1. Jan. 20041 Java Basics - II Yangjun Chen Dept. Business Computing University of Winnipeg

2. Jan. 20042 Outline: Java Basics - II Modifiers (Specifiers) Statements Array Control flow -condition statements: if switch -loop structure: for while do others: String, print, new, constructor

3. Jan. 20043 Modifiers Modifiers are special keywords that modify the definition of a class, method, or variables. modifiers Modifiers for methods Modifiers for variables Modifiers for classes

4. Jan. 20044 Modifiers for Methods and Variables static final private protected public access modifiers

5. Jan. 20045 Modifiers for Methods and Variables Static and final Modifiers -The static keyword is used in the declaration of class variables and class methods. -At times we want to associate a variable or a method with the class rather than with every instance (every object) of the class. -Within a class, a static variable can be accessed by referring to its name. -From outside of the class, a static variable can be visited using the dot notation: className.variableName. -A static method can be visited from outside of the class using the dot notation: className.methodName.

6. Jan. 20046 Modifiers for Methods and Variables Class Circle { static double PI=3.141592635; double radius; double circumference; double diameter; … }

7. Jan. 20047 Modifiers for Methods and Variables Here, PI shouldn’t be a variable. (It is just a constant.) To make it not changeable, we add the keyword final to the declaration. Class Circle { final static double PI=3.141592635; … } -Any attempt to modify PI now will generate an error. -Final modifier can not be used for methods. Access modifiers allow you to control the visibility and access to variables and methods inside your class.

8. Jan. 20048 Difference between instance and class variables Class Circle { static double PI=3.141592635; double radius = 2.0; double circumference = 12.56; double diameter =4.0; … } Class ComputationCircles { double x, y, z, w; double computation() { x = Circle.PI; Circle o = new Circle(); y = o.radius; z = o.circumference; w = o.diameter; … …} }

9. Jan. 20049 Difference between instance and class methods Class Circle { …... static double m1( ) { … } double m2 ( ) { … } } Class ComputationCircles { double x, y, z, w; double computation() { …... x = Circle.m1(); Circle o = new Circle(); y = o.m2(); … …} }

10. Jan. 200410 Modifiers for Methods and Variables There are three access modifiers in Java: -private protected public private Access Modifier -The private modifier creates a non-accessible member. -A private method or variable restricts access only to those methods in the same class. -The private identifiers are unknown to other classes even if the extend (subclass) from the class. - To declare a member private, simply put the keyword private in front of its declaration. private int number;

11. Jan. 200411 Modifiers for Methods and Variables protected Access Modifier -The protected modifier allows access to a member within the same package and within any subclasses. -We won’t study packages at this point. (They are basically a collection of classes that you can define yourself and used as a library.) -You can declare an identifier protected by placing the protected keyword in front of its declaration. protected int number; public Access Modifier -The public modifier allows access to a member inside or outside of the class. public int number;

12. Jan. 200412 Modifiers for Classes There are three modifiers for classes in Java. -public This makes a class accessible to outside of the package. -final This prevents a class from being extended. -abstract In an abstract class, some abstract methods (with only method signature; no implementation) are defined. Therefore, an abstract class can not be instantiated.

13. Jan. 200413 Statements There are several types of statements: -Declaration statements, expression statements, return statements and compound statements. Declaration Statements -declare new variables by specifying the type, variable name, properties and optionally initialize the variable. -some example are: Button clear; private Point sp=new Point(45, 15), ep; final int Max=100000; int almostMax=Max - 500;

14. Jan. 200414 Statements Expression Statements -Expression statements cause the expression to be evaluated and then any other actions in the statement are executed. -some example are: clear = new Button(“Clear”); i++; p = q;

15. Jan. 200415 Statements Return Statements -statements in a method with non-void return types that return some information back to the location where the method was called. -A return statement is of the following form: return (expression); -The expression can be any expression that is of the same type as the header declaration of the method. -When a return is encountered, it forces the immediate exit from the method.

