Java Interfaces and Abstract Classes(1st)

Which of the following is a correct interface?

1 Mark(s)

abstract interface A { print(); }
interface A { void print() { } }
interface A { void print(); }
abstract interface A { abstract void print(); { }}

In Java, declaring a class abstract is useful

1 Mark(s)

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

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());
      }
}

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());
      }
}

1 Mark(s)

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

Suppose A is an abstract class, B is a concrete subclass of A, and both A and B have a default constructor. Which of the following is correct?
1. A a = new A();
2. A a = new B();
3. B b = new A();
4. B b = new B();

1 Mark(s)

1 and 2
3 and 4
2 and 4
1 and 3
2 and 3

Given the following piece of code:

public class School{
        public abstract double numberOfStudent();
}

which of the following statements is true?

Given the following piece of code:

public class School{
        public abstract double numberOfStudent();
}

which of the following statements is true?

1 Mark(s)

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

Which of the following statements regarding abstract classes are true?

1 Mark(s)

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

Which of the following declares an abstract method in an abstract Java class?

1 Mark(s)

public abstract void method();
public void method() {}
public void abstract Method();
public abstract method();
public abstract void method() {}

What is the output for the below code ?

interface A{
      public void printValue();
}

1. public class Test{
2.       public static void main (String[] args){
3.             A a1 = new A(){
4.                          public void printValue(){
5.                                System.out.println("A");
6.                          }
7.                    };
8.             a1.printValue();
9.       }
10. }

What is the output for the below code ?

interface A{
      public void printValue();
}

1. public class Test{
2.       public static void main (String[] args){
3.             A a1 = new A(){
4.                          public void printValue(){
5.                                System.out.println("A");
6.                          }
7.                    };
8.             a1.printValue();
9.       }
10. }

1 Mark(s)

A
null
Compilation fails due to an error on line 3
None of these
Compilation fails due to an error on line 8

interface Base{
      boolean m1 ();
      byte m2(short s);
}

which two code fragments will compile?

1. interface Base2 implements Base {}

2. abstract class Class2 extends Base
    { public boolean m1(){ return true; }}

3. abstract class Class2 implements Base {}

4. abstract class Class2 implements Base
    { public boolean m1(){ return (7 > 4); }}

5. abstract class Class2 implements Base
    { protected boolean m1(){ return (5 > 7) }}

interface Base{
      boolean m1 ();
      byte m2(short s);
}

which two code fragments will compile?

1. interface Base2 implements Base {}

2. abstract class Class2 extends Base
    { public boolean m1(){ return true; }}

3. abstract class Class2 implements Base {}

4. abstract class Class2 implements Base
    { public boolean m1(){ return (7 > 4); }}

5. abstract class Class2 implements Base
    { protected boolean m1(){ return (5 > 7) }}

1 Mark(s)

1 and 3
3 and 4
4 and 5
1 and 2
2 and 3

Which of the following class definitions defines a legal abstract class?

1 Mark(s)

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

Which two of the following are legal declarations for abstract classes and interfaces?
1. final abstract class Test {}
2. public static interface Test {}
3. final public class Test {}
4. protected abstract class Test {}
5. protected interface Test {}
6. abstract public class Test {}

1 Mark(s)

3 and 6
5 and 6
1 and 2
3 and 5
2 and 4

Runnable is a _____ .

1 Mark(s)

class
method
abstract class
vaiable
interface

What will happen after compiling this program code?

abstract class MyClass{ //line 1
      private int a, b;

      public void call(int a, int b){
            this.a = a;
            this.b = b;
            System.out.print(a+b);
      }
}

public class Test{
      public static void main(String args[]){
            MyClass m = new MyClass(); //line 2
            m.call(12,25);
      }
}

What will happen after compiling this program code?

abstract class MyClass{ //line 1
      private int a, b;

      public void call(int a, int b){
            this.a = a;
            this.b = b;
            System.out.print(a+b);
      }
}

public class Test{
      public static void main(String args[]){
            MyClass m = new MyClass(); //line 2
            m.call(12,25);
      }
}

1 Mark(s)

Compilation error due to line 1
None of these
Compilation error due to line 2
Successful run and print 37
Runtime error

What will be the output when the following program is compiled and executed?

abstract class TestAbstract{
      String my_name;
      String myName(){
            my_name = "Examveda";
            return my_name;
      }
      abstract void display();
}

public class Test extends TestAbstract{
      void display(){
            String n = myName();
	    System.out.print("My name is "+ n);
      }

      public static void main(String args[]){
            Test t = new Test();
	    t.display();
      }
}

What will be the output when the following program is compiled and executed?

abstract class TestAbstract{
      String my_name;
      String myName(){
            my_name = "Examveda";
            return my_name;
      }
      abstract void display();
}

public class Test extends TestAbstract{
      void display(){
            String n = myName();
	    System.out.print("My name is "+ n);
      }

      public static void main(String args[]){
            Test t = new Test();
	    t.display();
      }
}

1 Mark(s)

Compilation error as class can not be declared as abstract.
Compilation error occurs as the abstract class TestAbstract contains a non-abstract method.
None of these
Program will compile and execute successfully and prints
Program compiles but leads to runtime exception.

interface Test{
      int p = 10; //line 1
      public int q = 20; //line 2
      public static int r = 30; //line 3
      public static final int s = 40; //line 4
}

Which of the above line will give compilation error?

interface Test{
      int p = 10; //line 1
      public int q = 20; //line 2
      public static int r = 30; //line 3
      public static final int s = 40; //line 4
}

Which of the above line will give compilation error?

1 Mark(s)

4
None of these
2
1
3

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();		
	}
}

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();		
	}
}

1 Mark(s)

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

What happens if the following program is compiled and executed?

interface MyInterface{
      void display();
}

interface MySubInterface extends MyInterface{
      void display();
}

public class Test implements MySubInterface{
      public void display(){
            System.out.print("Welcome to Examveda.");
      }
      public static void main(String args[]){
            Test t = new Test();
	    t.display();
      }
}

What happens if the following program is compiled and executed?

interface MyInterface{
      void display();
}

interface MySubInterface extends MyInterface{
      void display();
}

public class Test implements MySubInterface{
      public void display(){
            System.out.print("Welcome to Examveda.");
      }
      public static void main(String args[]){
            Test t = new Test();
	    t.display();
      }
}

1 Mark(s)

The code will lead to a compilation error as declaration of the display method has been provided in two interface.
The code will lead to a compilation error as the display method is not declared as abstract.
The code will compile and execute successfully showing the output Welcome to Examveda.
The code will lead to a compilation error due to public modifier while declaring the display method.
None of these

What will be the output for the below code ?

public interface TestInf{
      int i =10;
}

public class Test{
      public static void main(String... args){
            TestInf.i=12;
	    System.out.println(TestInf.i);
      }
}

What will be the output for the below code ?

public interface TestInf{
      int i =10;
}

public class Test{
      public static void main(String... args){
            TestInf.i=12;
	    System.out.println(TestInf.i);
      }
}

1 Mark(s)

12
Runtime Exception
None of these
Compile with error
10

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");
        }
}

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");
        }
}

1 Mark(s)

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

Given the following piece of code:

public interface Guard{
        void doYourJob();
}
abstract public class Dog implements Guard{ }

which of the following statements is correct?

Given the following piece of code:

public interface Guard{
        void doYourJob();
}
abstract public class Dog implements Guard{ }

which of the following statements is correct?

1 Mark(s)

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

Java Interfaces and Abstract Classes(1st) : Question 1 of 20