2. Object Orientation

Question 1

What is good encapsulation?

Answer 1

private instance variables with public get and set (accessor) methods

Question 2

Can a subtype of an argument be passed?

Answer 2

Yes

Question 3

If car extends vehicle, what is the relationship?

Answer 3

Car IS-A vehicle

Question 4

If Car has a clutch variable, what is the relationship?

Answer 4

Car HAS-A clutch

Question 5

What can a reference variable refer to?

Answer 5

An object of the same type or an object of the subtype of the declared type ie Animal a1 = new horse();

Question 6

Can a reference variable be declared as a class type of interface type?

Answer 6

Both

Question 7

If a reference variable is referred to as an interface type, what objects can it reference?

Answer 7

Any objects of any class which implements the interface (it passes the IS-A relation test)

Question 8

What are the two ways to pass the IS-A relationship test?

Answer 8

If a class extends another class
If a class implements another class

Question 9

What does the compiler do at runtime with regard to object methods? For example:

Gamer p = new Player();
p.domethod();

Answer 9

The domethod() of Player object will be called at runtime rather than the Gamer version (This only applies to instance methods, not static methods and not variables)

Question 10

What will happen if a static method is called from an object declaration?

Answer 10

It will use the reference to get the method, and not the object method.

Question 11

What will happen is an variable is called from an object declaration?

Answer 11

It will get the reference variable, not the object variable.

Question 12

Which are called by the object and not by the reference during run time?

variables
methods
static methods?

Answer 12

methods only

Question 13

When can methods not be overriden?

Answer 13

When they are marked final

Question 14

Do you have to implement an abstract method?

Answer 14

The first concrete class must implement all parent abstract methods

Question 15

Can a method be invoked on the object side at runtime if it does not already exist and have been overridden on the reference side.

Answer 15

No, a reference version must exist for compilation to succeed.

Question 16

Can you override a method with a more restrictive access modifier?

Answer 16

No

Question 17

Can a overridden method have a different argument list?

Answer 17

No, that would cause it to be a overloaded method.

Question 18

What are the rules for the return type of an overridden method?

Answer 18

They must be the same, or subtype of, the overridden return type.

Question 19

Can the access level of an overridden method be less restrictive?

Answer 19

Yes

Question 20

Can overriding methods throw new checked exceptions?

Answer 20

No

Question 21

Can overriding methods throw new unchecked exceptions?

Answer 21

Yes

Question 22

Can you override methods that are marked static or final?

Answer 22

No

Question 23

How do you invoke the superclass method within the base class?

Answer 23

super.supermethod();

Question 24

Can you use super.super.supermethod() to invoke the grandparent class?

Answer 24

No.

Question 25

What must you make sure when overriding and calling a method that the original method has a checked exception?

Answer 25

That even if the base method isn't being called, that the checked exception is still caught by the overridden method

Question 26

What must a method to be an overloaded method?

Answer 26

It must change the argument list

Question 27

Can overloaded methods change the return type?

Answer 27

Yes

Question 28

Can overloaded methods change the access modifier?

Answer 28

Yes

Question 29

Can overloaded methods declare new or broader exceptions?

Answer 29

Yes

Question 30

public void doStuff(int y, String s){} public void doStuff(int y, long p){} Is the second method an override or an overload?

Answer 30

It's an overload, the argument list has changed

Question 31

When is the choice of which overloaded method decided?

Answer 31

At compile time - very nb | All about the reference, not the object

Question 32

Can main() be overloaded?

Answer 32

Yes, but the only special one is the one that is not overloaded

Question 33

if (animal instanceof Dog) { Dog d = (Dog) animal; d.playDead(); } What is this an example of?

Answer 33

Downcasting - casting from generics down to specifics

Question 34

if (animal instanceof Dog) { Dog d = (Dog) animal; d.playDead(); } Will this necessarily compile and execute?

Answer 34

It will compile but will probably not run. | Will probably throw a ClassCastException

Question 35

Can you upcast implicitly?

Answer 35

Yes ``` Dog d = new Dog(); Animal a1 = d; ``` Dog can do all Animal can do, hence no issue

Question 36

What is an interface?

Answer 36

A contract of what methods you must implement, but no rules on how they should be implemented

Question 37

Can you declare checked exceptions on implementation methods other than those declared in the interface?

Answer 37

No

Question 38

What happens if the class which is implementing the interface is abstract?

Answer 38

The buck to implement the methods is passed along to the next concrete class

Question 39

Do interfaces extend or implement other interfaces?

Answer 39

Extend

Question 40

Can an interface extend many interfaces?

