Time estimate: 90 min.

5.1. Anatomy of a Java Class¶

In Unit 2, we learned to use classes and objects that are built-in to Java or written by other programmers. In this unit, you will learn to write your own classes and make your own objects!

Remember that a class in Java defines a blueprint for creating objects. When you create objects, you create new instances of that class and what you can do with those instances is determined by what methods are defined in the class.

For example in Unit 2, we created yertle and myrtle, 2 Turtle variables and assigned them references to objects created from the class Turtle and we used instances of Java’s String class to assign values to different String variables.

Watch this short video to review the vocabulary of object-oriented programming:

5.1.1. Creating a Class¶

Most classes you write will have the keyword public before them though it is not required. The class definition itself always starts with the word class followed by the name of the class. Then the rest of the class, called the body, is defined inside a pair of {}s.

Since we’re talking about anatomy, let’s create a class called Person. Classes are almost always named with capitalized names though this is a matter of style, not a rule of the language. Here is the basic skeleton of a Person class:

public class Person
{
    // define class here - also called the “body” of the class

}

You can create instances of the Person class with new as in new Person() And you can declare variables that can hold a reference to a Person object with Person variableName.

Or put it altogether to declare some variables and initialize each one with a reference to a new Person as shown here.

Person ada = new Person();
Person charles = new Person();

So what makes up the body of the class—the stuff between the {}s?

Remember that objects have both attributes and behaviors. These correspond to instance variables and methods in the class definition.

The first things we define in a class are usually the instance variables. They are called that because each instance of the class (each object) has its own set of variables that aren’t shared with other instances. This is what allowed yertle and myrtle from Unit 2 to be at different positions at the same time; they each had their own x position and y position instance variables.

The next thing we define in a class is usually its constructors. We’ll talk about writing constructors in more detail in the next section but a constructor’s job is to initialize the instance variables when the object is created. Usually that will mean they need to take arguments. (The call to new Person() before is to a constructor that doesn’t take any arguments. In a moment we’ll see that our Person constructor will actually need arguments.)

The real meat of a class is in the methods which define the behaviors of the objects of that class. Recall from Unit 2 that most methods either do things (like the Turtle methods that moved the turtle on the screen) or return values like the getXPos and getYPos on Turtle.

The methods of the class share access to the object’s instance variables and when a method is called on an object it uses the instance variables for that object. For example in the Turtle class the forward method changes an instance variable xPos. When you call forward on yertle it changes xPos on the yertle object and when you call it on myrtle it changes the xPos on the myrtle object.

Putting it all together, the three main anatomical features of a class are the instance variables which hold values associated with each object, the constructors whose job is to initialize the instance variables, and the methods who contain the code that gives the objects their behavior and which can use the instance variables defined in the class.

public class Person
{
    // instance variables

    // constructors

    // methods

}

And finally one last bit of weird anatomy, kind of like the appendix: any Java class can have a main method which can be used to run that class as a program either to test that one class or sometimes as the entry point to a whole program made up of many classes and objects.

5.1.2. Designing a Class¶

Now that we know what the skeleton of a class looks like and the elements that make up the body of the class, we’re ready to create our own class. Let’s start with a quick look at how to design a class such as Person.

One important question we have to ask when designing a class is, what data does it represent? In this case we can ask, what would we want to know about a person? Our answer will depend on what problem we are trying to solve. In one program, perhaps an address book, we might want to know the person’s name and phone number and email. In another program, such as a medical application, we might need to know their vital signs such as their blood pressure, temperature, and pulse rate.

For now let’s go with the address book example. Here’s a fleshed out Person class with instance variables, a constructor, and methods. We’ll go through the details in the next few sections but for now you can run the code to see how it constructs 2 Person objects and fills in their data.

Remember that execution always starts in the main method. When a method like print is called, the code defined in the method runs but when it gets the values of name, email, and phoneNumber it gets the specific values of those variables that were set by the constructor when the particular object we called print on was created. Click on the Show CodeLens button below and the Next button to run the code step by step.

