Classes and Methods Jan 14 2019 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 2 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 3 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 4 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 5 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 6 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 7 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 8 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 9 Example constructor public class Person String name int age boolean male Constructor Person String aName boolean isMale name aName male isMale Parameters 10 Example use of a constructor The constructor looks like You can construct a new Person like this Person String aName boolean isMale aName and isMale are called formal parameters The formal parameters are used to receive values 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 11 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 12 A problem with names And have it mean public class Person String name boolean male Person String name boolean male name name male male 13 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 14 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 15 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 16 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 17 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 18 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 19 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 20 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 21 Putting it all together class Person methods String getName return name fields String name int age constructor Person String name this name name age 0 void birthday age age 1 System out println Happy birthday 22 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 23 null If you declare a variable to have a given object type for example and if you have not yet assigned a value to it for example with Person john String name 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 24 Classes form a hierarchy Classes
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