CS101: Compound Data

So far, we've seen primitive data like integers and booleans, but sometimes we want to treat a collection of data as one entity and carry it around as a single unit. This idea is called compound data.

Examples:

complex number (contains real and imaginary part)
couple (names of husband and wife)
family (mom, dad, kids, pets)
employee (name, SSN, salary, department, ...)

In other words, we want to define our own data types that are built from the other types we already have available. In Java, we define a type using something called a class. A class defines a kind of object. Once we define a class, we can instantiate objects of the class -- these are the data items of that type.

Each class can have

one or more constructors -- procedures for constructing (initializing) objects of that class;
instance variables -- variables used to store information inside the objects of that class;
instance methods -- procedures that can be invoked to perform operations on individual objects of the class. These methods can be
- accessors, which get information stored in the object, or
- mutators, which change the values stored in the object.

You can think of an object as a container. Its contents are the values of the instance variables. (You can think of each instance variable as a separate labeled compartment inside the container.) Let's clarify these concepts with some examples.

Example: `Couple`

Suppose we want to store the husband's name, wife's name, and year married. We would want three instance variables to store this information. We also want a constructor to create objects in this class, and some other methods to access information.

public class Couple {
  // Instance variables
  String husband;
  String wife;
  int    weddingYear;

  // Constructor
  Couple(String man, String woman, int yearMarried) {
    husband = man;
    wife = woman;
    weddingYear = yearMarried;
  }

  // Accessors
  public String getHusband() {
    return husband;
  }

  public String getWife() {
    return wife;
  }

  public int getWeddingYear() {
    return weddingYear;
  }

  // toString method (used by Java to convert an object into a string)
  public String toString() {
    return husband + " and " + wife + " were married in " + yearMarried;
  }
}

Now, in order to create an instance of the class, we use the operator new as follows:

Couple goldmans;
goldmans = new Couple("Ken", "Sally", 1984);

The first line declares a variable goldmans whose type is a reference to a Couple object. This variable initially has the value null because it does not yet refer to any object.

The expression new Couple("Ken", "Sally", 1984) causes the creation of a new Couple object somewhere in the heap memory. The constructor is called with the supplied parameters, and the instance variables are initialized accordingly.

Then the assignment makes the value of the variable goldmans be a reference to the newly created object. (You can think of the reference variable as containing the memory address of the object.) Note that, as with other variables, declaration and assignment can appear on one line:

Couple goldmans = new Couple("Ken", "Sally", 1984);

Given the reference variable, we can invoke methods on the object using the "dot" notation we've been using for the Terminal and Math classes. For example, the expression goldman.getWife() would evaluate to the string "Sally".

Within a program, you could use the Couple class as follows:

// Declaration and initialization
Couple goldmans = new Couple("Ken", "Sally", 1984);
Terminal.println("The husband is " + goldmans.getHusband());
Terminal.println(goldmans.getWife() + " is his wife.");
Terminal.println(goldmans.toString());
Terminal.println(goldmans);

(In the last line, Java sees that a conversion from Couple to String is necessary and automatically calls goldmans.toString().)

What would happen in the following?

Couple clintons;
Terminal.println("Bill and Hillary were married in " +
                 clintons.getWeddingYear());

Since the variable clintons is an uninitialized reference variable, its value is null. It does not refer to any object. Therefore, the expression clintons.getWeddingYear() cannot be evaluated.

If you're lucky, the compiler will detect this and inform you that

Variable clintons may not be initialized.

Otherwise, if the compiler does not detect a null reference, you'll get an error at run time, java.lang.NullPointerException, and it will show you the line number where the exception occurred.

CS101: Compound Data

Example: Couple

Example: `Couple`