Class design

Question 1

Inheritance

Answer 1

Inheritance is the process by which the new child subclass automatically includes any public or protected primitives, objects, or methods defned in the parent class.

Question 2

Java supports single inheritance, by which a class may inherit from only one direct parent class.

Answer 2

Java doesn’t support multiple inheritance!

Question 3

Java allows only one public class per file

Answer 3

true

Question 4

public access modifer

Answer 4

The public access modifer applied to a class indicates that it can be referenced and used in any class.

Question 5

default package private modifer

Answer 5

The default package private modifer, which is the lack of any access modifer, indicates the class can be accessed only by a subclass or class within the same package.

Question 6

java.lang.Object

Answer 6

java.lang.Object is the only class that doesn’t have any
parent classes.

Question 7

The first statement of every constructor is a call to another constructor within the class
using this(), or a call to a constructor in the direct parent class using super().

Answer 7

true

Question 8

The super() call may not be used after the first statement of the constructor.

Answer 8

true

Question 9

If no super() call is declared in a constructor, Java will insert a no-argument super()
as the first statement of the constructor.

Answer 9

true

Question 10

If the parent doesn’t have a no-argument constructor and the child doesn’t define any constructors, the compiler will throw an error and try to insert a default no-argument constructor into the child class.

Answer 10

true

Question 11

If the parent doesn’t have a no-argument constructor, the compiler requires an explicit
call to a parent constructor in each child constructor

Answer 11

true

Question 12

Hiding Static Methods

Answer 12

A hidden method occurs when a child class defnes a static method with the same name and signature as a static method defned in a parent class.

Question 13

Overriding vs. Hiding Methods

Answer 13

Unlike overriding a method, in which a child method
replaces the parent method in calls defned in both the parent and child, hidden methods only replace parent methods in the calls defned in the child class.

Question 14

final methods cannot be overridden.

Answer 14

true

Question 15

Hiding Variables

Answer 15

When you hide a variable, you defne a variable with the same name as a variable in a parent class.
This creates two copies of the variable within an instance of the child class: one instance defned for the parent reference and another defned for the child reference.

Question 16

abstract class

Answer 16

An abstract class is a class that is marked with the abstract keyword and cannot be instantiated

Question 17

abstract method

Answer 17

An abstract method is a method marked with the abstract keyword defned in an abstract class, for which no implementation is provided in the class in which it is declared.

Question 18

an abstract method may only be defned in an abstract class.

Answer 18

+

Question 19

concrete class

Answer 19

A concrete class is the frst nonabstract subclass that extends an abstract class and is required to implement all inherited abstract methods.

Question 20

A class may implement multiple interfaces, each separated by a comma

Answer 20

+

Question 21

interface

Answer 21

An interface is an abstract data type that defnes a list of abstract public methods that any class implementing the interface must provide.

Question 22

a class can implement an interface, a class cannot extend an interface.

Answer 22

but

an interface can extend another interface, an interface cannot implement another interface.

Question 23

Interface variables

Answer 23

Interface variables are assumed to be public, static, and final

Question 24

The value of an interface variable

Answer 24

The value of an interface variable must be set when it is declared since it is marked as final.

Question 25

Default Interface Methods

Answer 25

A default method is a method defned within an

interface with the default keyword in which a method body is provided

Question 26

Static Interface Methods

Answer 26

A static method defned in an interface is not inherited in any classes that implement the interface.

Question 27

Any field in an interface is implicitly public, static, and final, whether these keywords are specified or not.

Answer 27

+

Question 28

varialbles are hidden, methods are overriden.

Answer 28

true

Question 29

a interface can have static method but method must have body in that case

Answer 29

+

Question 30

Overriding a Method

Answer 30

You can override a method a method by declaring a new method with the signature and return type as the method in the parent class.

When you override a method, you may reference the parent version of the method using the super keyword.

Question 31

Hiding Variables

Answer 31

When you hide a variable, you defne a variable with the same name as a variable in a parent class. This creates two copies of the variable within an instance of the child class:
one instance defned for the parent reference and another defned for the child reference.

Question 32

Fields defined in an interface are ALWAYS considered as public, static, and final. Even if you don’t explicitly define them as such.

Answer 32

+

Question 33

An interface method cannot be default and static at the same time because a default method is always an instance method.

Answer 33

+

Question 34

Trying to override a static method with a non-static method (and vice-versa) in a class will result in a compilation error.

Answer 34

+