Run the following class. Try changing the Person p2 object in main to your name. Click on the Show CodeLens button and then Next to step through the code.

public class Person
{
    // instance variables
    private String name;
    private String email;
    private String phoneNumber;

// constructor: construct a Person copying in the data into the instance
    // variables
    public Person(String initName, String initEmail, String initPhone)
    {
        name = initName;
        email = initEmail;
        phoneNumber = initPhone;
    }

// Print all the data for a person
    public void print()
    {
        System.out.println("Name: " + name);
        System.out.println("Email: " + email);
        System.out.println("Phone Number: " + phoneNumber);
    }

// main method for testing
    public static void main(String[] args)
    {
        // call the constructor to create a new person
        Person p1 = new Person("Sana", "sana@gmail.com", "123-456-7890");
        // call p1's print method
        p1.print();
        Person p2 = new Person("Jean", "jean@gmail.com", "404 899-9955");
        p2.print();
    }
}

====
// Test for Lesson 5.1.0 - Person class - should pass if/when they run code
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect =
                "Name: Sana\n"
                    + "Email: sana@gmail.com\n"
                    + "Phone Number: 123-456-7890\n"
                    + "Name: Jean\n"
                    + "Email: jean@gmail.com\n"
                    + "Phone Number: 404 899-9955";

boolean passed = getResults(expect, output, "Expected output from main", true);
        assertTrue(passed);
    }
}

5.1.3. Instance Variables¶

As we’ve said, instance Variables hold the data for an object. They record what an object needs to know to play its role in the program. Instance variables are also sometimes called attributes, fields, or properties.

In general and definitely on the AP CSA exam, instance variables should be declared private. Think of private as like your diary. Only you should have direct access to it. Similarly, in Java a private instance variable can only be accessed by code in the class that declares the variable.

Note

Instance variables are declared right after the class declaration. They usually start with private then the type of the variable and then a name for the variable. Private means only the code in this class has access to it.

The Person class declares 3 private instance variables: name, email, and phoneNumber. These are things that you might want to know about a person. They are declared at the top of the class and they exist inside the { } of the class.

Once we have created a class like Person, we can create many instances (objects) of the class. The class is like a blueprint or cookie cutter that defines the variables and methods for that class. Each object will have their own copies of the same instance variables but with possibly different values in them (as seen in the cookie decorations below).

Person data encapsulation — Figure 2: Person Class and Objects¶

Good Java style stresses data encapsulation where the data (instance variables) and the code acting on the data (methods) are wrapped together into a single unit and the implementation details are hidden. Because only code in the class can access or change the values of private instance variables it is a lot easier to keep track of how your program works than if you had to worry that any code anywhere in a much larger program could possibly change the values of the variables.

Code in other classes can only interact with the public methods you provide and cannot directly access the private instance variables (shown in the pink box above).

When designing a class you get to decide what data to make accessible or modifiable from other classes by what public methods you provide. (Remember from Unit 2 how the Turtle class provided getXPos and getYPos to get the turtle’s position but no setXPos and setYPos since the only way to move a turtle is through forward and moveTo methods?)

Check Your Understanding

5-1-3: Click on all the instance variable declarations in the following classRemember, instance variables are private and are declared after the class declaration.

public class Name
{
    private String first;
    private String last;

    public Name(String theFirst, String theLast)
    {
        first = theFirst;
        last = theLast;
     }

     public void setFirst(String theFirst)
     {
        first = theFirst;
     }

     public void setLast(String theLast)
     {
        last = theLast;
     }

}

5.1.4. Methods¶

Now to methods which define what we can actually do with an object. The most important methods in a class are the public methods since they can be accessed from outside the class. You may also write private methods that are not accessible outside of the class and therefore can only be used by other methods inside the same class. As you’ve probably figured out, the public and private keywords determine the external access and visibility of classes, instance variables, constructors, and methods.

Note

Methods define what the object can do. They typically start with public then a type, then the name of the method followed by parentheses for optional parameters. Methods defined for an object can access and use its instance variables!

