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.
To prevent developers from further extending the class.
When it makes sense to have objects of that class.

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

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

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

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JAVA Interfaces and Abstract Classes
What will be the output?interface A{public void method();}class One{public void method(){System.out.println("Class One method");}}class Two extends One implements A{public void method(){System.out.println("Class Two method");}}public class Test extends Two{public static void main(String[] args){A a = new Two();a.method();}}

None of these
will print Class One method
compiles fine but print nothing
will print Class Two method
Compilation Error

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