1. Classes and Methods
15-Aug-19

2. Objects have behaviors
In old style programming, you had:
data, which was completely passive
functions, which could manipulate any data
In O-O programming, an object contains both data and methods that manipulate that data
An object is active, not passive; it does things
An object is responsible for its own data
It can protect that data (by marking it private)
It can expose that data to other objects (by not marking it, or by marking it public)
Data should not be exposed without good reason

3. Objects have state An object contains data
The data represent the state of the object
Data can also describe the relationship of the object to other objects
Example: a checkingAccount might have
A balance (the internal state of the account)
An owner (some object representing a person)
An accountNumber (used as an ID number)

4. Example: a “Rabbit” object
You could create an object representing a rabbit
It would have data:
How hungry it is
How healthy it is
Where it is
And methods:
eat, run, dig, hide

5. Classes A class describes a set of objects
The objects are called instances of the class
A class describes:
Fields that hold the data for each object
Constructors that tell how to create a new object of this class
Methods that describe the actions the object can perform
In addition, a class can have data and methods of its own (not part of the objects)
For example, it can keep a count of the number of objects it has created
Such data and methods are called static

6. Defining a class Here is the simplest syntax for defining a class:
class ClassName { // the fields (variables) of the object // the constructors for the object // the methods of the object }
You can put public, protected, or private before the word class
Things in a class can be in any order (I recommend the above order)

7. Defining fields
An object’s data is stored in fields (also called instance variables)
The fields describe the state of the object
Fields are defined with ordinary variable declarations:
String name; Double health; int age = 0;

8. Defining constructors
A constructor is code to create an object
You can do other work in a constructor, but you shouldn’t
The syntax for a constructor is:
ClassName(parameters) { …code… }
The ClassName has to be the same as the class that the constructor occurs in
The parameters are a comma-separated list of variable declarations

9. Parameters
We usually need to give information to constructors and to methods
A parameter is a variable used to hold the incoming information
A parameter must have a name and a type
You supply values for the parameters when you use the constructor or method
The parameter name is only meaningful within the constructor or method in which it occurs

10. Example constructor
public class Person { String name; int age; boolean male; Person (String aName, boolean isMale) { name = aName; male = isMale; } }
Constructor
Parameters

11. Example use of a constructor
The constructor looks like:
Person (String aName, boolean isMale) {…}
aName and isMale are called formal parameters
The formal parameters are used to receive values
You can construct a new Person like this:
Person john = new Person("John Smith", true);
"John Smith" and true are called actual parameters
The actual parameters are used to give values to the formal parameters
You must have the same number of actual parameters as formal parameters, in the same order, and they must have the same types

12. A problem with names It would be nice if we could say:
public class Person { String name; boolean male; Person (String name, boolean male) { name = name ; male = male ; } }

13. A problem with names And have it mean:
public class Person { String name; boolean male; Person (String name, boolean male) { name = name ; male = male ; } }

14. A problem with names But this is what it would really mean:
public class Person { String name; boolean male; Person (String name, boolean male) { name = name ; male = male ; } }

15. this to the rescue
A parameter may have the same name as an instance variable
The name always refers to the parameter
The keyword this refers to the current object
Putting this in front of a name means that the name is a field of this object (it isn't a parameter)

16. A problem with names—solved!
Here is how we do what we want:
public class Person { String name; boolean male; Person (String name, boolean male) { this.name = name ; this.male = male ; } }

17. A typical use of this If you write a constructor with parameters…
…and the parameters are used to set fields that have the same meaning…
…then use the same names!
Person (String name, boolean male) { this.name = name ; this.male = male ; }
In fact, this is the recommended way to do it

18. Defining a method A method has the syntax: Example:
return-type method-name ( parameters ) { method-variables code }
Example:
boolean isAdult( ) { int magicAge = 21; return age >= magicAge; }

19. Returning a result from a method
If a method is to return a result, it must specify the type of the result
boolean isAdult ( …
You must use a return statement to exit the method with a result of the correct type:
return age >= magicAge;
This is for methods only (constructors automatically return a result of the correct type)

