Final

Question 1

What is inheritance?

Answer 1

When a new class is derived from an existing class

Question 2

The superclass _______ the subclass

Answer 2

The superclass generalizes the subclass

Question 3

In the UML diagram what is used to describe a sub/super class relation?

Answer 3

an arrow pointing from the sub class to the super class

Question 4

What does the subclass inherit from the superclass?

Answer 4

The subclass inherits all methods and fields

Question 5

What does Overriding mean?

Answer 5

The subclass can have its own version of the super class’s method. The subclass method must have the exact same signature.
Can use ‘@Override’ to avoid errors.

Question 6

What is the Java syntax for Inheritance?

Answer 6

public class Shape{
protected String name;
}

public class Circle extends Shape{
private double radius;
}

Question 7

Benefits of class inheritance?

Answer 7

One superclass can be reused by many subclasses, allows for DRY code.

Improves the structure of large programs(hierarchical).

Modular Programming(encapsulation).

Question 8

Given a class called Circle that is a subclass of Shape.

if Shape has a method called draw() and Circle overrides that method, how would you reference the parent class overridden method?

Answer 8

super.draw() can be used to call the overridden parent method

Question 9

Can a subclass inherit private fields/methods from a method?

Answer 9

No, private fields and methods are only accessible to the class it is declared in

Question 10

Are the following subclasses the same?
a. public class A extends B{
}

b. public class A extends B{
Public A(){
super();
}
}

Answer 10

yes they are equivalent, in the first example Java will implicitly imply the super. keyword to invoke the parent constructor.

Question 11

In the UML diagram what is used to reference generalization?

Answer 11

an Arrowhead that points to the Superclass

Question 12

Are superclass constructors inherited?

Answer 12

No, superclass constructors are invoked either implicitly or explicitly.

Explicitly using the super() keyword

Question 13

Who do these visibility modifiers grant access to?

a. Public

b. Private

c. none

d. Protected

Answer 13

a. All objects anywhere

b. Only objects of the same class

c. All objects of the same package

d. Only objects of the same subclass( or same package)

So from most visibility to least visibility is the following:

Public -> Protected -> none -> Private

Question 14

Where does the super() keyword when being referenced in a child constructor?

Also given a parent class Person which has a protected field String name. Write the syntax for a constructor of the child class called Student with private int gpa.

Answer 14

The first line of the child constructor

public Student(String name, int gpa){
super(name)
this.gpa = gpa
}

Question 15

What is the difference between single and multilevel inheritance?

Answer 15

single level inheritance is when methods/fields are being inherited from one level above

Multilevel inheritance is when methods/fields are inherited from non parent classes(parents of their parents)

Question 16

Given the following class and constructor

public class A{
private int num;

public A(int num){
this.num = num;
}
}

Would a child class of this need to call the constructor?

Answer 16

public class B extends A{

public B(int num, String name){
super(num);
this.name = name;
}
}

Yes when the parent class doesn’t use the default constructor you must explicitly invoke the super() constructor.

Question 17

True or False?

When a derived object is created
the “super portion”
defined in the superclass
must also be
created and initialized?

Answer 17

True

Question 18

What is the Object class? Also provide all methods it provides

Answer 18

All classes in Java are subclasses of the Object class whether they extend it or not.

toString()
equals()
getClass()
hashCode()
finalize()
…
Note: these methods can be Overriden

Question 19

True or False?

All objects in Java instantiate the Object class.

Answer 19

True

Question 20

Applying the final modifier to a class, who can inherit it?

How about applying final to one of its methods instead?

Answer 20

When applying final to a class, that class can no longer be extended(inherited).

When applying final to a method that method can no longer be overridden.

Question 21

Why is the String class final?

Answer 21

The String class is immutable, therefore its objects cannot be modified after creation. Another reason for String being final is for security.

Question 22

Given a subclass that extends an abstract class, what must the subclass do otherwise a Java error occurs?

Answer 22

Assuming the subclass is not also an abstract class, the subclass must implement all abstract methods from the parent class, like so:

in parent: public abstract double calcArea()

in child: public double calcArea()

Question 23

What can hold abstract methods?

Answer 23

Abstract methods must be in an abstract class or interface, While an abstract class can hold non abstract methods

Question 24

Can a subclass override a method and make it abstract?

Answer 24

yes

Question 25

Can you instantiate a abstract class using the new keyword?

How about using it as a datafield?

Answer 25

You cannot instantiate an object of an abstract class

You can specify an abstract class as a data field.

Question 26

Explain the relation between java.util.Calendar
and java.util.GregorianCalendar

Answer 26

Calendar is an abstract class, and gregeorianCalendar is a concrete subclass of Calendar. Contains the get method for objects of the Calendar class.

GregorianCalender is formatted like year,month, date where month parameter is 0-based. January is 0.

Question 27

Why is an interface useful?

Answer 27

An interface is a blueprint that contains abstract methods and constants. The intent of an interface is to specify common behavior for objects.

note: Any fields declared in an interface are implicitly static and final

Examples of common interfaces(behaviours): Comparable, Edible, and Cloneable

Question 28

Create an interface called Edible, that has a string method called howToEat

Answer 28

public interface Edible {
public abstract String howToEat();
}

Question 29

Why are interfaces treated like special classes in Java?

Answer 29

Each interface is compiled into a separate bytecode file

Question 30

Given the following interface, what is implicitly implied?

public interface T1{
int NUM = 1;

void p();

}

Answer 30

public interface T1{
public static final int NUM = 1;

public abstract void p();
}

methods are always abstract and fields are always static and final.

Question 31

What does the Comparable interface consist of? and write the Comparable interface.

Also what does the method in it return?

