Abstract Classes, Interfaces and Polymorphism

Hello students, welcome to Kwickprep. One line of code says s dot area. Sometimes it finds the area of a circle, and sometimes of a square. How does Java know which one to run? Today we learn abstract classes, interfaces, run time polymorphism and the Object class.

Let us start with a real problem. Every shape, like a circle or a square, has an area. But a general shape has no formula, so you cannot find its area. A circle knows its own formula, and so does a square. So the parent class Shape should only promise an area method, and each child class should write the body.

This is how Java writes that promise. The keyword abstract before class makes Shape an abstract class. An abstract class is an incomplete class, so we cannot create its objects. The line abstract double area has no body, only a semicolon. That is an abstract method, which means a method with a heading but no body. The method show is a normal method with a body, and an abstract class may have both kinds.

Exams often ask these rules, so learn them well. First, writing new Shape gives a compile error, because an abstract class cannot make objects. Second, an abstract method ends with a semicolon and has no curly brackets. Third, a child class must override every abstract method, which means it writes its own body for it. If it does not, the child class must also be declared abstract. Finally, an abstract class can still have variables, constructors and normal methods.

Here is a complete program. To fit the screen, S is short for Shape, and Sq is short for Square. S is abstract, with an abstract method area. Sq extends S and overrides area, returning four into four. In main, we cannot write new S, but a variable of type S can hold a Sq object. So s dot area runs the square's version and prints sixteen.

Now, a second tool, called an interface. An interface is a list of methods that a class promises to have, like a contract. We write it with the keyword interface instead of class. A class accepts the contract with the keyword implements. Its plain methods are public and abstract by default, so we do not write those words. Any variable in an interface is public, static and final, which means it is a fixed constant.

Think of paying at the school canteen. Cash, card and UPI all pay money, but in different ways. The interface Pay says every payment method must have a pay method. The class Upi implements Pay and writes the body. Notice the word public before void pay. It is compulsory, because interface methods are public, and we cannot make them less visible. The program prints Paid fifty.

This comparison is a favourite exam question. A class uses extends for an abstract class, but implements for an interface. A class can extend only one class, but it can implement many interfaces at once. An abstract class can have any variables, while an interface has only constants. An abstract class can have constructors, but an interface cannot. From Java eight onwards, an interface may also have default and static methods with bodies.

Now the big idea that ties it together. Polymorphism means one name, many forms, from Greek words for many and forms. Compile time polymorphism is overloading, where methods share a name but take different parameters. Run time polymorphism is overriding, where a child class rewrites a parent method. It works because a parent type variable can hold a child object, which is called upcasting.

Here, the variable b has type Bird, but the object inside is a Crow. Crow overrides the sound method. When we call b dot sound, Java does not look at the variable type. At run time, it looks at the actual object, which is a Crow. So it prints Caw caw. This choice of method at run time is called dynamic method dispatch.

Pause the video and predict the output. The variable x is declared as type A. The object created is new B. Class B overrides the method hi. So which hi runs? The object is a B, so Java runs the B version, and it prints B. Many students wrongly answer A, because they look only at the variable type.

Finally, a class you use without knowing it. Object is the top parent of every class in Java, in the package java dot lang. If you write no extends, Java quietly makes your class extend Object. That is why every object has a to String method, which gives the object as text. Every object also has an equals method, which checks whether two objects are equal. The get Class and hash Code methods come from Object too.

When you print an object, Java calls its to String method. The version from Object prints the class name, an at sign and a code, which is not very useful. So class Stu, short for Student, overrides to String. It must be public, because it is public in Object. Now printing the object gives Student Riya. This is run time polymorphism again.

Some exams ask for this idea in pseudocode. One loop asks every shape for its area. A circle and a square each answer with their own formula.

Let us revise. An abstract class cannot make objects, and its abstract methods have no body. An interface is a contract, and a class implements it. A class can extend only one class, but it can implement many interfaces. In run time polymorphism, Java runs the method of the actual object. And every class extends Object, which gives to String and equals.