JAVA Interfaces and Abstract Classes
Which of the following class definitions defines a legal abstract class?

public class abstract A { abstract void unfinished(); }
abstract class A { abstract void unfinished(); }
class A { abstract void unfinished(); }
class A { abstract void unfinished() { } }

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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"); }}

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

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JAVA Interfaces and Abstract Classes
Which of the following statements regarding abstract classes are true?

An abstract class can be used as a data type.
All of these
A subclass can override a concrete method in a superclass to declare it abstract.
A subclass of a non-abstract superclass can be abstract.
An abstract class can be extended.

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JAVA Interfaces and Abstract Classes
Given the following piece of code:public interface Guard{ void doYourJob();}abstract public class Dog implements Guard{ }which of the following statements is correct?

This code will not compile, because in the declaration of class Dog we must use the keyword extends instead of implements.
This code will not compile, because class Dog must implement method doYourJob() from interface Guard.
This code will not compile, because method doYourJob() in interface Guard must be defined abstract.
This code will compile without any errors.

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