1. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 1 Chapter 5 l Programming with Methods l Static Methods and Static Variables l Designing Methods l Polymorphism l Constructors l Information Hiding Revisited l Packages (reading assignment) More about Objects and Methods

2. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 2 The this Parameter this refers to the calling object. l A method called inside an object definition file does not need a reference. You may either use this. or omit it because it is presumed. Example: answerOne() is a method defined in the class Oracle. public class Oracle {... //One way to invoke the answerOne method: this.answerOne(); //Another way is to omit this.: answerOne(); //this. is presumed.... }

3. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 3 When an Object Is Required l A method called outside an object definition file requires an object to precede the method name. l Example: Oracle myOracle = new Oracle(); //myOracle is not part of the definition code //for Oracle.... //dialog is a method defined in Oracle class. myOracle.dialog();...

4. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 4 null If the compiler requires you to initialize a class variable, you can set it to null if you have no other initial value. You can use == and != to see if a class variable is equal to null, because null is used like an address. Gotcha : Null Pointer Exception If you invoke a method using a variable that is initialized to null, you get an error message that says “Null Pointer Exception”. Species specialSpecies = null; specialSpecies.readInput(); Species specialSpecies = new Species(); specialSpecies.readInput(); Null Pointer Exception OK

5. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 5 What’s wrong? public class SpeciesDemo { public static void main(String[] args) { Species s1 = null; Species s2 = null; s1.set("aardvark", 50, 5); s2.set("wombat", 100, 3);... } l How can you fix it?

6. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 6 Static Methods l Some methods don’t need an object to do their job. »Example: A method to calculate area. Just pass the required parameters and return the area. l Use the class name instead of an object name to invoke a static method. l Example: » CircleFirstTry is a class with methods to perform calculations on circles. The area of a circle is CircleFirstTry.area(myRadius); »Notice that the the method invocation uses className. instead of circleObject.. l Static methods are sometimes called class methods.

7. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 7 Static Methods Declare static methods with the static modifier. Example: public static double area(double radius) {... l Since a static method doesn’t need a calling object, its definition cannot refer to an instance variable of the class because there is no instance variable to refer to. l Likewise, a static method’s definition cannot invoke a nonstatic method of the class (unless it creates a calling object). See page 328.

8. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 8 Uses for Static Methods l Static methods are commonly used to provide libraries of useful and related methods. Example: The Math class. »Automatically provided with Java. »Methods include pow, sqrt, max, min, etc. »See the next slide or page 335 for more details.

9. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 9 The Math Class Includes constants Math.PI (approximately 3.14159) and Math.E (base of natural logarithms, approximately 2.718). Includes three similar static methods: round, floor, and ceil. (Note the return types on page 335.) »Math.round returns the whole number nearest its argument. Math.round(3.3) returns 3 and Math.round(3.7) returns 4. »Math.floor returns the nearest whole number that is equal to or less than its argument. Math.floor(3.3) returns 3.0 and Math.floor(3.7) returns 3.0. »Math.ceil (short for ceiling) returns the nearest whole number that is equal to or greater than its argument. Math.ceil(3.3) returns 4.0 and Math.ceil(3.7) returns 4.0.

10. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 10 Java Tip : You Can Put a main in Any Class Usually main is by itself in a class definition. Sometimes it makes sense to have a main method in a regular class definition. When the class is used to create objects, the main method is ignored. Adding a diagnostic main method to a class makes it easier to test the class’s methods. Because main must be static, you can’t invoke nonstatic methods of the class in main unless you create an object of the class. Normally you wouldn’t put a main method in a class that is used to create objects unless it is for test purposes.

11. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 11 Wrapper Classes l Used to wrap primitive types in a class structure. l All primitive types have an equivalent class. l The class includes useful constants and static methods, including one to convert back to the primitive type.

12. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 12 Wrapper Class Example: Integer Declare an Integer class variable: Integer n = new Integer(42); Convert the value of an Integer variable to its primitive type int : int i = n.intValue();//intValue //returns an int Some useful Integer constants: »Integer.MAX_VALUE - the maximum integer value the computer can represent. (2 31 –1) »Integer.MIN_VALUE - the smallest integer value the computer can represent. (–2 31 )

13. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 13 Wrapper Class Example: Integer Some useful Integer methods: »Integer.parseInt("123") converts a string of numerals to an integer. We used this when obtaining data from a JOptionPane window. »Integer.toString(123) converts an Integer to a String. l The other wrapper classes have similar constants and methods. See page 341 for useful static methods in the class Character.

14. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 14 Usage of Wrapper Classes Wrapper Class l A variable contains the address of the value. Variable declaration example: Integer n; l Variable declaration and init: Integer n = new Integer(0); l Assignment: n = new Integer(99); Primitive Type l A variable contains the value. Variable declaration example: int n; Variable declaration and init: int n = 0; l Assignment: n = 99; There are some important differences in the code for wrapper classes and the code for primitive types.

15. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 15 Designing Methods: Top-Down Design l In pseudocode, write a list of subtasks that the method must do. l If you can easily write Java statements for a subtask, you are finished with that subtask. l If you cannot easily write Java statements for a subtask, treat it as a new problem and break it up into a list of subtasks. l Eventually, all of the subtasks will be small enough to design and code easily. l Solutions to subtasks might be implemented as private helper methods. l Top-down design is also known as divide-and-conquer or stepwise refinement.

16. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 16 Programming Tips for Writing Methods Apply the principle of encapsulation and detail hiding by using the public and private modifiers judiciously. »If the user will need the method, make it part of the interface by declaring it public. »If the method is used only within the class definition (a helper method), then declare it private. Create a main method with diagnostic (test) code within a class’s definition. »Run just the class to execute the diagnostic program. »When the class is used by another program, the class’s main method is ignored.

17. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 17 Testing a Method l Test programs are sometimes called driver programs. l Keep it simple. Test only one new method at a time. »A driver program should have only one untested method. l If method A uses method B, there are two approaches: l Bottom up »Test method B fully before testing A. l Top down »Test method A and use a stub for method B. »A stub is a method that stands in for the final version and does little actual work. It usually does something as trivial as printing a message or returning a fixed value. The idea is to have it so simple you are nearly certain it will work.

18. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 18 Overloading l The same method name has more than one definition within the same class. l Each definition must have a different signature. »Different argument types, a different number of arguments, or a different ordering of argument types. »The return type is not part of the signature and cannot be used to distinguish between two methods with the same name and parameter types.

19. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 19 Signature l The combination of method name and number and types of arguments, in order. equals(Species name) has a different signature from equals(String name). »Same method name, different argument types. myMethod(1) has a different signature from myMethod(1, 2). »Same method name, different number of arguments. myMethod(10, 1.2) has a different signature from myMethod(1.2, 10). »Same method name and number of arguments, but different order of argument types.

20. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 20 Overloading and Argument Type l Accidentally using the wrong datatype as an argument can invoke a different method. For example, see the Pet class in the text on page 357. »set(int newAge) sets the pet’s age. »set(double newWeight) sets the pet’s weight. »You want to set the pet’s weight to 6 pounds. –set(6.0) works because the argument is type double. –set(6) will set the age to 6, not the weight, because the argument is type int.

21. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 21 Gotcha: Overloading and Automatic Type Conversion If Java does not find a signature match, it attempts some automatic type conversions such as int to double. l An unwanted version of the method may execute. Consider the Pet example of overloading on page 357. What you want: name “Cha Cha”, weight 2, and age 3. l But you make two mistakes. 1. You reverse the age and weight numbers. 2. You fail to make the weight a type double. »set("Cha Cha", 2, 3) does not do what you want. –It sets the pet’s age = 2 and the weight = 3.0. »Why? –set has no definition with the argument types String, int, int. However, it does have a definition with String, int, double, and so it promotes the last number 3 to 3.0 and executes the method with that signature. l In other situations, automatic type conversions can make method invocations ambiguous.

22. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 22 Gotcha : You Cannot Overload Based on the Returned Type The compiler will not allow two methods with the same name, same types and number of parameters, but different return types in the same class: l In a situation like this, you would have to change the name of one method or change the number or types of parameters. public double getWeight() public char getWeight() Can’t have both in the same class

23. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 23 Constructors l constructor—a special method designed to initialize instance variables. Automatically called when an object is created using new. l Has the same name as the class. l Often overloaded (more than one constructor for the same class definition). »Different versions to initialize all, some, or none of the instance variables. »Each constructor has a different signature (a different number or sequence of argument types).

24. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 24 Defining Constructors Constructor headings do not include the word void. l In fact, constructor headings do not include a return type. l A constructor with no parameters is called a default constructor. l If no constructor is provided, Java automatically creates a default constructor. »If any constructor is provided, then no constructors are created automatically. Programming Tip l Include a constructor that initializes all instance variables. l Include a constructor that has no parameters. »Include your own default constructor.

25. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 25 Constructor Example from PetRecord (page 371) public class PetRecord { private String name; private int age; //in years private double weight; //in pounds... public PetRecord(String initialName) { name = initialName; age = 0; weight = 0; }... Initializes three instance variables: name from the parameter and age and weight with default initial values. Sample use: PetRecord pet1 = new Pet("Eric");

26. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 26 Using Constructors Always use a constructor after new. Example: The Pet class in text. Pet myCat = new Pet("Calvin", 5, 10.5); »Calls the Pet constructor with String, int, double parameters. l If you want to change values of instance variables after you have created an object, you must use other methods of the class. »You cannot call a constructor for an object after it is created. »set methods should be provided for this purpose.

27. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 27 Programming Tip : You Can Use Other Methods in a Constructor public PetRecord(String initialName, int initialAge, double initialWeight) { name = initialName; if ((initialAge < 0) || (initialWeight < 0)) { System.out.println("Error…"); } else { age = initialAge; weight = initialWeight; } shorter possibly more consistent with other constructors and methods avoids possible confusion about set parameters less method invocation overhead public PetRecord(String initialName, int initialAge, double initialWeight) { set(initialName, initialAge, initialWeight); }

28. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 28 Gotcha : Privacy Leaks l Using instance variables of a class type takes special care. l The problem stems from the fact that, unlike primitive types, object identifiers contain the object’s address, not its value. »Returning an object gives back the address, so the called method has direct access to the object. »The object is “unprotected” (usually undesirable). One solution: Use only primitive types ( int, char, double, boolean, etc. ) or String as instance variables. l Best solution: Cloning. (See Appendix 8.)

29. Chapter 5Java: an Introduction to Computer Science & Programming - Walter Savitch 29 Packages package - a collection of related classes that have been grouped together and given a name. Examples: Some packages in the Java library include: »java.util »java.awt »javax.swing The classes in a package are included in a program or class definition by putting an import statement at the beginning.