CSC 142 Computer Science II

1. CSC 142 Computer Science II Zhen Jiang West Chester University zjiang@wcupa.edu

2. Table of Contents • Introduction to field (attribute) • Public and private modifiers • Accessor and mutator methods • Constructor • Static method • Instance method • Overloading • Static variables • Summary

3. Fields • field: A variable inside an object that represents part of its internal state. • Each object will have its own copy of the data fields we declare. • Declaring a field, general syntax: public <type> <name>; or public <type> <name>= <value>; (with initialization)‏ • Example: public class Student { public String name; // each student object has a public double gpa; // name and gpa data field }

4. public class Point3D { public int x; public int y; public int z; } • Every object of type Point3D contains three integers. • Point3D objects (so far) do not contain any behavior. • Class declarations are saved in a file of the same name: Point3D.java

5. Public and private modifiers • P332 • Type specified as public • Any other class can directly access that object by name • Classes generally specified as public • Instance variables usually notpublic • Instead specify as private

6. Dog.java • Change field to private • http://www.cs.wcupa.edu/~zjiang/Dog2trial.pdf • DogDemo.java • http://www.cs.wcupa.edu/~zjiang/DogDemo.pdf

7. Error!

8. Demonstration of need for private variables • Statement such asbalto.name = "Balto";is illegal since name is private

9. Accessor and Mutator Methods • P337-338 • Must also provide methods to change the values of the private instance variable • Typically named setSomeValue • Referred to as a mutator method • When instance variables are private must provide methods to access values stored there • Typically named getSomeValue • Referred to as an accessor method

10. Consider the example class with accessor and mutator methods • http://www.cs.wcupa.edu/~zjiang/Dog2.pdf • Note the mutator method • setName , setBreed , setAge • Note accessor methods • getName, getBreed, getAge

11. Using the accessor and mutator methods • http://www.cs.wcupa.edu/~zjiang/DogDemo2.pdf

12. 1 public class DogDemo2 2 { 3 public static void main(String[] args) 4 { 5 Dog2 balto = new Dog2(); 6 balto.setName("Balto"); 7 balto.setAge(8); 8 balto.setBreed("Siberian Husky"); 9 balto.writeOutput();10 11 Dog2 scooby = new Dog2();12 scooby.setName("Scooby");13 scooby.setAge(42);14 scooby.setBreed("Great Dane");15 System.out.println(scooby.getName()+" is a "+scooby.getBreed()+".");16 System.out.print("He is " + scooby.getAge() + " years old, or ");17 int humanYears = scooby.getAgeInHumanYears();18 System.out.println(humanYears + " in human years.");19 }20 }21 Tedious

13. We want something more like: // better! Dog3 balto = new Dog3(“Balto”, “Siberian Husky”, 8);

14. Constructor • constructor: A special method that initializes the state of new objects as they are created. • Constructor syntax, p352: public <class name>(<parameter(s)>) { <statement(s)>; } • How does this differ from previous methods? • Method name • No return!

15. public class Dog3 { private String name; private String breed; private int age; public Dog3( String strName, String strBreed, int intAge) { name = strName; breed = strBreed; age = intAge; } }

16. http://www.cs.wcupa.edu/~zjiang/Dog3.pdf • http://www.cs.wcupa.edu/~zjiang/DogDemo3.pdf

18. Calling method from constructor: a constructor can call methods within its method body, in order to avoid unnecessarily repeating some code. • To avoid the problem with inheritance, which we will cover later, we need to make such a method private!

19. Static Methods • Some methods may have no relation to any type of object, p501 • One method each class • Example • Compute max of two integers • Convert character from upper- to lower case • Static method declared in a class • Can be invoked without using an object • Instead use the class name

20. Adding Method main to a Class • Method main used so far in its own class • Often useful to include method main within class definition • To create objects of other classes • To be run as a start of program • For any ordinary class, method main is ignored

21. public class Point3D { public int x; public int y; public int z; } • Every object of type Point3D contains three integers. • Point3D objects (so far) do not contain any behavior. • Class declarations are saved in a file of the same name: Point3D.java

22. How would we translate several points? p1.x += 11; p1.y += 6; p1.z + 2; p2.x += 2; p2.y += 4; p2.z += -2; p3.x += 1; p3.y += 7; p3.z += 17; What is unsettling about this code?

23. Write a static method in the client code to translate points. // Shifts the location of the given point. public static void translate( Point p, int dx, int dy, int dz) { p.x += dx; p.y += dy; p.z += dz; } Example: // move p2 translate(p2, 2, 4, 6); Question: Why doesn't the method need to return the modified point?

