1.8. Defining Classes in Java

You have already seen how to define classes in Java. It’s unavoidable for even the simplest of programs. In this section we will look at how we define classes to create our own data types. Lets start by creating a fraction class to extend the set of numeric data types provided by our language. The requirements for this new data type are as follows:

Here is a mostly complete implementation of a Fraction class in Python that we will refer to throughout this section:

The instance variables (data members) we will need for our fraction class are the numerator and denominator. Of course in Python we can add instance variables to a class at any time by simply assigning a value to objectReference.variableName, whereas in Java all data members must be declared up front.

The declarations of instance variables can come at the beginning of the class definition or the end. Cay Horstman, author of the “Core Java” books puts the declarations at the end of the class. I like them at the very beginning so you see the variables that are declared before you begin looking at the code that uses them. With that in mind the first part of the Fraction class definition is as follows:

public class Fraction {
    private Integer numerator;
    private Integer denominator;
}

Notice that we have declared the numerator and denominator to be private. This means that the compiler will generate an error if another method tries to write code like the following:

Fraction f = new Fraction(1,2);
Integer y = f.numerator * 10;

Direct access to instance variables is not allowed. Therefore if we legitimately want to be able to access information such as the numerator or denominator for a particular fraction we must have getter methods. It is very common programming practice to provide getter and setter methods for instance variables in Java.

 1public Integer getNumerator() {
 2    return numerator;
 3}
 4
 5public void setNumerator(Integer numerator) {
 6    this.numerator = numerator;
 7}
 8
 9public Integer getDenominator() {
10    return denominator;
11}
12
13public void setDenominator(Integer denominator) {
14    this.denominator = denominator;
15}

1.8.1. Writing a constructor

Once you have identified the instance variables for your class the next thing to consider is the constructor. In Java, constructors have the same name as the class and are declared public. They are declared without a return type. So any method that is named the same as the class and has no return type is a constructor. Our constructor will take two parameters: the numerator and the denominator.

public Fraction(Integer top, Integer bottom) {
    num = top;
    den = bottom;
}

There are a couple of important things to notice here. First, you will notice that the constructor does not have a self parameter. You will also notice that we can simply refer to the instance variables by name without the self prefix, because they have already been declared. This allows the Java compiler to do the work of dereferencing the current Java object. Java does provide a special variable called this that works like the self variable. In Java, this is typically only used when it is needed to differentiate between a parameter or local variable and an instance variable. For example this alternate definition of the the Fraction constructor uses this to differentiate between parameters and instance variables.

public Fraction(Integer num, Integer den) {
    this.num = num;
    this.den = den;
}

1.8.2. Methods

Now we come to one of the major differences between Java and Python. The Python class definition used the special methods for addition and comparison that have the effect of redefining how the standard operators behave: in Python, __add__ and __lt__ change the behavior of + and <, respectively. In Java there is no operator overloading. So we will have to write the method for addition a little differently.

A point of terminology: Python has both “functions” (def outside a class) and “methods” (def inside a class). Since Java requires all code to be inside classes, it only has “methods.” Those from a C++ background might refer to methods as “member functions.”

Let’s begin by implementing addition in Java:

1public Fraction add(Fraction otherFrac) {
2    Integer newNum = otherFrac.getDenominator() * this.numerator +
3                             this.denominator * otherFrac.getNumerator();
4    Integer newDen = this.denominator * otherFrac.getDenominator();
5    Integer common = gcd(newNum, newDen);
6    return new Fraction(newNum/common, newDen/common);
7}

First you will notice that the add method is declared as public Fraction The public part means that any other method may call the add method. The Fraction part means that add will return a fraction as its result.

Second, you will notice that the method makes use of the this variable. In this method, this is not necessary, because there is no ambiguity about the numerator and denominator variables. So this version of the code is equivalent:

1public Fraction add(Fraction otherFrac) {
2    Integer newNum = otherFrac.getDenominator() * numerator +
3                             denominator * otherFrac.getNumerator();
4    Integer newDen = denominator * otherFrac.getDenominator();
5    Integer common = gcd(newNum, newDen);
6    return new Fraction(newNum/common, newDen/common);
7}

The addition takes place by multiplying each numerator by the opposite denominator before adding. This procedure ensures that we are adding two fractions with common denominators. Using this approach the denominator is computed by multiplying the two denominators. The greatest common divisor method, gcd, is used to find a common divisor to simplify the numerator and denominator in the result.

Finally on line 6 a new Fraction is returned as the result of the computation. The value that is returned by the return statement must match the value that is specified as part of the declaration. So, in this case the return value on line 8 must match the declared value on line 1.

