OOP Preliminaries and Terminology

1
OOP Preliminaries and Terminology

• First some programming / Java concepts and
  terminology
• Everything is either data or function
• data are objects, things, nouns
• functions are algorithms, procedures, methods,
  verbs
• In Java we talk about objects (data) and methods
  (function)
• for example, in the single line of code we have
  written so far
• System.out.println(Hello world)

2
Classes and Objects

• A Java class represents an abstract collection of
  individuals (?)
• String the set of all possible strings
• PlayingCard the set of all possible playing
  cards
• Employee the set of all possible employees
• EmailMessage the set of all possible email
  messages
• Every object (every data object) is an instance
  of some class (?)
• "Hello world" is an instance of class String
• 1.25 is an instance of class Number (?)
• '' is an instance of class Character (?)
• System.out is a member of class PrintStream
• for many classes, there is no pre-defined
  "literal" way to refer to an object I can't say
  "Ace of Spades" is a member of class PlayingCard
• Any time you see an object in a program, you need
  to be aware of what its data type is!

3
Java Methods

• A method (subroutine, procedure, function) is a
  block of Java code that does some calculation or
  makes some change
• concatenate two strings
• extract the 4th character from a string
• print a line of text
• give an employee a raise
• extract the sender of an email message
• change
• A class defines all the methods for its member
  objects, and all methods are defined for some
  class, so there is no such thing as "disembodied
  calculation"
• You can't just print a line of text to the
  console. You say to System.out, (an instance of
  class PrintStream) "print this string"
• You can't just extract the 4th character of a
  string. You say to "Hello world" (and instance
  of class String) "give me your fourth character"
• You can't just give an employee a raise. You say
  to an instance of Employee "increase your
  salary by 500"
• You can't just concatenate two strings "Hello "
  and "world" to get the string "Hello world". You
  say to the String "Hello " concatenate the
  string "world" to yourself
• If you want to get some work done, you have to
  find some object to do it for you!

4
Java Methods and Mathematical Functions

• What programmers call methods or procedures are
  somewhat like mathematical functions
• This definition specifies
• the name of the function
• how many arguments
• what is returned
• implicitly, the types of the arguments and the
  type of the return value
• In programming terms, this is called the method's
  signature
• the method's name
• the number and data types of its arguments
  (inputs)
• the data type of its output
• But how would this function (minimum) work in the
  object-oriented way of thinking?
• What is the defining class and signature of Java
  methods that
• concatenate two strings
• extract the 4th character from a string
• extract the nth character from a string

5
To Repeat the Most Important Point of the Week

• Everything is either data or procedure
• In Java we call them objects and methods
• Every object has a data type
• in Java, this is often a Class
• the objects data type determines what methods
  apply to it
• for example, the String type supports
  concatenation and substring
• the numeric types support operations like
  addition and multiplication
• the PrintStream type supports the println
  operation
• your user-defined type Employee might support the
  salary() and raiseSalary(Number) methods
• Every method has a signature
• the name of the method
• the names and data types of the arguments
• the data type of the return result
• In the OOP way of computing, all computation is
  accomplished by doing a method call on an object

6
Classes, Objects, and Methods
Class String
Method concat(aString) substring(from,
to) charAt(position)
class constructs an instance
instance receives method calls "Hello
world".charAt(0)
Instance "Hello world"
7
Creating Objects

• Always keep in mind the distinction between the
  class, and instances of the class
• String is a class, "Hello world" is an instance
• Number is a class, 3.14159 is an instance (?)
• PrintStream is a class, System.out is an instance
• The only thing a class can do is create an
  instance of itself (?)
• be aware of the compile time versus run time
  distinction!
• There is only one way to create an instance of a
  class, via one of the class's constructors
• different classes have different constructors
  many classes have more than one constructor

8
Various Forms of the Constructor

• Classic constructors that "make sense"
• new String() // constructs an instance of
  String with 0 characters
• new PlayingCard("ace", "diamonds")
• new Employee(47, "John Smith", "Software
  Engineer", 85000)
• Constructors that you're not likely to see!
• new String("Hello world")
• new Character('A')
• new Float(3.14159)
• Why won't you see any of the constructors above
  in your Java code?

9
Literals

• Certain fundamental Java classes have literals
  associated with them they do the same thing as
  the classic constructor, but are easier (ONLY)
  for the sake of your convenience and code
  readability
• "Hello world" is exactly the same as new
  String("Hello world")
• 3.14159 is exactly the same as new
  Float(3.14.19) (?)
• '' is exactly the same as new Character('')
  (?)
• true is exactly the same as new Boolean(true)
  (?)
• Of course you're going to use the literal form of
  the constructor when you can just be aware that
  it really is a shortcut, and the fundamental
  process here is that of a class creating an
  instance of itself.

10
Summary

• All computation is about data and function
• data holds state (a number, a string, an
  Employee)
• functions perform calculations or produce or
  change values
• add two numbers, extract the 3rd character in a
  string, change an employee's salary
• in Java, data classes, function methods
• In Java and other object-oriented languages
  classes
• are collections of individuals (Employee,
  PlayingCard, String)
• a class versus instances of that class
• define methods that all instances can respond do
• the PlayingCard class defines the getSuit method
• the Employee class defines the setSalary method
• the String class defines the concatenate method
• Although a class defines the methods, members of
  the class actually respond to the method calls
• aPlayingCard.getSuit()
• anEmployee.setSalary(100000)
• "Hello".concat(" world")

11
Summary (cont.)

• The connection between a class and instances of
  the class the constructor
• literals (constructors for a few Java-defined
  types)
• String "hello"
• numbers 12 3.14
• boolean true false
• char 'a'
• explicit constructor
• new Employee(123, "Joe", 100000)
• new PlayingCard("Ace", "Clubs")
• new Scanner(System.in)
• new Calendar()
• classes can have multiple constructors

12
Summary (cont.)

• A class defines
• its constructors (all the ways you can create an
  instance)
• methods that all instances respond to
• A method is described by its signature
• its name
• its arguments
• its return result
• for example the String class defines
  charAt(position)
• the method is defined in class String, which
  means that all instances of String (and only
  instances of String) can respond to this method
• the method's name is charAt
• it has a single argument, and the argument's data
  type is int
• its return result is char
• To put it all together, explain what happens here
• System.out.println("Hello world".charAt(5))

13
Summary (cont.)

• Since a method is defined by its signature
  (arguments and their types, return type, and
  name) there can be multiple methods that happen
  to have the same name.
• You have to be careful because although your
  intuition says that two methods must somehow be
  the same if they have the same name, Java says
  that they must be different methods if their
  signatures differ in any way.
• and Java always wins these arguments
• One example we will see frequently
• System.out.println("Hello")
• System.out.println(3.25)
• System.out.println()