Answer 31

Comes with the compareTo abstract method
package java.lang;

public interface Comparable<E> {
public int compareTo (E o);
}</E>

compareTo returns negative if object is less than o. returns 0 if object is equal to o. returns positive if object is greater than o.

Question 32

Write a simple class called Dog that implements the comparable class and has a name field and has the compareTo method

Answer 32

public class Dog implements Comparable< Dog > {
private String name;

@Override
public int compareTo(Dog other) {

Question 33

What is the bytesize of the following datatypes?

a. int
b. long
c. float
d. double
e. char

Answer 33

a. 4
b. 8
c. 4
d. 8
e. 2

Question 34

What are two classes we use that implement the Comparable interface by default.

Answer 34

The Integer and BigInteger Classes

Question 35

Write code in the main method to compare integers 3 and 5 without defining objects.

Answer 35

System.out.println(new Integer(3).compareTo(new Integer(5))

//This should output a negative int

Question 36

What does instanceof do and assume n is a Integer object write code checking n using instanceof

Answer 36

if(n instanceof Integer){
}

Question 37

What does the Cloneable interface do, and what are it’s contents?

Answer 37

Cloneable interface is used to clone an object.

Here are the contents:
package java.lang;
public interface Cloneable {
}

Yes the cloneable interface is empty. An empty interface is known as a marker interface.

Question 38

Write a class called Animal has a age field, has a explicit constructor and implements the cloneable interface.

Answer 38

public class Animal implements Cloneable {
private int age;

public Animal(int age) {
    this.age = age;
}

@Override
public Animal clone() {
    try {
        return (Animal) super.clone();
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

Question 39

What is conflicting interfaces?

Answer 39

On rare occasions, a class may implement two interfaces with conflict information (e.g., two same constants with different
values or two methods with same signature but different
return type). This type of errors will be detected by the
compiler.

Question 40

What is Polymorphism

Answer 40

Essentially the idea of polymorphism is that an object of a subtype can be used wherever it’s supertype is required.
Example1 : Animal a1 = new Dog()

or

public static void m(Object x){
}

m(a1)
Notice the m method is stated to take the Object type, but since Object is the superclass of all subtypes we can use any class to substitute.

Question 41

How does dynamic binding work?

Answer 41

If an object invokes a method, Java will first search the most specific subclass, and then go up the hierarchy of more general classes(parents and parents of parents, etc) until it finds a class that defines the method.

Question 42

Does Java support Multiple Inheritance?

Answer 42

Java doesn’t support traditional Multiple Inheritance, as attempting to extend more than one class will cause a compile error.

Multiple Inheritance can be achieved through Interfaces.

Question 43

A java Class will implement interfaces, but what is it called when an interface is trying to use another interface?

Answer 43

Interface extends other interfaces. Note that an interface can extend multiple interfaces.

Question 44

What does a Collection mean? and provide examples

Answer 44

A collection is a container object that holds
a group of objects, often referred to as
elements.

Sets
Lists(Arraylist, LinkedList)
Maps(Key value pairs)
Stacks(First in, Last out)
Queues(First in, first out)
PriorityQueues

Question 45

What are some of the methods of the Collection interface?

Answer 45

clear() - removes all elements from this collection
add() - adds a new element to collection
addAll() - adds all elements from a collection to this one
remove() - remove element from collection
removeAll() - remove all elements that are in that collection from current collection
retainAll() - Only keep elements that are common in both collections

Note: AbstractCollection is an abstract class that partially implements Collection

Question 46

Given Collection A and Collection B,
how would you:
a. Remove all elements from A?

b. Add all elements from B to A?

c. Remove all elements in A from B

d. Only keep common elements that both A and B have

Answer 46

a. A.clear()

b. A.addAll(B)

c. B.removeAll(A)

d. A.retainAll(B)

Question 47

What are the classes of the List interface? and common methods?

Answer 47

ArrayList and LinkedList

Common Methods:
add(index, element)
addAll(index, collection)
remove(index)
set(index, element) - changes value of index(removes old value)
indexOf(element)
lastIndexOf(element)
subList(fromIndex, toIndex) - creates another list based on indexes, inclusive.

Question 48

Given the following:
List< String> list
Assuming it has values how would you output all elements of the list traversing forward?

Answer 48

ListIterator< String> iterator = list.listIterator();

while(iterator.hasNext()){
System.out.println(iterator.next());
}

Question 49

Given the following:
List< String> list
Assuming it has values
How to traverse the list backwards using ListIterator?

Answer 49

ListIterator< String> iterator = list.listIterator(list.size());

while(iterator.hasPrevious()){
System.out.println(iterator.previous());
}

Question 50

Given two versions of the same class

A. public class Circle{
double radius
public boolean equals(Circle c){
return this.radius == c.radius
}
}

B. public class Circle{
double radius
public boolean equals(Object o){
return this.radius == ((Circle)o.radius)
}
}

If this is the main:
Object o1 = new Circle();
Object o2 = new Circle ();
System.out.println(o1.equals(o2))

What would output of versions A and B be?

Answer 50

A. would be False
In the main o2 is defined as Object class, so with polymorphism it can still be passed in as a Circle, when being compared using == , without being casted using (Circle) the == operator returns false

B. would be True
since the o2 being passed into the equals method is casted as Circle, the comparison would be between two of the same classes.

Question 51

What is an inner class?

Answer 51

A class in a class, the inner class can use private fields/methods of the class it exists in.

It is compiled like so: OuterClass$InnerClass.class.

An inner class can be declared static

Question 52

True or False?
If a method is declared private in the superclass, you may declare the method protected in the subclass.

Answer 52

True,
the methods would be unrelated to each other, not overridden. The private modifier would prevent the subclass from inheriting it, allowing the subclass to create it’s own protected version of the method.