lecture09.pdf

Transcript

1. Session 9 ICS 211 ADT II
2. Memory Upload • Variables • Data fields • Methods
3. Variable • A variable is a name for a place in memory where a value is stored String n1 = new String("Nami"); //n1 is a variable • Variables can be used to store an object’s address • An object’s address (reference) is the place in memory where an object is stored
4. Data Fields • Data fields are variables used to store the data (attributes) of a specific object • Also called “instance variables” • Belong to an object • Declared (defined) in a class, but outside a method
5. Data Field Example • For example, String objects have data fields (attributes) that store the characters and length of a String String n1 = new String("Hana"); //"Hana" is stored //in a data field. //4 is stored //in another data field.
6. Defining Data Fields • When declaring (defining) data fields, put them outside all methods, but inside the class • Usually private, so data fields cannot be used by code outside the class • Data fields are global variables, so they can be used by any method in the same class as the data fields
7. Example Code • See program Fraction.java public class Fraction{ private Integer numerator; private Integer denominator; public Fraction(Integer num, Integer den){...} //other methods public String toString(){...}
8. Scope of Data Fields • The scope of a variable is where a variable is visible (where it can be used) • Scope refers to a program’s line numbers in a text file
9. Scope of Data Fields • For data fields, the scope begins after the opening delimiter (curly bracket) of a class and ends at the closing delimiter (curly bracket) of a class • Scope of variables numerator and denominator are lines 12-235, as they can be used within any method in the Fraction.java program
10. Comparison • Static Variables • Syntax to use a static variable outside a class: Class.VARIABLE • Example: double area = radius * radius*Math.PI; • Data Fields • Syntax to use a data field outside a class: cannot access directly • Example with accessor method: Integer num = f1.getNumerator();
11. Comparison • Static Variables  Syntax to use a static variable inside a class: VARIABLE  Global scope, so can use in any method in the same class • Data Fields  Syntax to use a data field within a class: variable  Global scope, so can use in any method in the same class
12. Comparison • Static Variables • Declaration has keywords “public static final” • By convention, all letters are capitalized • ALL_CAPS • Data Fields • Declaration only has keyword “private” (NO “static final”) • By convention, use “camel notation” • aCamelNotation
13. Comparison • Static Variables • Example declaration: public static final String MESSAGE = new String("ERROR"); • Data Fields • Example declaration: private String name = new String("Fred");
14. Comparison • Static Variables • Only have one copy of a static variable • Because classes do not use their static variables to create objects • Data Fields • Possible to have multiple copies of data fields • Because each object has one set of data fields
15. Methods • Methods are used to define the behavior of a class • Used to get the object to do something • See the Java API for methods of the String class and many, many other classes
16. Method Calls • To call (invoke) a method means to send a message (command) to an object • Syntax for (non-static) method call: variableName.methodName(); • Example: String upper = n1.toUpperCase();
17. Comparison • Static Methods • Belong to a class as a whole • DO NOT access or modify the data fields of an object • Non-static Methods • Access or modify the data fields of an object
18. Comparison • Static Methods • Method call syntax: Class.method() • Example: Double x = Math.pow(2.0,3.0); • Non-static Methods • Method call syntax: variable.method() • Example: String display = f1.toString();
19. Comparison • Static Methods • Method definition has keyword “static” • Non-static Methods • Method definition DOES NOT have keyword “static”
20. Method Call Terms String str = f1.toString(); method call variable variable of the same class as the method’s return type class
21. Method Definition Terms public String toString() {String fractionString = numerator + " / " + denominator; return fractionString;} method prototype method body
22. Method Definition Terms public String toString() { String fractionString = numerator + " / " + denominator; return fractionString; } access modifier return value return type method name local variable
23. Method Call Examples • Java API class String method call String n1 = new String("Nami"); int length = n1.length(); //length is 4 • ICS 211 class Fraction method call Fraction f1=new Fraction(3,5); String output = f1.toString(); //output is "3 / 5"
24. Method Definition • A non-static method has direct access to the class’s data fields • We can refer to the data fields by their names (see Fraction.java code) public String toString(){ String fractionString = numerator + " / " + denominator; return fractionString; }
25. Access Modifier • The public access modifier allows access to a class’s methods from code outside the class • If the method’s access modifier is private, then we CANNOT use a class’s methods from code outside the class
26. Access Modifier • In Fraction.java, because method greatestCommonDivisor() of class Fraction is private, we CAN ONLY call this method INSIDE the class • See reduceToLowestTerms() code Integer gcd = this.greatestCommonDivisor(numerator, denominator);