The Person class above has a print method that prints out all the data stored for a person object. Notice that it is marked as public and after public comes the return type. The void return type, as you may recall from Unit 2, is used to indicate that the method does not return anything but has some effect such as printing to the screen. After the return type comes the method name followed by parentheses containing the list of parameters. In this case there are no parameters but we still need the ()s. The body of the method is in {}s. As we’ve discussed, the method can access and use the instance variables defined in the class: name, email, and phoneNumber but will get the values specific to the object we called print on.

public void print()
{
  System.out.println("Name: " + name);
  System.out.println("Email: " + email);
  System.out.println("Phone Number: " + phoneNumber);
}

To call a method, we need an object that is an instance of the class such as we get by calling its constructor. Then we use the dot (.) operator to call its public methods, for example p1.print() means call the print method on the object p1.

// call the constructor to create a new person
Person p1 = new Person("Sana", "sana@gmail.com", "123-456-7890");
// call p1's print method
p1.print();

Check Your Understanding

5-1-4: Click on all the lines of code that are part of a method in the following class.Methods follow the constructor. They include a return type in case they returns something from the method.

public class Name
{
    private String first;
    private String last;

    public Name(String theFirst, String theLast)
    {
        first = theFirst;
        last = theLast;
    }

    public void setFirst(String theFirst)
    {
        first = theFirst;
    }

    public void setLast(String theLast)
    {
        last = theLast;
    }

}

5.1.5. Object-Oriented Design¶

So far we’ve just talked about designing one class. In object-oriented design (OOD), programmers often start by deciding which classes are needed to solve a problem and then figure out the data and methods in each class.

When you are given a problem specification, you can identify classes you’ll need by looking for the nouns in the specification. For instance, the specification for the turtle graphics system from Unit 2 probably contained a sentence that said something like, “there are turtles that can exist on a 2-dimensional world and can draw lines by moving around the world”. The main nouns in that description are “turtle” and “world” and indeed the classes in the system are Turtle and World. (The one noun that was not turned into a class was “line”. Do you think it would have made sense to create a Line class? Why or why not?)

Once you’ve determined the classes you need, then you can go through the process we described above to design the individual classes. Note that you can often identify methods that should exist on classes by looking for verbs in the specification like “move”.

Sometimes it’s useful, when designing a complex system with lots of classes, to make diagrams of the classes that show you at a glance what instance variables and methods they have. Often these can just be sketches in your notebook or on a whiteboard but there are also more formal systems such as the Unified Modeling Language (UML) for drawing these diagrams.

For example, here is a UML class diagram for the Turtle class. The - in front of the attributes indicate that they are private, and the + in front of the methods indicate that they are public. Here is a tutorial on class diagrams that explains it in more detail if you are curious (Class diagrams are not on the AP CSA exam). If you want to draw your own, app.diagrams.net or Creately.com are good free online drawing tools for UML class diagrams.

Turtle class diagram — Figure 3: Turtle Class Diagram¶

Check Your Understanding

5-1-5: You’ve been hired by your school to create a program that keeps track of “students at your school and the courses they are taking”. Name 2 classes that you would create in your program. Name 2 attributes (data kept in instance variables) for each class.

The two nouns in the problem description above, Student and Course would make good class names! Then, you can think about what data you need to keep track of for students and courses and what methods you need. Note that the instance variables in the Person class could also work for a Student class!

Check Your Understanding

5-1-6: Say you wanted to make a computer game from a board game that you are playing. Think about what objects are in the game. For example, here is the description for Monopoly (trademark Hasbro games): “Buy, sell, dream and scheme your way to riches. Players buy, sell and trade to win. Build houses and hotels on your properties and bankrupt your opponents to win it all. Chance and Community Chest cards can change everything.” What classes would you need to create a computer version of this game? (Remember to look for the nouns). Take one of the classes you listed, and try to come up with 2 pieces of data in that class that will be the instance variables.

5.1.6. Programming Challenge : Riddle Class¶

In this project, you will create a class that can tell riddles like the following:

Riddle Question: Why did the chicken cross the playground?
Riddle Answer: To get to the other slide!

