Interfaces

Question 1

What access modifiers do interfaces accept?

Answer 1

Only package and public

Question 2

Why interfaces don’t have protected methods?

Answer 2

Because they are not extended by classes

Question 3

Why interfaces don’t have package access methods?

Answer 3

Legacy reasons

Question 4

Can interfaces have concrete methods?

Answer 4

Yes - default, static, private, private static

Question 5

When overriding an abstract method, what are rules for return type?

Answer 5

Return type must be the same, or covariant

Question 6

How do interfaces differ from abstract classes?

Answer 6

Interfaces have implicit modifiers, abstract classes do not

Question 7

What is implicit regarding interfaces variables?

Answer 7

They are “public final”

Question 8

What is implicit regarding interfaces?

Answer 8

They are abstract

Question 9

What is implicit regarding interfaces methods?

Answer 9

Methods without bodies are “abstract”
Methods without “private” access modifier are “public” - if something else is defined -> compile error

Question 10

What if different interfaces try to implement methods with same signatures?

Answer 10

Compile error because compiler doesn’t know which method to override or implement
eg.
interface A -> void int a()
interface B -> int eat()

Question 11

What are methods inside interface?

Answer 11

They are abstract and public

Question 12

What attributes do static methods inside interface have?

Answer 12

They are public, and cannot be abstract or final

Question 13

How do we run static methods in interfaces?

Answer 13

Via interface name -> Interface.method()

Question 14

Can default methods be overridden?

Answer 14

Yes

Question 15

How can we access overridden (or hidden) default method from implemented interface?

Answer 15

InterfaceName.super.method()

Question 16

Can static methods be overridden?

Answer 16

No

Question 17

What if two different interfaces that are being implemented have same default method?

Answer 17

Then that method needs to be implemented in class

Question 18

What if two different interfaces that are being implemented have default method with same signature, but different return type?

Answer 18

Compiler error, since it doesn’t know which method to implement, and both of them need to be implemented.

Question 19

What if two different interfaces that are being implemented have method with same signature, but different return type?

Answer 19

Compiler error, since it doesn’t know which method to implement, and both of them need to be implemented.

Question 20

What if two different interfaces that are being implemented don’t have same default methods?

Answer 20

Then none of default methods don’t need to be implemented in class

Question 21

What are rules for default methods?

Answer 21

• Can be overridden
• Cannot be static, final, abstract
• Must have a method body
• Implicitly public, cannot be private or protected

Question 22

What do we need to watch out when implementing multiple interfaces?

Answer 22

For double methods