2 Object Orientation

Question 1

How do you use encapsulation in your code?

Answer 1

Keep instance variables hidden (often with private). If necessary provide public accessor methods according to the naming convention.

Question 2

What are the fancy names for getters and setters?

Answer 2

accessors and mutators

Question 3

What are the advantages of providing ‘empty’ getters and setters?

Answer 3

Future maintainability: You can add logic to them later without changing the public API of the class

Question 4

Every Java class is a subclass of …

Answer 4

Object

Question 5

Mention some methods which every Java class inherits from Object.

Answer 5

equals, hashCode, toString, clone, notify, wait

Question 6

How can you create inheritance relationships in Java?

Answer 6

extending a class; implementing an interface (since Java 8)

Question 7

Which elements of a class can be inherited and which not?

Answer 7

instance variables, static (class) variables, methods: abstract, instance, static; NOT: constructors, initialization blocks

Question 8

Which elements of an interface can be inherited?

Answer 8

constants = static final variables, methods: abstract, default, static

Question 9

What determines which methods of an object you can call?

Answer 9

the declared type. And NOT the runtime type.

Question 10

Which operator checks for an ‘IS-A’ relationship?

Answer 10

instanceof

Question 11

What is a polymorphic object?

Answer 11

An object which passes instanceof for multiple types. Thus any object which is not declared as Object.

Question 12

What can change and what can not with a reference variable?

Answer 12

not changeable: its type; changeable: the object it refers to (exception: final variables)

Question 13

What does polymorphic method invocation mean?

Answer 13

JVM knows the real type of the executing object and executes the overridden version of an instance method, if there is one. Scope: only instance methods. No static methods, no variables.

Question 14

Which methods can be overridden?

Answer 14

Every inherited method except the final ones.

Question 15

What about the access level of overriding methods?

Answer 15

It can be the same or less restrictive than the superclass’ method.

Question 16

Can a subclass in the same package override a default / package method of the superclass?

Answer 16

yes

Question 17

Which exceptions can an overriding method throw?

Answer 17

arbitrary runtime exceptions. The checked exceptions thrown by the super method or narrower exceptions.

Question 18

What happens, if a class implements two interfaces which both have a default method with the same signature?

Answer 18

The class has to override the method. Otherwise compiler error.

Question 19

Can you implement an interface method with a different return type?

Answer 19

Yes, with a subtype.

Question 20

What’s the term for reusing the name of a method?

Answer 20

overloading

Question 21

What does covariant return mean?

Answer 21

Overriding a method and declaring a subtype as return type

Question 22

What can a method with a primitive return type return?

Answer 22

Any value or variable which can be * implicitly converted * explicitly cast to the declared return type

Question 23

Can an abstract class have a constructor?

Answer 23

Yes. It must.

Question 24

What does constructor chaining mean?

Answer 24

A constructor always calls a super constructor. Default: implicitly the one without arguments. Directly or via this and another constructor of the same class.

Question 25

What can be the 1st statement of a constructor?

Answer 25

this, super

Question 26

What can you use as an argument to a super or this call?

Answer 26

only static variables and methods

Question 27

What is the access modifier of the default constructor?

Answer 27

The same as for the class.

Question 28

What happens in the case of circular reference?

Answer 28

StackOverflowError

Question 29

When does the code in an instance initialization block run?

Answer 29

after all super constructors have been called

Question 30

Where can you put code that performs operations?

Answer 30

methods, constructors, initialization blocks

Question 31

When does the code in a static initialization block run?

Answer 31

Once. When the class is first loaded.

Question 32

Can you have multiple initialization blocks in a class?

Answer 32

Yes. They are run in the order of appearance.

Question 33

What happens if an Exception is thrown in a static init block?

Answer 33

java.lang.ExceptionInInitializerError