JAVA Interfaces and Abstract Classes
In Java, declaring a class abstract is useful

To force developers to extend the class not to use its capabilities.
When default implementations of some methods are not desirable.
When it doesn't make sense to have objects of that class.
When it makes sense to have objects of that class.
To prevent developers from further extending the class.

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JAVA Interfaces and Abstract Classes
What will be the output?1. public interface InfA{2. protected String getName();3. }public class Test implements InfA{ public String getName(){ return "test-name"; } public static void main (String[] args){ Test t = new Test(); System.out.println(t.getName()); }}

None of these
Compilation fails due to an error on lines 1
test-name
Compilation fails due to an error on lines 2
Compilation succeed but Runtime Exception

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JAVA Interfaces and Abstract Classes
Determine output of the following code.interface A { }class C { }class D extends C { }class B extends D implements A { }public class Test extends Thread{ public static void main(String[] args){ B b = new B(); if (b instanceof A) System.out.println("b is an instance of A"); if (b instanceof C) System.out.println("b is an instance of C"); }}

b is an instance of
b is an instance of
Nothing.
b is an instance of A followed by b is an instance of

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JAVA Interfaces and Abstract Classes
Given the following piece of code:public class School{ public abstract double numberOfStudent();}which of the following statements is true?

The keywords public and abstract cannot be used together.
Class School must be defined abstract.
The method numberOfStudent() in class School must have a body.
You must add a return statement in method numberOfStudent().

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