24. Every client code (application class) that declares Point3D and wants to translate points would have to write their own static translate method! Also, the call syntax doesn't match the way we're used to interacting with objects: translate(p2, 2, 4); We want something more like: p2.translate(2, 4);

25. Instance methods instance method: A method inside an object that operates on that object, p347 Declaring an object's method, general syntax: public <type><name> (<parameter(s)>) { <statement(s)>; } How does this differ from previous methods?

26. An object's instance methods can refer to its fields. public void translate(int dx, int dy, int dz) { x += dx; y += dy; z += dz; } How does the translate method know which x, y, z to modify?

27. Each instance method call happens on a particular object. Example: p1.translate(11, 6, 2); The code for an instance method has an implied knowledge of what object it is operating on. implicit parameter: The object on which an instance method is called.

28. Think of each Point3D object as having its own copy of the translate method, which operates on that object's state: Point3D p1 = new Point3D(7, 2, -2); Point3D p2 = new Point3D(4, 3, 2); public void translate(int dx, int dy, int dz){ ... } 7 2 x: y: p1: public void translate(int dx, int dy, int dz){ ... } 4 3 x: y: p2: -2 z: 2 z:

29. What happens when the following calls are made? p1.translate(11, 6, 3); p2.translate(1, 7, -10); p1: p2: public void translate(int dx, int dy, int dz){ ... } 8 1 7 2 -2 18 x: y: z: public void translate(int dx, int dy, int dz){ ... } 5 2 -8 4 3 10 z: x: y:

30. public class Point3D { public int x; public int y; public Point3D( int initialX, int initialY, int initialZ) { x = initialX; y = initialY; z = initialZ; } // Changes the location of this Point3D object. public void translate(int dx, int dy, int dz){ x += dx; y += dy; z += dz; } }

31. public class PointMain3 { public static void main(String[] args) { // create two Point3D objects Point3D p1 = new Point3D(5, 2, -2); Point3D p2 = new Point3D(4, 3, 2); // print each point System.out.println("p1 is (" + p1.x + ", " + p1.y + ")"); System.out.println("p2 is (" + p2.x + ", " + p2.y + ")"); // move p2 and then print it again p2.translate(2, 4, -2); System.out.println("p2 is (" + p2.x + ", " + p2.y + ")"); } } Output: p1 is (5, 2, -2)‏ p2 is (4, 3, 2)‏ p2 is (6, 7, 0)‏

32. Instead of System.out.println("p1 is (" + p1.x + ", " + p1.y + ")"); It would be nice to have something more like: System.out.println("p1 is " + p1); What does this line currently do? Does it even compile? It will print:p1 is Point@9e8c34

33. When an object is printed or concatenated with a String, Java calls the object's toString method, p509-516. System.out.println("p1 is " + p1); is equivalent to: System.out.println("p1 is " + p1.toString()); Note: Every class has a toString method.

34. The default toString behavior is to return the class's name followed by gibberish (as far as you are concerned). You can replace the default behavior by defining a toString method in your class.

35. The toString method, general syntax: public String toString() { <statement(s) that return a String>; } NB: The method must have this exact name and signature (i.e., public String toString()). Example: // Returns a String representing this Point. public String toString() { return "(" + x + ", " + y + “, “ + z + ")"; }

36. The Math Class • Provides many standard mathematical methods • See in examples

38. Recall that arguments of primitive type treated differently from those of a class type • May need to treat primitive value as an object • Java provides wrapper classes for each primitive type, p562 • Methods provided to act on values

39. Contain useful predefined constants and methods, for instance: • Integer.MAX_VALUE • Double.parseDouble(“199.88”) • Double.toString(199.88) • Have no default constructor • Programmer must specify an initializing value when creating new object • Have no set methods

40. Overloading • When two or more methods have same name within the same class • Java distinguishes the methods by number and types of parameters • If it cannot match a call with a definition, it attempts to do type conversions • A method's name and number and type of parameters is called the signature

41. Overloading and automatic type conversion can conflict • Remember the compiler attempts to overload before it does type conversion • Use descriptive method names, avoid overloading

42. P376 • http://www.cs.wcupa.edu/~zjiang/Overload.pdf

43. You must not overload a method where the only difference is the type of value returned

44. Static Variables • Are shared by all objects of a class • Variables declared static final are considered constants – value cannot be changed • e.g., public static final double FEET_PER_YARD = 3; • Variables declared static (without final) can be changed, p498 • Only one instance of the variable exists • It can be accessed by all instances of the class • e.g., public static int numberOfInvocations;

45. Static variables also called class variables • Contrast with instance variables • Do not confuse class variables with variables of a class type • Both static variables and instance variables are sometimes called fields or data members

46. Summary • Public and private modifier, p332 • Accessor and mutator method, p337 • Constructor, p352 • Calling private method from constructor • Static method, p501 • Instance method, p347 • Adding method main to a class • Math class • Wrapper class, p562 • Overloading, p376 • Static Variables, p498