Starting Classes and Methods 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 But it can expose that data to other objects 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 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 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 Defining a class Here is the simplest syntax for defining a class class NameOfClass 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 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 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 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 Example constructor public class Person String name int age boolean male Person String aName boolean isMale name aName male isMale Example use of a constructor The constructor looks like Person String aName boolean isMale aName and isMale are called formal parameters You can construct a new Person like this Person john new Person John Smith true John Smith and true are called actual parameters You must have the same number of actual parameters as formal parameters in the same order and they must have the same types 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 A problem with names And have it mean public class Person String name boolean male Person String name boolean male name name male male 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 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 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 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 Defining a method A method has the syntax return type method name parameters method variables code Example boolean isAdult int magicAge 21 return age magicAge 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 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 Sending messages We send a message to an object by 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 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 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 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 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 Example of inheritance Example 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
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