Methods Complexity The programmer s biggest adversary is complexity Primitive types and the basic statements assignment while if etc are theoretically enough to perform any computation Everything else in Java is designed to organize actions and data in order to control complexity In particular we try to organize everything into relatively small independent parts with few interactions among them Why objects Object oriented programming models the world as a collection of objects This organization allows us to think about one object at a time allows us to use pre written objects It works best when the objects are relatively independent rather than deeply intertwined Why methods There are two main reasons for using methods 1 Objects have behaviors and we try to have each method perform one behavior 2 Methods are also a way of breaking a complex behavior down into simpler components Divide and conquer Modeling behaviors Example 1 class Animal int hunger 10 int fatigue 10 void eat hunger void sleep fatigue 0 void hide Modeling behaviors Example 2 class Document String text boolean saved void open void save void edit Divide and conquer Example 1 class Animal void eat Food f findFood chew f swallow f digest f Divide and conquer Example 2 class Document void open File inFile String fileName askUserForName if fileExists fileName loadFile fileName saved true else complainToUser When do you write a method Write a new method If there is a particular behavior of an object that you need to implement Typically methods like this are used from outside to communicate something to the object To break a complex problem into simpler parts The new methods should each perform a single clearly defined task Typically these sub methods are not made available outside the class Kinds of methods There are two kinds of methods instance methods the default kind Can only be executed by an individual object May use this May use class variables and class methods May use its own instance variables and other instance methods class methods denoted by the keyword static Executed by the class itself not by an object May not use this why not May use class variables and class methods May not use instance variables or instance methods why not A common error class Test int fudge 0 fudge is an instance not static variable each object of type Test will have its own copy but the class itself does not public static void main String args System out println fudge This means the class does the work not any particular object non static variable fudge cannot be referenced from a static context When to make a method static Instance methods are more capable than static methods they can access their own instance variables and methods as well as class variables and methods They are intended to be used by instances Class static methods can only access class variables and methods They are appropriate when what you are doing does not depend on any particular object Examples of static methods You re tired of writing System out println so you write this method static void println String s System out println s You want to perform a task that isn t specific to the individual object int nextCard return random nextInt 10 1 I didn t make this static but I should have Defining a method A method has the syntax return type method name formal parameters method variables code Example int add int number1 int number2 int sum number1 number2 return sum Information flow We call a method like this result add 3 5 Actual parameter values are copied into formal parameters int add int number1 int number2 int sum number1 number2 return sum A result is returned Formal parameters are used Formal parameters int add int number1 int number2 int sum number1 number2 return sum When you enter a method the formal parameters are created and assigned initial values You must specify the types of the formal parameters The formal parameters are variables that you can use however you like When the method returns the formal parameters are discarded Method variables int add int number1 int number2 int sum number1 number2 return sum Within a method you may create additional variables You can use these variables within the method When the method returns the variables are discarded Formal parameters get their values from outside method variables are created inside but they are otherwise alike Primitive parameters are copied in int m 3 int n 5 result add m n The values of m and n that is 3 and 5 are copied to the formal parameters int add int number1 int number2 while number1 0 number1 Changing number1 and number2 number2 does not affect m and return number2 n The value of number2 is returned but the This is call by value variable number2 is thrown away Objects are different Person p new Person John Person p actually allocates space to hold a reference to a Person new Person John allocates space for the actual Person and initializes it The assignment makes p refer to the new Person p John Object references are copied in p holds a reference Person p new Person John to a Person changeName p void changeName Person per per name per name Jack Jack John Jack It is the reference that is copied in not the Person This means that p and per refer to the same object p is not changed but changes made to the object referenced by per remain changed when the method returns This is call by reference Assignments When using a method values are assigned to the formal parameters Normal assignment statements work the same way int m 5 int n n 5 the value 5 is copied from m to n Person p1 new Person John Person p2 p2 p1 the reference is copied from p1 to p2 Hence p1 and p2 are different names for the same Person changes made to one change the other Object references are copied in Person p new Person John changeName p void changeName Person per per name Jack per new Person Mary Changes made to the object referenced by per remain changed when the method returns But changes to per itself are not copied back p still refers to the Jack object This is call by reference John Jack Mary Using instance methods I You don t call a method you send a message to an object Suppose I know about an object john that is an instance of class Person Suppose further that the Person class has an instance method goAway I can tell john to go away like this john goAway Notice that I say who I m talking to and what message I want to convey This is always the form the object a dot then the message Using instance methods II Suppose
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