Understanding Object Instantiation and Access Modifiers in Java Programming

1. CIS3023: Programming Fundamentals for CIS Majors II Summer 2010 Objects and Classes (contd.) Course Lecture Slides 19 May 2010 Ganesh Viswanathan

2. Objects and Classes Credits: Adapted from CIS3023 lecture slides (Spring 2010) by Dr SeemaBandyopadhyay,University of Florida, Gainesville.

3. Object Instantiation • Declaring and Creating object in two steps • Declaring and Creating object in single step • // <ClassName> <objectRefVar>; • Circle myCircle; • myCircle = new Circle(); • // <ClassName> <objectRefVar> = new <ClassName>(); • Circle myCircle = new Circle();

4. Trace Code animation Declare myCircle Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; myCircle no value

5. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; myCircle no value Create a circle

6. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; myCircle reference value Assign object reference to myCircle

7. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; myCircle reference value yourCircle no value Declare yourCircle

8. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; myCircle reference value yourCircle no value Create a new Circle object

9. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle( ); yourCircle.radius = 100; myCircle reference value yourCircle reference value Assign object reference to yourCircle

10. Accessing Objects • Referencing the object’s data: • Invoking the object’s method: // objectRefVar.data myCircle.radius //objectRefVar.methodName(arguments) myCircle.getArea()

11. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle( ); yourCircle.radius = 100; myCircle reference value yourCircle reference value Assign object reference to yourCircle

12. Trace Code, cont. animation Circle myCircle = new Circle(5.0); Circle yourCircle = new Circle(); yourCircle.radius = 100; myCircle reference value yourCircle reference value Change radius in yourCircle

13. Value Type vs. Reference Type

14. Copying Variables

15. Information Hiding • In a well designed OO application, a class publicizeswhat it can do i.e. its method signatures • but hides the internal details both of • how it performs these services (method bodies) and • the data (attributes) that it maintains in order to support these services

16. Access Modifiers class Circle { private double radius; public Circle() { this(1.0); } public Circle(double newRadius) { radius = newRadius; } public double getArea() { return radius * radius * 3.14159; } } Typically: Attributes are declared private Methods are declared public

17. Visibility Modifiers • public • The class, data, or method is visible to any class • private • The data or methods can be accessed only by the declaring class.

18. Accessing private attributes public class Driver { public static void main(String[] args) { Circle s1, s2; s1 = new Circle(14); s2 = new Circle(7); System.out.println(“Radius = “, s1.radius); outcome = s2.getArea(); //ok! } } Illegal:because attribute radius is hidden or private!

19. Accessing private attributes • How can code in any other class access them? • Programmer can provide methods to get and set them.

20. Get/Set Methods • Get method or Accessor • A method that returns the value of an attribute e.g., getUFID( ) { … return studentUFID; } • Set method or Mutator • A method that changes the value of an attribute e.g., getUFID( ) { … return studentUFID; } • Typically, have one get method and one set method per attribute.

21. Example Class Code public class Circle { private double radius; public double getRadius() { return radius; } public void setRadius(double radius){ this.radius = radius; } public Circle() { this(1.0); } public Circle(double r) { setRadius(r); } public getArea() { return 3.14*radius*radius; } }

22. Example Driver Code public class Driver { Circle c1 = new Circle(14); Circle c2 = new Circle(7); System.out.println(c1.getRadius()); //ok! c1.setRadius(5); //ok! boolean outcome = s2.getArea(); //ok! }

23. Some more code public class Circle { private double radius; public double getRadius() { return radius; } public void setRadius(double r){ if (r>0) radius = r; } public Circle() { this(1.0); } public Circle(double r) { setRadius(r); } public getArea() { return 3.14*radius*radius; } }

24. Still more code public class Student { private String name; private String ssn; private float gpa; public Student(inti, String n) { setSsn(i); setName(n); setGpa(0.0f); } // other constructors and methods, not shown here public String getName () { return name; } public void setName(String newName) { name = newName;} public String getSsn () { return ssn; } public void setSsn(String s) { ssn = s;} public float getGpa () { return gpa; } public void setGpa(float newGpa) { gpa = newGpa;} }

25. Lots more code public class Student { private String name; private String ssn; private float gpa; private intnumDsFs; public Student(inti, String n) { setSsn(i); setName(n); setGpa(0.0f); setNumDsFs(0); } // other methods, not shown here public booleanisOnProbation() { if(numDsFs > 3) return true; else return false; } public String getName () { return name; } public void setName(String newName) { name = newName;} public String getSsn () { return ssn; } public float getGpa () { return gpa; } public void setGpa(float newGpa) { gpa = newGpa;} public void setNumDsFs(int n) { numDsFs = n; } }

26. Benefits of Information Hiding • Allows Data Validation • Allows control over the level of access given for an attribute • Simplifies Code Maintenance

27. Class attributes • Each instanceof a class (called an object) has a copy of the attributes • Changing an attribute in one object doesn’t affect the attribute of another object

28. Class Attributes, cont. • Sometimes you may want some data to be shared among all instances. • Example: we want all Student objects to have a shared access to the total student enrollment count at the university.

29. Static Attributes public class Student { private String name; private String ssn; private float gpa; private staticinttotalNumStudents = 0; public Student(inti, String n) { setSsn(i); setName(n); setGpa(0.0f); totalNumStudents++; } // other constructors, accessors/mutators, and methods, not shown here public intgetTotalNumStudents() { return totalNumStudents; } }

30. Static Attributes, cont. • A static attributeis one whose value is shared by all instances of that class. • It in essence belongs to the class as a whole.

31. Static Attributes, cont. // Client code: Student s1 = new Student(); Student s2 = new Student(); Student s3 = new Student(); System.out.println(s1.getTotalNumStudents()); System.out.println(s2.getTotalNumStudents()); System.out.println(s3.getTotalNumStudents()); All of these println statements will print the value 3.

32. Static Methods public class Student { private String name; private static inttotalNumStudents = 0; // other attribute details omitted ... public staticintgetTotalNumStudents() { return totalNumStudents; } } • // Client code • Student s1 = new Student(); Student s2 = new Student(); Student s3 = new Student(); System.out.println(Student.getTotalNumStudents()); System.out.println(s1.getTotalNumStudents());

33. Static Methods • may only access static attributes • class Student { • private String name; // NOT static • private staticinttotalStudents; • public static void print() { • System.out.println(name + " is one of " + • totalStudents + " students."); // ILLEGAL! • } • }

34. Get more info! • Value Types and Reference Types:http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html • Creating and initializing objects:http://docstore.mik.ua/orelly/java-ent/jnut/ch03_02.htm • (Online) Search keywords: • JAVA classes and objects • JAVA pass by reference or value