1.8.2.1. Method Signatures and Overloading

Our specification for this project said that we need to be able to add a Fraction to an Integer. In Python we can do this by checking the type of the parameter using the isinstance function at runtime. Recall that isinstance(1,int) returns True to indicate that 1 is indeed an instance of the int class. See the __add__ and toFract methods in the Python version of the Fraction class to see how our Python implementation fulfills this requirement.

In Java we can do runtime type checking, but the compiler will not allow us to pass an Integer to the add method since the parameter has been declared to be a Fraction. The way that we solve this problem is by writing another add method with a different set of parameters. In Java this practice is legal and common we call this practice method overloading.

This idea of method overloading raises a very important difference between Python and Java. In Python a method is known by its name only. In Java a method is known by its signature. The signature of a method includes its name, and the types of all of its parameters. The name and the types of the parameters are enough information for the Java compiler to decide which method to call at runtime.

To solve the problem of adding an Integer and a Fraction in Java we will overload both the constructor and the add method. We will overload the constructor so that if it only receives a single Integer it will convert the Integer into a Fraction. We will also overload the add method so that if it receives an Integer as a parameter it will first construct a Fraction from that integer and then add the two Fractions together. The new methods that accomplish this task are as follows:

1public Fraction(Integer num) {
2    this.numerator = num;
3    this.denominator = 1;
4}
5
6public Fraction add(Integer other) {
7    return add(new Fraction(other));
8}

Notice that the overloading approach can provide us with a certain elegance to our code. Rather than utilizing if statements to check the types of parameters we just overload methods ahead of time which allows us to call the method we want and allow the compiler to make the decisions for us. This way of thinking about programming takes some practice.

Our full Fraction class to this point would look like the following. You should compile and run the program to see what happens.

1.8.3. Inheritance

If you ran the program above you probably noticed that the output is not very satisfying. Chances are your output looked something like this:

Fraction@6ff3c5b5

The reason is that we have not yet provided a friendly string representation for our Fraction objects. Just like in Python, whenever an object is printed by the println method it must be converted to string format. In Python you can control how that looks by writing an __str__ method for your class. If you do not then you will get the default, which looks something like the above.

1.8.3.1. The Object Class

In Java, the equivalent of __str__ is the toString method. Every object in Java already has a toString method defined for it because every class in Java automatically inherits from the Object class. The Object class provides default implementations for the following methods.

  • clone

  • equals

  • finalize

  • getClass

  • hashCode

  • notify

  • notifyAll

  • toString

  • wait

We are not interested in most of the methods on that list, and many Java programmers live happy and productive lives without knowing much about most of the methods on that list. However, to make our output nicer we will implement the toString method for the Fraction class. A simple version of the method is provided below.

public String toString() {
    return numerator.toString() + "/" + denominator.toString();
}

The other important class for us to implement from the list of methods inherited from Object is the equals method. In Java, when two objects are compared using the == operator they are tested to see if they are exactly the same object (that is, do the two objects occupy the same exact space in the computer’s memory?). This is also the default behavior of the equals method provided by Object. The equals method allows us to decide if two objects are equal by looking at their instance variables. However it is important to remember that since Java does not have operator overloading if you want to use your equals method you must call it directly. Therefore once you write your own equals method:

object1 == object2

is NOT the same as

object1.equals(object2)

Here is an equals method for the Fraction class:

1public boolean equals(Fraction other) {
2    Integer num1 = this.numerator * other.getDenominator();
3    Integer num2 = this.denominator * other.getNumerator();
4    if (num1 == num2)
5        return true;
6    else
7        return false;
8}

One important thing to remember about equals is that it only checks to see if two objects are equal – it does not have any notion of less than or greater than. We’ll see more about that shortly.

1.8.3.2. Abstract Classes and Methods

If we want to make our Fraction class behave like Integer, Double, and the other numeric classes in Java then we need to make a couple of additional modifications to the class. The first thing we will do is plug Fraction into the Java class hierarchy at the same place as Integer and its siblings. If you look at the documentation for Integer you will see that Integer’s parent class is Number. Number is an abstract class that specifies several methods that all of its children must implement. In Java an abstract class is more than just a placeholder for common methods. In Java an abstract class has the power to specify certain methods that all of its children must implement. You can trace this power back to the strong typing nature of Java.

Here is code that makes the Fraction class a child of Number:

public class Fraction extends Number {
    ...
}

The keyword extends tells the compiler that the class Fraction extends, or adds new functionality to the Number class. A child class always extends its parent.

