Week 10

Question 1

What is an interface in Java and what does it contain?

Answer 1

A collection of abstract methods and constants, it does not contain any method body’s

Question 2

T/F - All methods in an interface are abstract by default

Answer 2

True

Question 3

What is an interface in Java and what does it contain?

Answer 3

No, an interface cannot be instantiated in Java

Question 4

What is the visibility of methods in an interface by default?

Answer 4

Public

Question 5

How does a class formally implement an interface in Java?

Answer 5

By stating “implements” in the class header and implementing each method

Question 6

What happens when a class states that it implements an interface?

Answer 6

It must define all the methods in the interface, or there will be a compilation error

Question 7

Can an interface contain constants? How do they work in a class that implements the interface?

Answer 7

Yes. When class implements an interface it gains access to all the constants defined in that interface

Question 8

Why would a class implement multiple interfaces? How would the class do this?

Answer 8

class MyClass implements interface1, interface 2

Question 9

All objects of a class that implements multiple interfaces have what type fo a relationship?

Answer 9

An is-a relationship with each implemented interface type

Question 10

4 Benefits of interfaces?

Answer 10

Help achieve abstraciton
Support dynamic method resolution (at runtime)
Achieve loose coupling
Separate definition of a method from the inheritance hierarchy

Question 11

How are interfaces different from classes?

Answer 11

They contain only constants and abstract methods AND all interface members are implicitly public

Question 12

How can interfaces be used for polymorphism?

Answer 12

2 classes implement an interface = Allow objects of unrelated classes to be processed polymorphically, they respond to the same method calls

Question 13

Interface vs abstract class

Answer 13

Interface can only contain constants and abstract methods, an abstract class can contain concrete methods and instance variables

Question 14

T/F - In an abstract class access modifiers do not need to be explicitly defined

Answer 14

False

Question 15

T/F - Interfaces standardize operations, objects figure out how to implement them

Answer 15

True

Question 16

T/F - an interface must be declared in a file with the same name as the interface, and a .java filename extension

Answer 16

True

Question 17

Can an abstract class have default and static methods?

Answer 17

Yes

Question 18

In an interface all variables declared are by default _____, an abstract class does not have this restriction

Answer 18

Final

Question 19

T/F - An abstract class can have final, non-final, static and non-static varaibles

Answer 19

True

Question 20

T/F - an interface has final, non-final, static and non-static variables

Answer 20

False. Interfaces only have static and final

Question 21

Can an interface extend another interface? How about an abstract class?

Answer 21

Yes. An abstract class can extend another class and implement multiple interfaces?

Question 22

T/F - When naming interface methods, you should choose a general name for it, as many unrelated classes may use the same method when implemented

Answer 22

True

Question 23

In a UML, what is special about an interface?

Answer 23

«interface» will be above the interface name

Question 24

How does UML express the relationship between a class and an interface? What does this look like?

Answer 24

Through realization. It’s a dashed arrow with a hollow arrowhead, point from the implemented class to the interface

The class is said to “realize” methods of the interface

Question 25

T/F - A subclass inherits its superclass’ realization relationships

Answer 25

True

Question 26

What is a functional interface in Java and what is its significant in Java SE 8?

Answer 26

Functional interface = interface that contains only 1 abstract method. Significant in Java SE 8 because it allows the use of “lambda” expressions

Question 27

Give 3 examples of functional interfaces

Answer 27

ActionListener, Comparator, Runnable

Question 28

This functional interface defines a method that is called when a user clicks a GUI button

Answer 28

ActionListener

Question 29

This functional interface defines a method that can compare 2 objects of a given type (can be useful for sorting and ordering)

Answer 29

Comparator

Question 30

This functional interface defines tasks that can be executed concurrently with other parts of the program, often used in multi-threaded applications

Answer 30

Runnable

Question 31

What is the purpose of the default keyword in interface methods?

Answer 31

A method has a default implementation in case it is not overridden in the implementing class. Allows for backward compatibility when new methods are added to an interface

Question 32

“Program to an interface, not an implementation” means what?

Answer 32

Write code that relies on interfaces rather than concrete implementations of classes

Question 33

Interface inheritance vs implementation inheritance

Answer 33

Interface inheritance = you must implement an interfaces abstract methods

implementation inheritance = extending classes, super/subclasses

Question 34

Althought interface inheritance requires more work than implementation inheritance, which one is more flexible?

Answer 34

Interface inheritance, as it eliminates tight coupling between classes

Question 35

When you use a varaible of an interface type, you can assign it an object of any type that implements the interface ________ or _________

Answer 35

directly or indirectly

Question 36

______ _______ are a good example of how interfaces enable systems to be modified easily

Answer 36

Device drivers

Question 37

What type of relationship do subclass objects have with the superclass?

Answer 37

An “is-a” relationship