16. Jan. 200416 Statements - Example: double magnitude(double x, double y) { double sqrSum = x*x + y*y; return Math.sqrt(sqrSum); } Compound Statements -a collection of statements enclosed in curly braces. -Example: { Integer obj = new Integer(8); System.out.println(obj.value); }

17. Jan. 200417 Array An array is a collection of items. Each array has some number of slots, each of which can hold an individual item. -An item can be any object or primitive variable; but an array can be only of one type of items; -You can’t have an array with different types in it. Steps to create an array: -1. Declare a variable to hold the array int temp[]; -2. Create new array object and assign it: temp = new int[10]; -3. Store things in the array temp[0] = 6;

18. Jan. 200418 Array Note that the array index starts from zero, so if the array is of size 10, the index values will run from 0 to 9. It is also possible to initialize the array during declaration: -int temp[] = {1, 2, 3, 4, 5};

19. Jan. 200419 Multidimentional Array This is just an array of arrays and is declared in the same manner as regular arrays. -Example: int temp[][] = new int[10][5]; -This will create an array that has 10 rows and 5 columns. -This will allow us to store 50 values in temp. -You would access each item of the array by specifying both indices, for example, temp[6][3] = 12; This will store the value 12 into location with row index 6 and column index 3. -int temp[][] = {{33, 71}, {-16, 45}, {99, 27}};

20. Jan. 200420 if Statement The if statement -written as: if (boolean expression) statement; //controlled statement -If the boolean expression evaluates true, then the controlled statement is executed. If it is false, then the controlled statement is skipped. -The controlled statement can also be a compound statement.

21. Jan. 200421 if Statement The if … else statement -This is another form of the if statement. -It allows some code to be executed if the boolean expression is true and some other code if false. -written as: if (boolean expression) statement1//expression is true else statement2;//expression is false

22. Jan. 200422 switch Statement The switch statement -Instead of chaining if …else statements, a switch statement can be used for testing equality. -This is like a case statement in some other languages. -The expression must evaluate to a primitive type and the constants must be literals of final variables. -The defaul case is evaluated if the expression fails to match any of the constants. (The default may be omitted, but it is a good idea to have it here.)

23. Jan. 200423 switch Statement Switch (expression) { case constant1: some statements; case constant2: some statements; … default: some statements: } //end of switch

24. Jan. 200424 switch Statement Whenever a label (case constant:) is matched, execution of the program continues from there through the switch statement executing each line. -This is the fall through behavior. To avoid this, a statement is used. -A break statement will force the exit from a switch statement -In almost all cases, you would write a switch statement with a break statement separating each case statement.

25. Jan. 200425 switch Statement Switch (expression) { case constant1: some statements; break; case constant2: some statements; break; … default: some statements: } //end of switch

26. Jan. 200426 for Statement The for loop in Java looks as follows: for( init ; expression ; increment) { some statements; } //end of the for loop The init is an expression that will introduce the start of the for loop. - Example: int i = 0; -declares and initializes the index i for this loop.

27. Jan. 200427 for Statement The expression is a boolean expression that is evaluated before each pass of the loop. -If it is true, then the loop is executed. -Example: i < 10; The increment expression is any expression of function call. The increment is commonly used to bring the state of the loop closer to returning false and completing. - Example: int i ++;

28. Jan. 200428 for Statement So the for loop looks like this for (int i = 0; i < 10; i ++) { System.out.println( i ); } //end of the for loop What is the output?

29. Jan. 200429 while Statement while loops like for loops will repeat the execution of a block of code until a specific condition is met. A while loop looks like the following: while (boolean expression) { statements } //end of the while loop As long as the boolean expression evaluates true, the code in the loop is executed.

30. Jan. 200430 do Statement Another loop that is quite similar to the while loop is called the do loop. A do loop takes the form: do { statements } while (boolean expression) The difference between this loop and the while loop is that the condition is tested at the end of the loop. -do loop always execute the loop at lease once.