The methods we must implement if Fraction is going to be a child of Number are:

  • longValue

  • intValue

  • floatValue

  • doubleValue

This really isn’t much work for us to implement these methods, as all we have to do is some type conversion and some division:

 1public double doubleValue() {
 2    return numerator.doubleValue() / denominator.doubleValue();
 3}
 4
 5
 6public float floatValue() {
 7    return numerator.floatValue() / denominator.floatValue();
 8}
 9
10
11public int intValue() {
12    return numerator.intValue() / denominator.intValue();
13}
14
15
16public long longValue() {
17    return numerator.longValue() / denominator.longValue();
18}

By having the Fraction class extend the Number class we can now pass a Fraction to any Java method that specifies it can receive a Number as one of its parameters. For example many Java user interface methods accept any object that is a subclass of Number as a parameter. In Java the class hierarchy and the “is-a” relationships are very important. Whereas in Python you can pass any kind of object as a parameter to any method or function, the strong typing of Java makes sure that you only pass an object as a parameter that is of the type specified in the method signature, or one of the children of the type specified. When you see a parameter of type Number it’s important to remember that an Integer is-a Number and a Double is-a Number and a Fraction is-a Number, because these classes are children of Number.

However, and this is a big however, it is important to remember that if you specify Number as the type of a particular parameter then the Java compiler will only let you use the methods of a Number: longValue, intValue, floatValue, and doubleValue.

Suppose you try to define a method as follows:

public void test(Number a, Number b) {
    a.add(b);
}

The Java compiler would give an error because add is not a defined method of the Number class. You will still get this error even if all your code that calls this test method passes two Fractions as parameters (remember that Fraction does implement add).

1.8.4. Interfaces

Lets turn our attention to making a list of fractions sortable by the standard Java sorting method Collections.sort. In Python, we would just need to implement the __cmp__ method. But in Java we cannot be that informal. In Java, things that are sortable must be Comparable. Your first thought might be that Comparable is superclass of Number, but that is actually not the case. Java only supports single inheritance, that is, a class can have only one parent. Although it would be possible to add an additional layer to the class hierarchy it would also complicate things dramatically, because not only are Numbers comparable, but Strings are also Comparable as would many other types. For example, we might have a Student class and we want to be able to sort students by their GPA. But Student might already extends the class Person for which there would be no natural comparison method.

Java’s answer to this problem is the Interface mechanism. Interfaces are like a combination of “inheritance” and “contracts” all rolled into one. An interface is a specification that says any object that claims it implements this interface must provide the following methods. It sounds a little bit like an abstract class, however it is outside the inheritance mechanism. You can never create an instance of Comparable. Many objects, however, do implement the Comparable interface. What does the Comparable interface specify?

The Comparable interface says that any object that claims to be Comparable must implement the compareTo method. Here is an excerpt from the official documentation for the compareTo method as specified by the Comparable interface.

int compareTo(T o)

Compares this object with the specified object for order. Returns a
negative integer, zero, or a positive integer as this object is less
than, equal to, or greater than the specified object. The
implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for
all x and y. (This implies that x.compareTo(y) must throw an exception
iff y.compareTo(x) throws an exception.)

...

To make our Fraction class Comparable we must modify the class declaration line as follows:

public class Fraction extends Number implements Comparable<Fraction> {
    ...
}

The specification Comparable<Fraction> makes it clear that Fraction is only comparable with another Fraction. The compareTo method could be implemented as follows:

1public int compareTo(Fraction other) {
2    Integer num1 = this.numerator * other.getDenominator();
3    Integer num2 = this.denominator * other.getNumerator();
4    return num1 - num2;
5}

1.8.5. Static member variables

Suppose that you wanted to write a Student class so that the class could keep track of the number of students it had created. Although you could do this with a global counter variable that is an ugly solution. The right way to do it is to use a static variable. In Python we could do this as follows:

In Java we would write this same example using a static declaration.

In this example notice that we create a static member variable by using the static modifier on the variable declaration. Once a variable has been declared static in Java it can be accessed from inside the class without prefixing the name of the class as we had to do in Python.

1.8.6. Static Methods

We have already discussed the most common static method of all, main. However in our Fraction class we also implemented a method to calculate the greatest common divisor for two fractions (gdc). There is no reason for this method to be a member method since it takes two Integer values as its parameters. Therefore we declare the method to be a static method of the class. Furthermore, since we are only going to use this gcd method for our own purposes we can make it private.

1private static Integer gcd(Integer m, Integer n) {
2    while (m % n != 0) {
3        Integer oldm = m;
4        Integer oldn = n;
5        m = oldn;
6        n = oldm%oldn;
7    }
8    return n;
9}

1.8.7. Full Implementation of the Fraction Class

Here is a final version of the Fraction class in Java, which includes all the features we discussed:

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