27. Access Modifier • In FractionCalculator.java, because method greatestCommonDivisor() of class Fraction is private, we CANNOT call this method OUTSIDE the class • This code at the bottom of the main() method (on line #74) will NOT compile Fraction f = new Fraction(1,2); f.greatestCommonDivisor(1,2);
28. Local Variables • The scope of local variables starts where the local variable is declared and ends at the last delimiter ("}") of the method • In the Fraction.java program, the scope of local variable fractionString starts at the declaration on line 45 and ends at the closing delimiter on line 50
29. Comparison • Data Fields • Variables that are created within a class and outside all methods • Examples: numerator, denominator • Local Variables • Variables that are created within a method • Examples: fractionString, numerator2, denominator2, result
30. Constructor • A constructor is a special method that initializes the values (data fields) of an object • The name of a constructor is the same as the name of its class • For example, the constructor’s name for class Fraction is “Fraction”
31. Constructor Call • A call to a constructor must be preceded by keyword “new”, which creates space in memory (on the heap) for the object • Syntax: ClassName variable = new ClassName(); • Example: Fraction f1 = new Fraction(3, 4);
32. Constructor Call • Classes that we write on our own can instantiate objects just like the classes from the Java API • Java API class String constructor String n2 = new String("Nami"); • ICS 211 class Fraction constructor Fraction f2=new Fraction(5,7);
33. Constructor Call Terms Fraction f2 = new Fraction(5,7); (actual) parameters, also called arguments class variable constructor
34. Constructor Definition • Just like other methods, a constructor definition has parameters • Unlike other methods, a constructor definition has neither a return type nor the keyword “void”
35. Constructor Definition Terms public Fraction(Integer num, Integer den){ numerator = num; denominator = den; } access modifier constructor name parameter’s type (formal) parameter
36. Parameters • The scope for parameters is similar to local variables • Starts at the method prototype, which is the first line of a method • Ends at the closing delimiter ("}") at end of the method
37. Parameters • For example, in the Fraction.java program, the scope of parameter numeratorParameter in the constructor definition is from line 29 to line 38 • So we can only use the variable numeratorParameter from line 29 to line 38
38. NullPointerException • A NullPointerException will happen whenever you try to call a method on a variable which is “null” • See NullPointerProgram.java code on the class web site • When you run the program, the exception will be thrown on line 17
39. NullPointerException • If you do not initialize your data fields in your constructor, you may also get this exception • For example, set the data field denominator to “null” in the constructor in the Fraction.java program and then run the FractionCalculator.java program
40. NullPointerException • To avoid forgetting to initialize my data fields in my constructor, I usually initialize my data fields when I first declare them private Integer numerator = new Integer(0); private Integer denominator=new Integer(1); • Of course, you can choose any way you wish to initialize your data fields
41. Method toString() • Whenever a String is needed, the toString() method is automatically called • For example, when a variable is put in a print() or println() method
42. Method toString() Fraction f1=new Fraction(5, 7); System.out.println(f1); //output is "5 / 7" System.out.println(f1.toString()); //output is "5 / 7"
43. Method toString() • Because all classes in Java are subclasses of class Object, which has method toString(), all classes automatically inherit the toString() method • See the Java API for class Object for the toString() method that is inherited by all classes in Java
44. Method toString() • Unless you write the code for the toString() method for your class, your toString() method will output the default value for the toString() method, which looks funky • Try it yourself at home: comment out the toString() method in the Fraction.java program and run the FractionCalculator.java program
45. Accessor Methods • Accessor methods are simple methods that return a data field • By convention, the name of an accessor method starts with “get” • Have a return value, but no parameters • Used to return data to the calling method
46. Accessor Methods • Method call Fraction f1=new Fraction(5,7); Integer num=f1.getNumerator(); //num is 5 • Method definition public String getNumerator(){ return numerator; }
47. Mutator Methods • Mutator methods are simple methods that modify a data field • By convention, the method name starts with the word “set” • Have parameters, but no return value • Used to change the value of a data field
48. Mutator Methods • Method call Fraction f1=new Fraction(5,7); f1.setNumerator(100); String str = f1.toString(); //str is "100 / 7" • Method definition public void setNumerator(Integer numeratorParameter){ numerator = numeratorParameter; }
49. Other Methods • You can create any methods that you deem appropriate for any class • For example, the Fraction class has many more methods, such as add(), subtract(), divide(), multiply(), reduceToLowestTerms(), and greatestCommonDivisor()
50. Memory Defragmenter • Variables • Data fields • Constructors • Accessor methods • Mutator methods
51. Task Manager • Before the next class, you need to: 1.Do the assignment corresponding to this lecture 2.Email me any questions you may have about the material 3.Turn in the assignment before the next lecture 4.Admire your local scenery!