Java Inheritance

Question 1

Output of following Java Program?

class Base {
    public void show() {
       System.out.println("Base::show() called");
    }
}
 
class Derived extends Base {
    public void show() {
       System.out.println("Derived::show() called");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Base b = new Derived();;
        b.show();
    }
}

• A. Derived::show() called
• B. Base::show() called

Answer: (A)

In the above program, b is a reference of Base type and refers to an abject of Derived class. In Java, functions are virtual by default. So the run time polymorphism happens and derived fun() is called.

Question 2

class Base {
    final public void show() {
       System.out.println("Base::show() called");
    }
}
 
class Derived extends Base {
    public void show() {
       System.out.println("Derived::show() called");
    }
}
 
class Main {
    public static void main(String[] args) {
        Base b = new Derived();;
        b.show();
    }
}

• A. Base::show() called
• B. Derived::show() called
• C. Compiler Error
• D. Runtime Error

Answer: (C)

Final methods cannot be overridden. See the compiler error here.

Question 3

Java

class Base {
    public static void show() {
       System.out.println("Base::show() called");
    }
}
 
class Derived extends Base {
    public static void show() {
       System.out.println("Derived::show() called");
    }
}
 
class Main {
    public static void main(String[] args) {
        Base b = new Derived();
        b.show();
    }
}

• A. Base::show() called
• B. Derived::show() called
• C. Compiler Error

Answer: (A)

Like C++, when a function is static, runtime polymorphism doesn't happen.

Question 4

Which of the following is true about inheritance in Java?

1) Private methods are final.
2) Protected members are accessible within a package and 
   inherited classes outside the package.
3) Protected methods are final.
4) We cannot override private methods. 

• A. 1, 2 and 4
• B. Only 1 and 2
• C. 1, 2 and 3
• D. 2, 3 and 4

Answer: (A)

See http://www.geeksforgeeks.org/can-override-private-methods-java/ and http://www.geeksforgeeks.org/comparison-of-inheritance-in-c-and-java/

Question 5

Output of following Java program?

class Base {
    public void Print() {
        System.out.println("Base");
    }         
}

class Derived extends Base {    
    public void Print() {
        System.out.println("Derived");
    }
}

class Main{
    public static void DoPrint( Base o ) {
        o.Print();   
    }
    public static void main(String[] args) {
        Base x = new Base();
        Base y = new Derived();
        Derived z = new Derived();
        DoPrint(x);
        DoPrint(y);
        DoPrint(z);
    }
}

• A.

  Base
  Derived
  Derived

• B.

  Base
  Base
  Derived

• C.

  Base
  Derived
  Base

• D. Compiler Error

Answer: (A)

See question 1 of http://www.geeksforgeeks.org/output-of-java-program-set-2/

Question 6

Predict the output of following program. Note that fun() is public in base and private in derived.

class Base {
    public void foo() { System.out.println("Base"); }
}
 
class Derived extends Base {
    private void foo() { System.out.println("Derived"); } 
}
 
public class Main {
    public static void main(String args[]) {
        Base b = new Derived();
        b.foo();
    }
} 

• A. Base
• B. Derived
• C. Compiler Error
• D. Runtime Error

Answer: (C)

It is compiler error to give more restrictive access to a derived class function which overrides a base class function.

Question 7

Which of the following is true about inheritance in Java. 1) In Java all classes inherit from the Object class directly or indirectly. The Object class is root of all classes. 2) Multiple inheritance is not allowed in Java. 3) Unlike C++, there is nothing like type of inheritance in Java where we can specify whether the inheritance is protected, public or private.

• A. 1, 2 and 3
• B. 1 and 2
• C. 2 and 3
• D. 1 and 3

Answer: (A)

See Comparison of Inheritance in C++ and Java

Question 8

Predict the output of following Java Program

// filename Main.java
class Grandparent {
    public void Print() {
        System.out.println("Grandparent's Print()");
    }
}
 
class Parent extends Grandparent {
    public void Print() {
        System.out.println("Parent's Print()");
    }
}
 
class Child extends Parent {
    public void Print() {
        super.super.Print(); 
        System.out.println("Child's Print()");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Child c = new Child();
        c.Print();
    }
}

• A. Compiler Error in super.super.Print()
• B.

  Grandparent's Print()
  Parent's Print()
  Child's Print()

• C. Runtime Error

Answer: (A)

In Java, it is not allowed to do super.super. We can only access Grandparent's members using Parent. For example, the following program works fine.

// Guess the output
// filename Main.java
class Grandparent {
    public void Print() {
        System.out.println("Grandparent's Print()");
    }
}
 
class Parent extends Grandparent {
    public void Print() {
    	super.Print();  
        System.out.println("Parent's Print()");
    }
}
 
class Child extends Parent {
    public void Print() {
        super.Print();  
        System.out.println("Child's Print()");
    }
}
 
class Main {
    public static void main(String[] args) {
        Child c = new Child();
        c.Print();
    }
}

Question 9

final class Complex {

    private final double re;
    private final double im;

    public Complex(double re, double im) {
        this.re = re;
        this.im = im;
    }

    public String toString() {
        return "(" + re + " + " + im + "i)";
    }
}

class Main {
  public static void main(String args[])
  {
       Complex c = new Complex(10, 15);
       System.out.println("Complex number is " + c);
  }         
}

• A.

  Complex number is (10.0 + 15.0i)

• B. Compiler Error
• C.

  Complex number is SOME_GARBAGE

• D.

  Complex number is Complex@8e2fb5

  Here 8e2fb5 is hash code of c

Answer: (A)

See http://www.geeksforgeeks.org/overriding-tostring-method-in-java/