Answer 40

Yes

Question 41

Can an interface extend a class?

Answer 41

No

Question 42

Can you change the return type of an overloaded method?

Answer 42

Yes

Question 43

What must be done to an overloaded method?

Answer 43

The argument list must be changed

Question 44

``` void go(){}; String go(){}; ``` Will this compile if the second is declared in a subclass?

Answer 44

No, it's not overloading properly, it needs to change the argument list. Or it needs to use the same return type to override

Question 45

Can an overrode method return anything except for what is return in the super method?

Answer 45

Yes, it can return a subtype of the return of the overrode method.

Question 46

Will this compile? public Button doStuff(){ return null; }

Answer 46

Yes, you can return null to object references

Question 47

Can you return an array?

Answer 47

Yes

Question 48

Will this compile? public int foo(){ char c = 'c'; return c; }

Answer 48

Yes, you can return this because chars can implicitly be converted to ints

Question 49

Will this compile? public int foo(){ float f = 32.5f; return (int) f; }

Answer 49

Yes, you can return values that have been explicitly cast to the return type

Question 50

Will this work? public void bar(){ return; }

Answer 50

Yes, this will work

Question 51

Will this work? ``` public Animal getAnimal(){ return new Horse(); } ```

Answer 51

``` Yes, you can return any object that can be explicitly cast to another Also works with interfaces as the class will therefore pass the IS-A test ```

Question 52

What classes must have constructors?

Answer 52

Every class, including abstract classes

Question 53

Can a constructor have a return type?

Answer 53

No

Question 54

If only constructor is: Foo(String name){} Will this work: Foo f1 = new Foo();

Answer 54

No, because there is no explicit no-arg constructor.

Question 55

What happens when a constructor of a subclass Horse is invoked?

Answer 55

The constructor of the superclass is called (Animal) which is turn calls the constructor of the Object class, then it comes back down, giving out all values as needed

Question 56

What access modifiers can constructors use?

Answer 56

Any

Question 57

Can you have a method with the same name as a constructor?

Answer 57

Yes, be careful of this in the exam

Question 58

What is the default constructor that is generated if their is no constructor typed in?

Answer 58

A no-arg constructor

Question 59

How do you get a no-arg constructor if you've typed in many other overloaded constructors?

Answer 59

You must type it in yourself

Question 60

What does every constructor inherently contain?

Answer 60

A call to an overloaded constructor (this()) or to the superclass constructor (super())

Question 61

Within a constructor, what is being called here (super.(Animal.NAME))

Answer 61

The static variable NAME

Question 62

Can an interface have a constructor?

Answer 62

No

Question 63

How is a constructor invoked?

Answer 63

Only through the call by another constructor

Question 64

If the superclass does not have a no arg constructor, what needs to happen in the base class.

Answer 64

The super method must be fleshed out to pass the required overloaded method parameters

Question 65

What can be called from the constructor?

Answer 65

Only static methods and static variables

Question 66

What is happening here? Animal(){ this(makeRandomName()); }

Answer 66

Animal no arg constructor is making call to other overloaded Animal constructor.

Question 67

What is this and when does it run? static { x= 8; }

Answer 67

A static init block which runs when the class is first loaded.

Question 68

What is this and when does it run? { o = 0 }

Answer 68

An instance init block that runs with each instance of the class

Question 69

What is the order of init blocks?

Answer 69

As they appear in the class, right after the call to super.

Question 70

Do static variables need to be initialised?

Answer 70

No, they get initialised automatically, like instance variables.

Question 71

Can static methods be overridden in a subclass?

Answer 71

No, they would be redefined and different from the static method which has the same name.

Question 72

What are the two features of encapsulation?

Answer 72

- Instance variables are kept protected, usually with the private modifier - Getter and setter methods

Question 73

How is an IS-A relationship expressed?

Answer 73

Through extends or implements

Question 74

How is a HAS-A relationship expressed?

Answer 74

An instance of one class has a reference to an instance of another

Question 75

What determines which methods are called, the reference variable type of the object type?

Answer 75

The reference variable type.

Question 76

What is a covarient return with regards to overridden methods?

Answer 76

Return a subtype of the return type

Question 77

What determines what overridden method is used at runtime?

Answer 77

The object type

Question 78

What determines which overloaded method will be used at compile time?

Answer 78

The reference type

Question 79

How do you explicitly downcast from an object to a String?

Answer 79

Object o = "a String"; | String s = (String) s;

Question 80

How do you properly implement an interface?

Answer 80

By implementing all of it's methods

Question 81

For a method that has a void return type, how do you escape the method quickly?

Answer 81

Simple issue a return;