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2015-03-17_classes-part-1.md

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Objectives

  • How to instantiate a class.
  • How to use a class's methods.

What you should know by now...

  • How to print out using System.out.println.
  • How to use primitive types, e.g. int, char, boolean, etc.
  • How to use Strings.
  • How to read user input with Scanner.
  • How to use while loops.
  • How to use an if statement.
  • How to use for loops
  • How to call methods
  • How to declare a method

Review

Objects and classes

Today's lesson
  • Object-oriented programming
  • Creating classes
  • Using static methods
Thursday's lesson
  • Creating our own classes
  • References vs. values

Object-oriented programming

Object-oriented programming is a style of computer programming in which the programmer represents the real world using "objects." An object is a very generic term in software development and will have different meanings depending on the context and language. For now, this definition is enough:

Definition: An object is a group of primitive types, methods, and other objects.

Real world objects have state and behavior. For example, a car has state (color, make, model) and behavior (turning, accelerating). Object-oriented programming is an attempt to model this combination of state and behavior in a modular, comprehensible way.

Object-oriented programming

Figure: A car can be thought of as an object with certain state and behavior.

We've already talked about methods. A method models the behavior of an object--it does something.

But why?

NYC

Objects help us to build big programs.

More technically:

  • Modularity: Objects can be maintained independent of other objects in the system.
  • Information-hiding: You can use an object without knowing how it is implemented!
  • Code reuse: Many objects may share attributes or behavior.

Classes

You may notice that many real world objects share attributes and behavior. For example, a motorcycle, sedan, truck, and semi-truck are all vehicles. They can be thought of as the same kind of object.

Object-oriented programming

In Java, objects of the same kind are said to share a "type", a word we already know! For example, we might say that motorcycle and semi-truck are both of type vehicle.

What if we'd like to make lots of a particular type of object? We can create a class:

Definition: A class is a blueprint for creating objects.

Thus, we can amend our previous definition:

Definition: An object is an instance of a class.

What's an instance? "Instance" and "object" mean the same thing; typically, "instance" is used to refer to an object in reference to its class. For example, "The sedan is an instance of a vehicle."

Why classes?

suburbs

Creating a class

You've already used classes and objects. They're impossible to avoid, because in Java, nearly everything is an object. Some things aren't and we'll talk about those later.

For example, when you created a new Scanner instance, you wrote:

Scanner input = new Scanner(System.in);

Definition: We create an instance of a class, also called "instantiating a class", using the new keyword.

Classes are types

In the above code, the new keyword tells Java to create a new instance of type Scanner. Remember how we always put a type in front of our variable names...

int myNumber = 9;
boolean isTrue = 99 < 100;

...? This is the same as when we say that input is of type Scanner. We'll talk later about why int and boolean do not require the new keyword, but for now, think of them as just simpler objects.

Classes have methods

We've also already used methods on a class.

String queens = new String("Queens!");
char c = queens.charAt(0); // 'Q'
Composition

Perhaps the most important notion of object-oriented programming is simply that once you have an object, you can compose it with other objects. Today, we're going to play around with some custom classes to get a feel for this.

In-class assignment

Use two custom classes, SecretWord and Drawing to implement the game Hangman. Hint: you need to write your own class in Hangman.java and then use then other two classes to build your program! Some things to consider:

  • Users can miss 5 times, after which they lose.
  • User input should be uppercase letters.

You can choose how to precisely to implement hangman yourself, but here is an example game:

Let's play Hangman!

Word  : _ _ _ 
  |
Misses: 0
Guess : E
Word  : _ _ _ 
  |
 \
Misses: 1
Guess : G
Word  : G _ _ 
  |
 \
Misses: 1
Guess : I
Word  : G _ _ 
  |
 \0
Misses: 2
Guess : L
Word  : G _ _ 
  |
 \0/
Misses: 3
Guess : M
Word  : G _ M 
  |
 \0/
Misses: 3
Guess : U
Word  : G _ M 
  |
 \0/
  |
Misses: 4
Guess : T
Word  : G _ M 
  |
 \0/
  |
 /
Misses: 5
Guess : Y

You won!
The word was: G Y M

You can look at the .java files directly to see what methods are available to you but in summary:

SecretWord

Type Method Description
boolean isLetter(char letter) Returns true if letter is in the secret word, false otherwise.
void set(char letter) Sets letter as correctly guessed but throws an error if letter is not in the secret word.
String toString() Returns a String of the secret word with underscores for un-guessed letters.
String reveal() Returns a full String of the secret word.
boolean isGuessed() Returns true if the secret word has been guessed, false otherwise.

Drawing

Type Method Description
String get(int misses) Returns the drawing as a String, dependent on the number of misses.
Bonus questions:

Have you already finished the assignment? Here are a few other things to try:

  • SecretWord expects uppercase letters, but the user can input anything they'd like. Check user input and gracefully report back to the user ("Please only use uppercase letters") rather passing the input on to SecretWord.
  • If the user guesses the same letter twice, we don't care. Can you instead report a specific message, for example, "You've already guessed that letter"? You can do this without any new tools, i.e. without arrays.
  • So long as you do not effect the misses-to-output relationship of Drawing, the object can return whatever you'd like. Change how the stick figure is drawn.