”); • else • System.out.println(“one not true”);

30. || and | • if ((a>b) | (c<=7)) • System.out.println(“at least one true”); • else • System.out.println(“both false”); • if ((a>b) || (c<=7)) • System.out.println(“at least one true”); • else • System.out.println(“both false”);

31. if ((a>b) && (a++)) • System.out.println(“(true)a=” + a); • else • System.out.println(“(false)a=” + a); • if (a>b) • if (c!=7) • System.out.println(“123456”); • is the same as • If((a>b) & (c!=7)) • System.out.println(“123456”);

32. Relational operators • == equal to • != not equal • > greater than • < less than • >= greater than or equal to • <= less than or equal to

33. The loops • For a loop you need 3 things • a counter of some kind • a comparison statement - a test • increment the counter some time • the loops are

34. for loop • for (int i = 6; i<5; i++) • { • System.out.println(“counter is “ + i); • } • the ‘for’ statement contains the counter, increment, and test all together

35. example • For (int k=0; k<3; k++) • System.out.println(k); • for (int d=5; d<3; d++) • System.out.println(k); • for (int d=3, int e=7; d<e; d++, e--) • System.out.println(d + “ and e=“ + e);

36. for loop – using a comma • for (a =1, b=4; a<b; a++, b--) • { • System.out.println(“a = “ + a); • System.out.println(“b = “ + b); • } • Gives a = 1 • b = 4 • a = 2 • b = 3

37. for loop – some variations • boolean done = false; • for (a =1; !done; a++) • { ……… done = true; } • boolean done = false; • for ( ; !done; ) • { ……… done = true; }

38. while loop • int x=0; // counter • while (x<5) // test • { • System.out.println(“counter = “ + x); • x++; // increment • }

39. Example #1 • int x=4; // counter • while (x<5) // test • { • System.out.println(“counter = “ + x); • x++; // increment • }

40. Example #2 • int x=4; // counter • while (x<5) // test • { • System.out.println(“counter = “ + x); • }

41. Example #3 • int x=14; // counter • while (x<5) // test • { • System.out.println(“counter = “ + x); • x++; // increment • }

42. do/while loop • int c=3; // counter • do { • System.out.println(“counter = “ + c); • c++; // increment • } while (c<5); // test • what’s ‘special’ about a do/while loop??

43. Example #1 • int c=3; // counter • do { • System.out.println(“counter = “ + c); • } while (c<5); // test

44. Example #2 • int c=13; // counter • do { • System.out.println(“counter = “ + c); • c++; // increment • } while (c<5); // test

45. Switch statement • switch(expression) • { • case value01: statement; • break; • case value02: statement; • break; • default: statement; • } • The expression is evaluated. Jump to the corresponding case statement with = value

46. expression must be a byte, short, int, or char • When you jump to the proper case value, you execute code until the end of the switch statement or a break statement is executed • No matches – jump to default • No default – out of switch

47. Switch(month) • { • case 1: days=31; • break; • case 2: days=28; • break; • case 3: days=31; • break; • case 4: days=30; • break; • | • case 12: days=31; • break; • default: days=0; • }

48. switch(month) • { • case 9: • case 4: • case 6: • case 11: days=30; break; • case 2: days=28; • break; • default: days=31; • }

49. More use of “break” • hitting a ‘break’ will terminate a loop • for(int x=0; x<100; x++) { • if (x==5) break; • System.out.println(“X=“ + x); • } • X=0 • X=1 • X=2 • X=3 • X=4

50. Using ‘break’ in nested loops • for(int x=1; x<4; x++) • { • System.out.println(“X=“ + x); • for (int y=0; y<30; y++) • { • if (y==x) break; • System.out.println} (“Y=“ + y); • } • }