Classes

Question 1

What is the implicit import of every class?

Answer 1

java.lang.object

Question 2

What type of inheritance model Java supports?

Answer 2

Single inheritance model, multiple inheritance model is not allowed for classes (only interfaces)

Question 3

What modifiers do classes have?

Answer 3

final, abstract, sealed, non-sealed, static

Question 4

What is relationship of “this” or “super” and static?

Answer 4

“this” and “super” is instance keyword, so it is not available to static members of a class

Question 5

How do we create immutable objects?

Answer 5

class needs to be final, all constructors must be private, no setters, all instance variables must be private and all objects related to class must be secure from changes

Question 6

What is defensive copy in context of classes?

Answer 6

Creating copy of a object via new instantiation and passing to it a object that is being “protected”

Question 7

What is a default constructor?

Answer 7

If we dont define any constructor, compiler auto. adds default constructor to class, which auto. instantiates member fields. It is a constructor with same name as class and contains no parameters.

Question 8

What is “this()”?

Answer 8

It is a call to constructor from current instance. It can only be used from within constructor, and can call constructor from instance, but with different signature.

Question 9

What do we need to watch out for when one of the parent classes is not using default constructor?

Answer 9

If one of the parent classes has defined a different constructor, all child classes must implement constructors that have same signature as that parent’s constructor class

Question 10

Is it allowed to have multiple explicit calls from constructor to another constructor?

Answer 10

No, only one - super() or this()

Question 11

Is var allowed as constructor parameter type?

Answer 11

No

Question 12

What is super()?

Answer 12

Call to parent class constructor. It is auto. added as first line in every constructor, but we can add it yourself also.h

Question 13

What do we need to watch out regarding classes signatures?

Answer 13

When using abstract keyword, it must come before class keyword

Question 14

What can only be abstract in classes?

Answer 14

Classes and methods, no variables, no static members or constructors

Question 15

When do we must mark a class as abstract?

Answer 15

When we have at least one abstract method inside a class

Question 16

What is the rule for abstract class signature?

Answer 16

It cannot be static or final, must contain abstract, and it can be placed before or after access modifier

Question 17

What do we need to watch out regarding method signatures when using abstract?

Answer 17

When using abstract keyword, it must come before method return type. abstract keyword can be omitted if abstract is used in class signature

Question 18

What are rules for abstract classes instantiation?

Answer 18

It cannot be instantiated

Question 19

What are rules regarding constructors in abstract class?

Answer 19

Abstract class has them, all rules apply same as concrete class,

Question 19

What are rules for abstract classes inheritance?

Answer 19

They can extend other abstract or concrete classes.
Class that extends abstract class must implement all methods from its parent abstract classes, except if this class is being extended by another class. In other words, any concrete class must implement abstract methods

Question 20

What are rules regarding non-abstract methods in abstract class?

Answer 20

Abstract classes can have normal non-abstract methods

Question 21

Can private methods be overridden?

Answer 21

No

Question 22

Can static methods be overridden?

Answer 22

No, but they can be hidden with the same name and signature in child class. So each static method in parent class needs to be static in child class

Question 23

Can final methods and variables be overridden or hidden?

Answer 23

No

Question 24

Can variables be overridden?

Answer 24

No, they can be hidden. Child class defines variable with same name as in parent class.

Question 25

What are rules on overriding a method?

Answer 25

• method signature must be the same
• return type must be the same or a covariant (broader to narrower)
• if it throws a exception, it can be omitted, or it must be the same or covariant of it
• access modifier must be the same or broader (protected -> public)

Question 26

How can final fields be initialized?

Answer 26

Via init blocks or constructor

Question 27

Do final fields need to be initialized?

Answer 27

Yes, before code exits constructor

Question 28

Can fields be null?

Answer 28

Yes, only if they are objects

Question 29

When is main method ran?

Answer 29

After class initialization

Question 30

Class is being initialized in two phases …

Answer 30

1. Class initialization - static variables in all parent classes and current class
2. Instance initialization - all other

Question 30

How is class being initialized?

Answer 30

Class initialization goes as:
Upmost parent class is being initialized first - all static fields and static blocks are being initialized from top to bottom, as they appear.
Then initialization proceeds to next child class and does the same all down to the starting child class.
Static variables are initialized once.
super() are not being called.

Output would be eg.:
3 static block
2 static block
1 static block

Question 31

How is class instance being initialized?

Answer 31

Instance initialization goes as:
Upmost parent class is being initialized first - all static fields and static blocks are being initialized from top to bottom, as they appear.
Then initialization proceeds to next child class and does the same all down to the starting child class.
Static variables are initialized once.
Then init blocks are being run from upmost parent class from top to bottom, as they appear. After that, constructor is being run. This process proceeds to next child class and does the same all down to the starting child class.
super() are being called before init blocks in upper classes.

Output would be eg.:
3 static block
2 static block
1 static block
3 init block
3 constructor
2 init block
2 constructor
1 init block
1 constructor