Kevins MCQs

Question 1

What is a direct superclass?

Answer 1

The	superclass from which	the	
subclass	explicitly	inherits.

Question 2

What is an indirect superclass?

Answer 2

Any	class	above	the	direct	superclass	
in	the	class	hierarchy.

Question 3

What is single inheritance?

Answer 3

Java supports only single inheritance, in which each class is derived from exactly one direct superclass.

Question 4

Is-a and Has-A: Which represents inheritance and which represents composition?

Answer 4

Is-a	represents	inheritance - In an	is-a	relationship,	an	object	of	a	subclass	can	also	be	treated	as	an	object	of	its	superclass	
– i.e.	Lecturer	is	a	Faculty	member	

Has-a represents composition - In a has-a relationship, an object contains as members references to other objects.

Question 5

What are the members of a class?

Answer 5

Instance variables & methods.

Question 6

What is difference between aggregation and composition?

Answer 6

In a Has-A relationship:

Composition is a strong relationship: A Composes B, means A contains and owns B.
Aggregration between A and B means, A contains but not owns B.

Question 7

What is polymorphism?

Answer 7

When you define a supertype for a group of classes, any subclass of that supertype can be substituted where the supertype is expected.

Question 8

What has an overloaded method got to do with inheritance and polymorphism?

Answer 8

Nothing.

Question 9

What are the four access modifier levels?

Answer 9

Public, Protected, Default (Package-Private), Private.

Question 10

Explain four levels of access modifiers:

Answer 10

Public (all classes can access), Protected(Classes in the same package as well as ‘extends’), Default (Classes in the same package), Private (just the class it is in).

Question 11

What can interfaces contain (5 things)

Answer 11

Constants, method signatures, default methods, static methods, nested types. (Method bodies exist only for default and static methods).

Question 12

Can interfaces be instantiated?

Answer 12

No, they can only be implemented by a class or extended by another interface.

Question 13

What is the difference between a concrete and an abstract class?

Answer 13

An abstract class can not be instantiated. It is declared using the abstract keyword. A concrete class is on one that is not abstract.

Question 14

How do you create an iterator for an Arraylist called employees with abstract superclass called SalesEmployee?

Answer 14

import java.util.Iterator;
Iterator itr = employees.iterator();
while (itr.hasNext()){
System.out.println(itr.next());

Question 15

How do you print out toString of objects in an Arraylist called employees of type SalesEmployee, using an enhanced for loop?

Answer 15

for (SalesEmployee temp : employees) {

System.out.println(“Now: “ + temp);

Question 16

How do you declare an interface?

Answer 16

interface MyInterface { }

Interfaces are explicitly public and abstract so there is no need to declare them as such.

Question 17

How do you abstract classes and interfaces differ?

Answer 17

A class can implement many interfaces but extend just 1 abstract class. An abstract class can have code that implements, an interface cannot.

Question 18

Create an ArrayList of type SalesEmployee.

Answer 18

import util.java.ArrayList;
import util.java.List;
List employees = new ArrayList();

Question 19

When do you use serialization over writing a plain text file?

Answer 19

If your data will be used only by the java program that generated it use serialization.

Question 20

Why is the serializable interface known as a marker or tag interface?

Answer 20

It doesn’t have any methods to implement.

Question 21

What is the keyword for an instance variable that can’t be saved (serializable).

Answer 21

transient

Question 22

What happens to transient variable when they are deserialized?

Answer 22

null for objects, default for primitives.

Question 23

Write out code for serializing an arraylist called employees

Answer 23

Object must implement Serializable and import java.io.Serializable;

import java.io.FileOutputStream;
import java.io.ObjectOutputStream;

try {
		FileOutputStream fileStream = new FileOutputStream("EmployeeInfo.dat");
		ObjectOutputStream os = new ObjectOutputStream(fileStream);
		os.writeObject(employees);
		os.close();

	} catch (Exception e) {
		e.printStackTrace();
	}

Question 24

Write out code deserializing an arraylist of type SalesEmployee.

Answer 24

import java.io.FileInputStream;
import java.io.ObjectOutputStream;

try {
			FileInputStream fileStream = new FileInputStream("EmployeeInfo.dat");
			ObjectInputStream os = new ObjectInputStream(fileStream);
			List employees2 = (List)os.readObject();
			for (SalesEmployee element : employees2) {
				System.out.println(element);
			}
			os.close();
		} catch (Exception e) {
			e.printStackTrace();
		}

Question 25

Can an inner class use it’s outer class private methods and variables?

Answer 25

Yes.

Question 26

Why use inner classes (3 reasons)?

Answer 26

It increases encapsulation.
It leads to more readable and maintainable code.
It is a logical way to embed a class that is only used by one other class.

Question 27

When should you choose an ArrayList over a LinkedList?

Answer 27

If you want fast random access of the elements.

Question 28

When would you choose a LinkedList?

Answer 28

If fast adding and removing of elements at the end of the list is desired.

Question 29

What does Vector have that ArrayList and LinkedList don’t?

Answer 29

It is thread-safe.

Question 30

How do you sort an ArrayList called employees by implementing the collections sort method?

Answer 30

import java.util.Collections;

Collections.sort(employees);

Question 31

How do you make sure SalesEmployee class is comparable? Also make sure abstract method compareTO is implemented.

Answer 31

public class SalesEmployee implements Comparable{
  public int compareTo(SalesEmployee p){
  return firstName.compareTo(p.getFirstName());
  }
}

Question 32

Add a comparator to a Tester class fro comparing type SalesEmployess using getPPS() method.

Answer 32

import java.util.Comparator;

Nested class:
public static class EmployeeCompare implements Comparator {
		public int compare(SalesEmployee a, SalesEmployee b) {
			return a.getPPS().compareTo(b.getPPS());
		}

In the main method:
EmployeeCompare byPPS = new EmployeeCompare();
Collections.sort(employees, byPPS);