First, brainstorm in pairs to create an object-oriented design for a riddle asking program. What should we call this class? What data does it need to keep track of in instance variables? What is the data type for the instance variables? What methods do we need? (You could draw a Class Diagram for this class using app.diagrams.net or Creately.com, although it is not required).
Using the Person class above as a guide, write a Riddle class in the Active Code template below that has 2 instance variables for the riddle’s question and answer, a constructor that initializes the riddle, and 2 methods to ask the riddle and answer the riddle. Don’t name your instance variables initQuestion and initAnswer since they are used as constructor parameter variables. If you came up with other instance variables and methods for this class, you can add those too! Don’t forget to specify the private or public access modifiers. Use the outline in the Active Code below. You will learn how to write constructors and other methods in detail in the next lessons.
Complete the main method to construct at least 2 Riddle objects that call the Riddle constructor with the arguments for the riddle question and answer and call their printQuestion and printAnswer methods to ask and answer the riddle. You can look up some good riddles online.

Complete the Riddle class below and complete the main method to construct 2 Riddle objects and call their printQuestion() and printAnswer() methods.

public class Riddle
{
    // write 2 instance variables for Riddle's question and answer: private type
    // variableName;

// constructor
    public Riddle(String initQuestion, String initAnswer)
    {
        // set the instance variables to the init parameter variables

}

// Print riddle question
    public void printQuestion()
    {
        // print out the riddle question with System.out.println

}

// Print riddle answer
    public void printAnswer()
    {
        // print out the riddle answer with System.out.println

}

// main method for testing
    public static void main(String[] args)
    {
        // call the Riddle constructor to create 2 new Riddle objects
        // with the arguments for the riddle question and answer.

// call the riddle objects' printQuestion() and printAnswer methods

}
}

====
// Test Code for Lesson 5.1.5 - Riddle
// @author Kate McDonnell
// Test Code for Lesson 5.1.5 - Riddle
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    public RunestoneTests()
    {
        super("Riddle"); // class name / location of main

Object[] values = new Object[] {"Question", "Answer"};
        setDefaultValues(values);
    }

@Test
    public void testPrintQuestion()
    {
        String output = getMethodOutput("printQuestion");
        String expect = "Question";

boolean passed = getResults(expect, output, "Checking method printQuestion()");
        assertTrue(passed);
    }

@Test
    public void testPrintAnswer()
    {
        String output = getMethodOutput("printAnswer");
        String expect = "Answer";

boolean passed = getResults(expect, output, "Checking method printAnswer()");
        assertTrue(passed);
    }

@Test
    public void testCallConstructors()
    {
        String code = getCodeWithoutComments();
        String search = "= new Riddle(";

int num = countOccurences(code, search);

String expect = search + "...) x 2";
        String actual = search + "...) x " + num;

boolean passed = getResults(expect, actual, "Checking that you made 2 Riddle objects");
        assertTrue(passed);
    }

@Test
    public void testVariableTypes()
    {
        String varTypes = "String String";
        String output = testInstanceVariableTypes(varTypes.split(" "));

boolean passed = getResults(varTypes, output, "Checking Instance Variable Type(s)");
        assertTrue(passed);
    }

@Test
    public void testPrivateVariables()
    {
        String expect = "2 Private";
        String output = testPrivateInstanceVariables();

boolean passed = getResults(expect, output, "Checking Private Instance Variable(s)");
        assertTrue(passed);
    }

@Test
    public void testMain()
    {
        String output = getMethodOutput("main");

String expect = "2+ line(s) of text";
        String actual = " line(s) of text";

int len = output.split("\n").length;

if (output.length() > 0)
        {
            actual = len + actual;
        }
        else
        {
            actual = output.length() + actual;
        }
        boolean passed = len >= 2;

getResults(expect, actual, "Checking main method", passed);
        assertTrue(passed);
    }
}

5.1.7. Design a Class for your Community¶

In this unit, you will design a class of your own choice that is relevant to your community. You will improve the class in each lesson of this unit. We suggest that you work in pairs on this project. If you would rather work on this project as 1 stand-alone lab project rather than a piece at a time at the end of each lesson, you can do this in Lesson 5.19 at the end of Unit 5.

