Inheritance, super, Abstract Classes and Polymorphism (ISC Class 12 Computer Science)

Understand Inheritance, super, Abstract Classes and Polymorphism

Inheritance, super, Abstract Classes and Polymorphism: key facts

Inheritance syntax

• •class Derived extends Base
• •One super class only
• •Constructors are not inherited
• •Every class extends Object

The derived class is-a Base.

Constructor order

• •Super constructor runs first
• •super(args) must be the first statement
• •No explicit call means super() is called implicitly
• •Order: top class down

Output of print statements follows this order.

Overriding versus overloading

• •Overriding: same signature in derived class, chosen at run time
• •Overloading: same name, different parameters, chosen at compile time
• •Fields are hidden, not overridden

Dynamic binding applies to methods only.

Access levels

private	own class only
protected	derived classes and same package
public	everywhere
	Abstract class cannot be instantiated

A concrete subclass implements all abstract methods.

How to answer Inheritance, super, Abstract Classes and Polymorphism questions

1. 1Draw the class hierarchy and mark each overridden method.
2. 2For object creation, list the constructors in the order super class first, then derived.
3. 3For a method call, find the actual class of the object and use its version (or the nearest parent version).
4. 4For a variable access, use the declared type of the reference.
5. 5Check what the declared type allows: calling a derived-only method needs a cast.
6. 6Write the output exactly; a trailing space after print is part of it.

Inheritance, super, Abstract Classes and Polymorphism in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Inheritance, super, Abstract Classes and Polymorphism

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Parents first

  Constructors run from the top class down, so parent output comes first.

  Use it when: Constructor chaining output.

• Object picks the method

  Methods follow the real object; fields follow the declared type.

  Use it when: Dynamic binding questions.

• Is-a test

  Say "Dog is an Animal"; if the sentence is false, do not inherit.

  Use it when: Designing inheritance.

• Abstract = unfinished

  An abstract class has an unfinished method; subclasses must finish it.

  Use it when: Abstract class questions.

Inheritance, super, Abstract Classes and Polymorphism: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

class A {
    A()      { System.out.print("A "); }
    A(int x) { System.out.print("A" + x + " "); }
}
class B extends A {
    B()      { System.out.print("B "); }
    B(int y) { super(y); System.out.print("B" + y + " "); }
}
// in main
new B();
new B(7);

class Shape {
    double area()  { return 0; }
    String name()  { return "Shape"; }
}
class Sq extends Shape {
    double s;
    Sq(double s) { this.s = s; }
    double area()  { return s * s; }
    String name()  { return "Square"; }
}
class Circ extends Shape {
    double r;
    Circ(double r) { this.r = r; }
    double area()  { return 3.0 * r * r; }
}
// in main
Shape[] a = { new Sq(4), new Circ(2), new Shape() };
for (int i = 0; i < a.length; i++)
    System.out.println(a[i].name() + " " + a[i].area());

class P {
    int v = 10;
    int get() { return v; }
}
class Q extends P {
    int v = 20;
    int get() { return v + super.v; }
}
// in main
P o = new Q();
System.out.println(o.v + " " + o.get() + " " + ((Q) o).v);

abstract class Account {
    abstract double bonus();
    void show() { System.out.println("Bonus " + bonus()); }
}
class Savings extends Account {
    double bal;
    Savings(double b) { bal = b; }
    double bonus() { return bal * 0.25; }
}

Where students lose marks in Inheritance, super, Abstract Classes and Polymorphism

The slips examiners see most often, and what to write instead.

• ✗Mistake: Child output before parent output

  The super class constructor runs first.

  Wrong working

  new B() prints B A

  Right working

  new B() prints A B

  ✓Do this: Trace constructors from the top.

  Usually costs: 1 mark

• ✗Mistake: Using the declared type for a method call

  Overridden methods are chosen by the actual object.

  Wrong working

  Shape s = new Sq(4); s.area() gives 0

  Right working

  Shape s = new Sq(4); s.area() gives 16.0

  ✓Do this: Look at what follows new.

  Usually costs: 1 mark

• ✗Mistake: super call not first

  super(...) must be the first statement of a constructor.

  Wrong working

  B(int y) { print("B"); super(y); }

  Right working

  B(int y) { super(y); print("B"); }

  ✓Do this: Put super(...) on the first line.

  Usually costs: 1 mark

Practice questions on Inheritance, super, Abstract Classes and Polymorphism

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   A class that cannot be instantiated is called
   (a) final class (b) abstract class (c) static class (d) derived class

   Hint

   It may have methods with no body.

   Answer

   (b) abstract class

2. Q2 · 2 marks · Short answerBoard level

   What does the protected access specifier allow? Compare it with private.

   Hint

   Think of derived classes.

   Answer

   A protected member can be used inside its own class, in derived classes and in classes of the same package. A private member is accessible only inside its own class, so a derived class cannot use it directly.

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   class Vehicle {
       String move() { return "moves"; }
   }
   class Car extends Vehicle {
       String move() { return "drives and " + super.move(); }
   }
   // in main
   System.out.println(new Car().move());

   Hint

   super.move() is the parent's version.

   Answer

   Car's move() runs and adds the parent's text: drives and moves

4. Q4 · 4 marks · Short answerHarder board level

   Write the output:

   class A {
       void f(int x) { System.out.print("A.int "); }
   }
   class B extends A {
       void f(double x) { System.out.print("B.double "); }
       void f(int x)    { System.out.print("B.int "); }
   }
   // in main
   A a = new B();
   B b = new B();
   b.f(1);
   b.f(1.0);
   a.f(1);

   Say which methods are overloaded and which overridden.

   Hint

   f(int) is redefined; f(double) is new.

   Answer

   b.f(1) picks f(int): B.int. b.f(1.0) picks f(double): B.double. a.f(1) is a call to f(int); the object is a B, so the overriding version runs: B.int. Output: B.int B.double B.int. In B, f(int) overrides A.f(int); f(int) and f(double) in B are overloaded.

5. Q5 · 4 marks · Case-basedHOTS / competency

   Given class Animal { void speak() {} } and class Dog extends Animal { void fetch() {} }, state for each statement whether it is valid, a compile-time error or a run-time error: (i) Animal a = new Dog(); a.speak(); (ii) Animal a = new Dog(); a.fetch(); (iii) Animal a = new Dog(); ((Dog) a).fetch(); (iv) Dog d = (Dog) new Animal();

   Hint

   The declared type decides what can be called; the object decides what a cast can succeed on.

   Answer

   (i) Valid; Dog inherits speak(). (ii) Compile-time error: the type Animal has no fetch(). (iii) Valid: the cast is correct because the object is a Dog. (iv) Compiles because Dog is a subtype, but fails at run time with ClassCastException, since the object is only an Animal.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which keyword derives a class from another?Show answer

   extends.

2. 2.Which class is the parent of every class in Java?Show answer

   Object.

3. 3.Can an abstract class be instantiated?Show answer

   No.

4. 4.What must be the first statement when super(...) is used?Show answer

   It must be the first statement of the constructor.

5. 5.What is dynamic binding?Show answer

   The version of an overridden method is chosen at run time by the actual object.

6. 6.Is a private member inherited for direct use?Show answer

   No, it is not accessible in the derived class.

Inheritance, super, Abstract Classes and Polymorphism: frequently asked questions