Polymorphism Practice

Sample questions

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1. Sample 1difficulty 2/5

   public class A {
       public String name() { return "A"; }
   }
   public class B extends A {
       public String name() { return "B"; }
   }
   // Usage:
   A obj = new B();
   System.out.print(obj.name());

   What is printed?

   • A

     AB

   • B

     B

     check_circle
   • C

     Compile error

   • D

     A

   Why

   Method dispatch in Java is based on the runtime (actual) type of the object, not the declared type. obj refers to a B object, so B's name() is called, printing "B".

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2. Sample 2difficulty 2/5

   public class Animal {
       public String sound() { return "generic"; }
   }
   public class Dog extends Animal {
       public String sound() { return "Woof"; }
   }
   Animal a = new Dog();
   System.out.println(a.sound());

   What is printed?

   • A

     Woof

     check_circle
   • B

     generic

   • C

     Compile-time error

   • D

     Runtime error

   Why

   Java uses dynamic dispatch: even though <code>a</code> is declared as <code>Animal</code>, the actual object is a <code>Dog</code>, so <code>Dog.sound()</code> is invoked at runtime.

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3. Sample 3difficulty 3/5

   public class Animal {
       public String sound() { return "?"; }
   }
   public class Dog extends Animal {
       public String sound() { return "woof"; }
   }
   public class Zoo {
       public static String hear(Animal a) { return a.sound(); }
   }
   // Usage:
   System.out.print(Zoo.hear(new Dog()));

   What is printed?

   • A

     Animal

   • B

     Compile error

   • C

     ?

   • D

     woof

     check_circle

   Why

   A method that takes Animal can accept any subclass. Dynamic dispatch invokes Dog's sound, returning "woof".

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4. Sample 4difficulty 3/5

   ArrayList<Object> list = new ArrayList<Object>();
   list.add("hello");
   list.add(42);
   list.add(3.14);
   int count = 0;
   for (int i = 0; i < list.size(); i++) {
       if (list.get(i) instanceof String) count++;
   }
   System.out.print(count);

   What is printed?

   • A

     2

   • B

     0

   • C

     3

   • D

     1

     check_circle

   Why

   Only "hello" is a String. 42 autoboxes to Integer, 3.14 to Double. So instanceof String is true exactly once.

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5. Sample 5difficulty 3/5

   import java.util.*;
   public class A { public String f() { return "A"; } }
   public class B extends A { public String f() { return "B"; } }
   List<A> list = new ArrayList<>();
   list.add(new B());
   A x = list.get(0);
   System.out.println(x.f());

   What is printed?

   • A

     B

     check_circle
   • B

     AB

   • C

     Compile-time error

   • D

     A

   Why

   Stored as B, accessed via A reference; dynamic dispatch picks B.f.

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