What will be the output of the following Java program?

package pkg;
class output 
{
    public static void main(String args[])
    { 
        StringBuffer s1 = new StringBuffer("Hello");
        s1.setCharAt(1, x);
        System.out.println(s1);
    }
}

What is an interface in Java?

A. A contract specifying a set of methods that a class must implement

B. A class that cannot be instantiated

C. A class that contains only static methods

D. A subclass of the Object class

In Java, can a class implement multiple interfaces?

A. Interfaces cannot be implemented by classes

B. Only abstract classes can implement interfaces

C. No, a class can only implement one interface

D. Yes, a class can implement multiple interfaces

Which keyword is used to declare an interface in Java?

A. class

B. abstract

C. interface

D. extends

What is the purpose of an abstract class in Java?

A. To declare abstract methods

B. To provide a base class with common methods and fields

C. To make a class final

D. None of These