1. Methods
16-May-19

2. 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

3. 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

4. A fruity analogy
Programs can be like grapes—each part is complete in itself, and only loosely connected to the other parts
Grapes are better
Programs can be like pomegranates—all wrapped up together and impossible to get to one part without disturbing many other parts

5. Why methods? There are two main reasons for using methods:
Objects have behaviors, and we try to have each method perform one behavior
Methods are also a way of breaking a complex behavior down into simpler components (“Divide and conquer”)

6. Modeling behaviors: Example 1
class Animal {
int hunger = 10;
int fatigue = 10;
void eat() { hunger--; }
void sleep() { fatigue = 0; }
void hide() { ... } }
When you define methods, you are defining a vocabulary with which to talk about your problem

7. Modeling behaviors: Example 2
class Document { String text; boolean saved; void open() { ... } void save() { ... } void edit() { ... } }

8. Divide and conquer: Example 1
class Animal {
...
void eat() { Food f = findFood(); chew(f); swallow(f); digest(f); } }

9. Divide and conquer: Example 2
class Document {
...
void open(File inFile) { String fileName = askUserForName(); if (fileExists(fileName)) { loadFile(fileName); saved = true; } else complainToUser(); } }

10. 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
To do the same thing in more than one place
It’s always a bad idea to copy code

11. 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 not use instance variables or instance methods (why not?)

12. A common error
fudge is an instance (not static) variable—each object of type Test will have its own copy, but the class itself does not
class Test { int fudge = 0; public static void main(String args[]) { System.out.println(fudge); } }
non-static variable fudge cannot be referenced from a static context
This means the class does the work, not any particular object

13. 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 (=objects)
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
If a method never makes use of the instance variables or instance methods of the object, it should probably be static

14. 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
static int nextCard() { return random.nextInt(10) + 1; }

15. Defining a method A method has the syntax: Example:
return-type method-name ( formal-parameters ) { method-variables code }
Example:
int add ( int number1, int number2 ) { int sum = number1 + number2; return sum; }

16. Information flow We call a method like this: result = add( 3 , 5 ) ;
Actual parameter values are copied into formal parameters
We call a method like this:
result = add( 3 , 5 ) ;
int add ( int number1, int number2 ) { int sum = number1 + number2 ; return sum ; }
A result is returned
Formal parameters are used

17. 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

18. 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

19. Primitive parameters are copied in
The values of m and n (that is, 3 and 5) are copied to the formal parameters
int m = 3; int n = 5; result = add( m , n ) ;
int add ( int number1 , int number2 ) { while (number1 > 0) { number1--; number2++; } return number2 ; }
The value of number2 is returned, but the variable number2 is thrown away
Changing number1 and number2 does not affect m and n
• This is call by value

20. Objects are different Person p = new Person("John");
The assignment makes p refer to the new Person
Person p actually allocates space to hold a reference to a Person p:
new Person("John") allocates space for the actual Person, and initializes it
"John"

21. Object references are copied in
p holds a reference to a Person
"John"
Person p = new Person("John"); changeName( p );
void changeName(Person per ) { per.name = "Jack"; }
"Jack"
It is the reference that is copied in, not the Person
per.name = "Jack";
• 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

22. 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 = m; // 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

23. Object references are copied in
Person p = new Person("John"); changeName( p );
void changeName(Person per ) { per.name = "Jack"; per = new Person("Mary"); }
"John"
"Jack"
"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

24. 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

25. Using instance methods II
Suppose I want to tell john to read a book
I also need to say which book
This is additional information
Additional information is provided by actual parameters
I can do this: john.readBook("Moby Dick");
The form is: object . message (actual-parameters)
Suppose I want to tell john to kiss someone
I can do this: john.kiss(mary);
Is mary affected by this?
Yes! A reference to the mary object is passed to kiss

26. Using instance methods III
Suppose I want to tell myself to do something
(I’m pretending to be an object of type Person)
I refer to myself by using the keyword this
I can talk to myself like this:
this.readBook("Mirror Dance");
Or, as a convenience, I can leave out the explicit reference to myself:
readBook("Mirror Dance");
That is, any object can use its own methods and variables (the ones declared by its class) without explicitly naming itself

27. Using class methods I
If a method is a class method, any object of that class can refer to it directly, without using this
Example:
class DumbExample { static void println(String s) { // class method System.out.println(s); }
void doSomething( ) { // instance method, // in the same class println("I did it!"); } }

28. Using class methods II
To use a class method from outside the class, you can send a message to the class
Examples:
x = Math.abs(y);
DumbExample.println("I can do it too");
System.out.println("So that's what this means!");

29. Vocabulary
instance method – a method that can only be executed by objects (instances) of a class
class method – a method that can be executed by the class itself
actual parameter – a value given to a method
formal parameter – a variable used by a method to refer to the value it has been given
call by value – to copy the value of an actual parameter into a formal parameter
call by reference – to copy the reference to an object from the actual parameter into the formal parameter

30. The End