Object-Oriented Programming in Java. Topic : Objects and Classes (cont) Object Oriented Design

Transcription

1 Electrical and Computer Engineering Object-Oriented in Java Topic : Objects and Classes (cont) Object Oriented Maj Joel Young Joel.Young@afit.edu 11-Sep-03 Maj Joel Young

2 Using Class Instances Accessing an instance method: newguy.raisesalary( 10.0 ); Accessing an instance attribute: double hissalary = newguy._salary; newsalary = this._salary * ( 1 + percent/100); // this refers to current instance Note: direct access to other object s attributes is discouraged (and in fact it is not allowed in above case due to private attribute) 2

3 Constructors Automatically called when object is created Same name as class -- e.g. Date(); No return value May be more than one constructor per class Default constructor with no arguments should be specified initialize to reasonable default value public Date() _day = 1; _month = 1; _year = 1; Constructor with arguments -- initialize to user specified values public Date( int d, int m, int y) _day = d; _month = m; _year = y; Cannot be invoked on existing objects 3

4 Constructors A quick quiz on constructors can you spot the error? class Employee public Employee( String n, double s ) String _name = n; double _salary = s; private String _name; private double _salary; 4

5 class Person // Attributes int m_age; double m_gpa; String m_name; // Operations public Person(String name, int age, double gpa) m_age = age; m_name = name; m_gpa = gpa; private void printname() System.out.print(m_name); Java Example static public void main(string args[]) // Create a Person instance Person p1 = new Person("Pyle",27,1.9); Person p2; // Print the name of // the person System.out.print("Name: "); p1.printname(); p2 = p1; // Print the same name again System.out.print(" Name: "); p2.printname(); // End class Person 5

6 Class Attributes/Methods Class methods/attributes Use static keyword in front of declaration Class MyMath static void add( int val1, int val2 ); May be accessed by using ClassName.method() or ClassName.attribute notation MyMath.add( 1, 3 ); An instance of the class is not required for access Class-wide Attributes An attribute shared by all instances of a class If one instance changes it all others see the change Analog: Global Variable use SPARINGLY Class-wide Operation A meta operation operates on the class, not instances of the class Typical application: Creating new class members, assigning class ID numbers, etc. 6

7 Packages Java code is distributed in separate packages (you can also create your own packages) To access code in a package refer to class by full name java.util.vector v = new java.util.vector(); or import class into your program import java.util.vector Vector v = new Vector(); or import entire package import java.util.* Package paths map to directory structure java.util.vector <CLASSPATH>\java\util\<vector package files> 7

8 Electrical and Computer Engineering Object-Oriented in Java Topic : Object Oriented Maj Joel Young Joel.Young@afit.edu 11-Sep-03 Maj Joel Young

9 Development Process Analysis Transform vague understanding of a problem into a precise description of the tasks to be solved Concerned with description of what must be done, not how it should be done Structure programming tasks into classes and packages (logically grouped clusters of objects) Specify operations and attributes of each class Specify relationships with other classes in the system Implementation Classes and operations are coded, tested, and integrated Object-orientation encourages evolutionary development since behavior and state is encapsulated into objects with predefined interfaces thus objects or packages can be incrementally added and tested more easily 9

10 Object-Oriented Goal: decompose a programming task into data types or class and define the functionality of these classes Sub-goals Identify classes Identify functionality of classes Identify relationships among classes A good design greatly reduces time required for implementation and testing 10

11 Object-Oriented Finding classes Look for nouns in problem analysis Many are good choices for classes Other classes may be necessary Finding operations Look for verbs in problem analysis Each operation must have exactly one class responsible for carrying it out Finding class relationships Association uses; a class uses another class if manipulates objects of that class in any way; should be minimized Aggregation has-a, contains; Inheritance is-a, specialization; useful in select places 11

12 Object-Oriented vs. Traditional Traditional Decomposition into sub-tasks Often results in numerous procedures or functions Difficult to manage and understand Task modules can simulate objects by organizing related tasks and attributes Only one instance (e.g. queue) Encapsulation achieved with opaque types (e.g. Unix file interface) No inheritance Object-Oriented Decomposition into objects with associated tasks Convenient clustering Data encapsulation helps debugging Multiple instances with similar behavior Inheritance captures commonalities among related classes of objects 12

13 Hints Always keep data private If user of the class has a need to both read and change a field (attribute), the following 3 items must be implemented in the class A private data field A public field accessor method A public field mutator method Benefits Internal implementation can be changed without affecting any code outside of the class Mutator methods can perform error-checking to ensure field is set properly Accessors to fields with mutable objects Should return clone of object so that calling object can not modify the field object contained in the class instance Always initialize data Avoids unexpected results Simplifies debugging 13

14 Hints Don't use too many basic types in a class Group related types into a separate class and use that class instead Bad Good class Employee class Employee private String _lname; private Name _name; private String _fname; private Address _address; private char _initial; private Date _hiredate; private String _street; private String _city; private String _state; private long _zip; Not all fields need individual field accessors and mutators Use a standard format for class definitions Break up classes with too many responsibilities Make the names of your classes and methods reflect their responsibilities 14

15 Homework Discussion Compile and run the Person example code Write and test a swap method Create a simple class with one int data member Write a swap method for that class that swaps values: : static public void swap(mytype a, mytype b); Such that after swap(a,b) is called a looks like b looked and b looks like a looked Analyze the requirements for a Calculator program Identify the classes, the operations, and the relationships between classes Make stub code for all your classes and operations with definitions for the has-a relationships Remember each class goes in a separate file 15

16 Homework Assignment Due Monday COMACC Calculator Question and Answer Session Analyze the requirements for a Calculator program Provide a narrative discussion of the requirements and behavior COMACC expects from the Calculator Prepare clarification questions. Proposing alternatives is a strong plus Discuss what is feasible in the next 3 weeks. Remember that you will know more in the next few weeks Provide me this in hard copy with your address on it Be prepared to: Identify the classes, the operations, and the relationships between classes Make stub code for all your classes and operations with definitions for the has-a relationships 16