20. Returning no result from a method
The keyword void is used to indicate that a method doesn’t return a value
void printAge( ) { System.out.println(name + " is " + age " years old."); return; }
The keyword return is not required in a void method
The method will return automatically when it reaches the end of the method (the final closing brace)

21. Sending messages We send a message to an object by: Example:
Naming the object
Naming the method we want to use
Providing any needed actual parameters
Example:
if (john.isAdult ()) { john.printAge(); }
john.isAdult() returns a value (subsequently used by the if statement)
john.printAge() does not return a value

22. Putting it all together
class Person {
// fields String name; int age;
// constructor Person(String name) { this.name = name; age = 0; }
// methods String getName() { return name; }
void birthday() { age = age + 1; System.out.println( "Happy birthday!"); } }

23. Using our new class Person john; john = new Person("John Smith");
System.out.print (john.getName());
System.out.println(" is having a birthday!");
john.birthday();
Of course, this code must also be inside a class!

24. null
If you declare a variable to have a given object type, for example,
Person john;
String name;
...and if you have not yet assigned a value to it, for example, with
john = new Person(); String name = “John Smith";
...then the value of the variable is null
null is a legal value, but there isn’t much you can do with it
It’s an error to refer to its fields, because it has none
It’s an error to send a message to it, because it has no methods
The error you will see is NullPointerException

25. Classes form a hierarchy
Classes are arranged is a treelike hierarchy
There is one class at the top, or root, of the hierarchy, named Object
In computer science we draw trees upside-down, with the root at the top
Every class except Object has one “parent” class, or superclass
Each class is a subclass of its superclass

26. What is the class hierarchy for?
Classes inherit from their superclasses
A class has not only its own fields and methods, but also:
Every field described in any class above it
Every method described in any class above it
Classes do not inherit constructors, however
Hence, a class may contain much more information than is obvious from the class description

27. Example of inheritance
class Employee extends Person { double hourlyWage; void pay(double hoursWorked) { System.out.println("Pay to the order of: " name + " $" hoursWorked * hourlyWage); } }
An Employee has a name, an age, an hourlyWage, and birthday and pay methods.

28. Class variables and methods
A class describes the variables and methods belonging to objects of that class
These are called instance variables and instance methods
A class may also have its own variables and methods
These are called class variables and class methods
Class variables and class methods are denoted by the keyword static
C programmers: same word as in C, different meaning

29. Why have class variables and methods?
Sometimes you want to keep information about the class itself
Example: Class Person might have a class variable population that counts people
This would not be appropriate data to keep in each Person!
Sometimes you want to do something relevant to the class as a whole
For example, find the average age of a population
Sometimes you don’t have any objects
For example, you want to start up a program

30. Example use of a class variable
class Person { // fields String name; int age; static int population;
// constructor Person(String name) { this.name = name; age = 0; population = population + 1; }
// methods String getName() { return name; }
void birthday() { age = age + 1; System.out.println( "Happy birthday!"); } }

31. Vocabulary I class – a description of a set of objects
object – a member of a class
instance – same as “object”
field – data belong to an object or a class
variable – a name used to refer to a data object
instance variable – a variable belonging to an object
class variable, static variable – a variable belonging to the class as a whole
method variable – a temporary variable used in a method

32. Vocabulary II
method – a block of code that can be used by other parts of the program
instance method – a method belonging to an object
class method, static method – a method belonging to the class as a whole
constructor – a block of code used to create an object

33. Vocabulary III
parameter – a piece of information given to a method or to a constructor
actual parameter – the value that is passed to the method or constructor
formal parameter – the name used by the method or constructor to refer to that value
return value – the value (if any) returned by a method

34. Vocabulary IV hierarchy – a treelike arrangement of classes
root – the topmost thing in a tree
Object – the root of the class hierarchy
subclass – a class that is beneath another in the class hierarchy
superclass – a class that is above another in the class hierarchy
inherit – to have the same data and methods as a superclass

35. The End
“Don't ask for the information you need to do the work; ask the object that has the information to do the work for you.”
--Allen Holub