Make your own copy of this worksheet from the File menu. For question 1, brainstorm and ask people in your community what would be important or useful for them to track in a community organization or activity or to help your community. For example, you could create a Java class to keep track of community events, club activities, athletic games or statistics, community leaders, performers or performances, health tracking, or another subject of your choice.
Come up with the class name for your community data-tracking class (it should be a category of people or things, not a specific person or thing). Come up with at least 3 instance variables that are attributes of things in that class. Think about what data type each variable should be.You can use the worksheet to design your class. Optionally, you may want to draw a UML class diagram for your class on paper or using app.diagrams.net or Creately.com (see tutorial on class diagrams).
Type in your class name and declare the instance variables for your class in the active code exercise below. Click on run. Your class will not print out anything yet, but it should run without errors. You will add constructors and other methods to this class in the next lessons.

Come up with the class name for your community data-tracking class (it should be a category of people or things, not a specific person or thing) and at least 3 instance variables that are attributes of things in that class. Your class will not print out anything yet, but it should run without errors.

5.1.8. Summary¶

Programmers use code to represent a physical object or nonphysical concept, real or imagined, by defining a class based on the attributes and/or behaviors of the object or concept.
Instance Variables define the attributes or data needed for objects, and methods define the behaviors or functions of the object.
Data encapsulation is a technique in which the implementation details of a class are kept hidden from the user. The data is kept private with access only through the public methods that can act on the data in the class.
The keywords public and private affect the access of classes, data, constructors, and methods.
The keyword private restricts access to the declaring class, while the keyword public allows access from classes outside the declaring class.
Instance variables are encapsulated by using the private access modifier.
Methods can be public or private. The set of public methods define what other classes can do with an instance of a class.

5.1.9. AP Practice¶

5-1-9: Consider the Cat class which will contain a String and an int attribute for a cat’s name and age and a constructor.

public class Cat
{
    /* missing code */
}

Which of the following replacements for /* missing code */ is the most appropriate implementation of the class?

public String name;
public int age;
private Cat(String name, int age)
{ /* implementation not shown */ }

Instance variables should be private.

public String name;
private int age;
private Cat(String name, int age)
{ /* implementation not shown */ }

Instance variables should be private.

private String name;
private int age;
public Cat(String name, int age)
{ /* implementation not shown */ }

Correct! The instance variables are private and the constructor is public.

public String name;
public int age;
public Cat(String name, int age)
{ /* implementation not shown */ }

Instance variables should be private.

private String name;
private int age;
private Cat(String name, int age)
{ /* implementation not shown */ }

Constructor should be public.

5-1-10: Consider the Party class below which will contain three int attributes for numOfPeople, volumeOfMusic, and numOfBoxesOfPizza, a constructor, and a startParty method. The startParty method is intended to be accessed outside the class.

public class Party
{
    /* missing code */
}

Which of the following replacements for /* missing code */ is the most appropriate implementation of the class?

private int numOfPeople;
private int volumeOfMusic;
private int numOfBoxesOfPizza;
public Party()
{ /* implementation not shown */ }
private void startParty()
{ /* implementation not shown */ }

Method startParty() should be public.

private int numOfPeople;
private int volumeOfMusic;
private int numOfBoxesOfPizza;
public Party()
{ /* implementation not shown */ }
public void startParty()
{ /* implementation not shown */ }

Correct, instance variables should be private and the methods should be public.

public int numOfPeople;
public int volumeOfMusic;
public int numOfBoxesOfPizza;
public Party()
{ /* implementation not shown */ }
public void startParty()
{ /* implementation not shown */ }

Instance variables should be private.

private int numOfPeople;
private int volumeOfMusic;
private int numOfBoxesOfPizza;
private Party()
{ /* implementation not shown */ }
private void startParty()
{ /* implementation not shown */ }

Methods should be public.

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