Explain the working of fraction calculator java.

Regarding the fraction calculator java, OOP is not only about Polymorphism and Inheritance towards which you intuitively designed your operation classes, although incompletely and it was pointed out by Simon André Forsberg. OOP is also about Encapsulation which is often neglected by many. Encapsulation simply says that you have to hide the details of your implementation behind the public interface. This has a good side effect which is one of the most important OOP features: it makes you keep your data and routines that operate on that data together in one place. Encapsulation is what differentiates OOP from Procedural programming. A related principle is called Tell, Don't Ask.

This said, a proper OOP-style design would be to convert the operation classes to methods of the Fraction class

public class Fraction {

    public void add(Fraction fraction) {}

    public void subtract(Fraction fraction) {}

    public void multiply(Fraction fraction) {}

    public void divide(Fraction fraction) {}

}

Another thing is that you should make use of a constructor other than the default one. The constructor will ensure that the newly created fraction is complete and good to go. What happens if someone tries to perform an operation on a fraction without setting the numerator and denominator first? This will force you to validate fractions before each operation. Constructor also is a good way to specify the required object properties

public class Fraction {

    public Fraction(int numerator, int denominator) {}

    public void add(Fraction fraction) {}

    public void subtract(Fraction fraction) {}

    public void multiply(Fraction fraction) {}

    public void divide(Fraction fraction) {}

}

Also, Fraction class should be immutable

public class Fraction {

    public Fraction(int numerator, int denominator) {}

    public Fraction add(Fraction fraction) {}

    public Fraction subtract(Fraction fraction) {}

    public Fraction multiply(Fraction fraction) {}

    public Fraction divide(Fraction fraction) {}

}

PS. If you want to have a set of operations, Enum would be a better choice.