31. Jan. 200431 Loop Constructs For all these loops, you can use the break statement to exit the loop at any time. There is also a continue statemment that forces the loop to start over at the next iteration. Label1: outer-iteration { inner-iteration { … break; //break out of the inner iteration … continue; //move back to the beginning of the inner iteration continue label1; //move all the way back to Lable1 break lable1; //break out of both iterations }}

32. Jan. 200432 int i = 0; label: for (int j = 0; j < 10; j++) { while (i < 10) { System.out.println(i); break; i++; }

33. Jan. 200433 label : for (int j = 0; j < 10; j++) { int i = 0; while (i < 10) { System.out.println(i); continue; i++; }

34. Jan. 200434 int k=0; label: for (int j = k; j < 10; j++) { int i = 0; while (i < 10) { System.out.println(i); k++; continue label; }

35. Jan. 200435 label: for (int j = 0; j < 10; j++) { int i = 0; while (i < 10) { System.out.println(i); break label; i++; }

36. Jan. 200436 big_loop: while (!done) { if (test(a, b) == 0) continue; //Control goes to point 2 try { for (int j = 0; j < 10; j++) { if (a[j] == null) continue; //Control goes to point 1 else if b[j] == null) continue big_loop; //Control goes to point 2 doit(a[j], b[j]); //point 1. Increment and start loop again with the test } finally {cleanup(a, b);} //point2. Start loop again with the (!done) test; }

37. Jan. 200437 break_test: if (check(i)) { try { for (int j =0; j<10; j++) { if (j > i ) break;//Terminate just this loop. if (a[i][j] == null) break break_test;//Do the finally clause and //terminate the if statement. } finally {cleanup(a, i, j)} }

38. Jan. 200438 Strings Strings are a combination of characters and instances of the class String in Java. -They are not simply an array of characters. String literals are a series of characters enclosed in quotation marks. A string declaration would look like: -String myString=“Brand new String”; Unlike many other language strings can not be indexed character by character. - There are methods in the String class that will retrieve characters for you. String s = “This is a string!”; char char_a = s.charAt[0];

39. Jan. 200439 Print Statements The System.out.println method prints massage out to the standard output of your system - to the screen, to a special window, or maybe to a log file. It also prints a new line character at the end of a string. System.err.println() prints on standard err instead. You can concatenate arguments to println() using the + operator. Example: -System.out.println(“Hello” + “World!”);

40. Jan. 200440 Print Statements Using print() instead of println() does not break the line. Example: System.out.print(“Hello”); System.out.print(“World!”); System.out.println(); This will yield the same output as the previous example. System.out.println() breaks the line and flushes the output.

41. Jan. 200441 new Operator The new operator is used for creating new instance of classes or objects. To create a new object, use the new operator with the name of the class you want to create an instance of, then parentheses after that. String str = new String(); Random r = new Random(); Triple origin = new Triple(); - The parentheses need not be empty; they can contain arguments that determine the initial values of instance variables or other properties. -Example: Point pt = new Point(0, 0);

42. Jan. 200442 new Operator The number and type of arguments that can be used in the parentheses with new are defined by the class’ constructor method. -Example: Date d1 = new Date(); Date d2 = new Date(71, 7, 1, 7, 30); Date d3 = new Date(“April 3 1993 3:24 PM”);

43. Jan. 200443 Class Date { … Date ( ) { … } Date (int x, int y, int z, int u, int v) { … } Date (String s) { … } …... } Constructors

44. Jan. 200444 Constructors In the above example, we have shown three different constructors for the Date class. This is called method overloading or in this case constructor overloading. Overloading is when you have methods with the same name but with different arguments. The compiler will distinguish between the methods by different arguments -different arguments mean different number of arguments or different type of arguments

45. Jan. 200445 Constructors Constructors are special methods that are used to initialize new object, set its variables, and perform any other functions the object needs to initialize itself. These methods are called by Java automatically when you create a new object. Constructors always have the same name as class with no return type. For example, say we had a class Circle and always wanted the center to start at (100, 100).

46. Jan. 200446 Constructors Class Circle { int x, y; Circle( ) { x = 100; y = 100; } … } So every time we create an object of class Circle, the constructor will be called automatically setting x and y to 100, respectively.

47. Jan. 200447 Backslash codes: CodeMeaningcodeMeaning \bBackspace\N Octal constant \nNewline(where N is 0 to 7) \rCarriage return\NNOctal constant \ffrom feed(where N is 0 to 7) \tHorizental tab\MNNOctal constant \”Double quote(where M is 0 to 3 and \’Single-quote characterN is 0 to 7) \0Null\uxxxxUnicode character \\Backslash(where xxxx are four hexadecimal constants)