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JavaTest > 2. Object Orientation > Flashcards

2. Object Orientation Flashcards

1
Q

What is good encapsulation?

A

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

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2
Q

Can a subtype of an argument be passed?

A

Yes

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3
Q

If car extends vehicle, what is the relationship?

A

Car IS-A vehicle

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4
Q

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

A

Car HAS-A clutch

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5
Q

What can a reference variable refer to?

A

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

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6
Q

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

A

Both

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7
Q

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

A

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

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8
Q

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

A 
If a class extends another class
If a class implements another class
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9
Q

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

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

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)

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10
Q

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

A

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

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11
Q

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

A

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

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12
Q

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

variables
methods
static methods?

A

methods only

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13
Q

When can methods not be overriden?

A

When they are marked final

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14
Q

Do you have to implement an abstract method?

A

The first concrete class must implement all parent abstract methods

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15
Q

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.

A

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

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16
Q

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

A

No

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17
Q

Can a overridden method have a different argument list?

A

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

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18
Q

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

A

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

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19
Q

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

A

Yes

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20
Q

Can overriding methods throw new checked exceptions?

A

No

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21
Q

Can overriding methods throw new unchecked exceptions?

A

Yes

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22
Q

Can you override methods that are marked static or final?

A

No

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23
Q

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

A

super.supermethod();

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24
Q

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

A

No.

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25
Q

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

A

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

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26
Q

What must a method to be an overloaded method?

A

It must change the argument list

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27
Q

Can overloaded methods change the return type?

A

Yes

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28
Q

Can overloaded methods change the access modifier?

A

Yes

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29
Q

Can overloaded methods declare new or broader exceptions?

A

Yes

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30
Q

public void doStuff(int y, String s){}
public void doStuff(int y, long p){}

Is the second method an override or an overload?

A

It’s an overload, the argument list has changed

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31
Q

When is the choice of which overloaded method decided?

A

At compile time - very nb

All about the reference, not the object

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32
Q

Can main() be overloaded?

A

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

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33
Q

if (animal instanceof Dog) {
Dog d = (Dog) animal;
d.playDead();
}

What is this an example of?

A

Downcasting - casting from generics down to specifics

34
Q

if (animal instanceof Dog) {
Dog d = (Dog) animal;
d.playDead();
}

Will this necessarily compile and execute?

A

It will compile but will probably not run.

Will probably throw a ClassCastException

35
Q

Can you upcast implicitly?

A

Yes

Dog d = new Dog();
Animal a1 = d;

Dog can do all Animal can do, hence no issue

36
Q

What is an interface?

A

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

37
Q

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

A

No

38
Q

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

A

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

39
Q

Do interfaces extend or implement other interfaces?

A

Extend

40
Q

Can an interface extend many interfaces?

A

Yes

41
Q

Can an interface extend a class?

A

No

42
Q

Can you change the return type of an overloaded method?

A

Yes

43
Q

What must be done to an overloaded method?

A

The argument list must be changed

44
Q 
void go(){};
String go(){};

Will this compile if the second is declared in a subclass?

A

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

45
Q

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

A

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

46
Q

Will this compile?

public Button doStuff(){
return null;
}

A

Yes, you can return null to object references

47
Q

Can you return an array?

A

Yes

48
Q

Will this compile?

public int foo(){
char c = ‘c’;
return c;
}

A

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

49
Q

Will this compile?

public int foo(){
float f = 32.5f;
return (int) f;
}

A

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

50
Q

Will this work?

public void bar(){
return;
}

A

Yes, this will work

51
Q

Will this work?

public Animal getAnimal(){
return new Horse();
}
A 
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
52
Q

What classes must have constructors?

A

Every class, including abstract classes

53
Q

Can a constructor have a return type?

A

No

54
Q

If only constructor is:

Foo(String name){}

Will this work:

Foo f1 = new Foo();

A

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

55
Q

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

A

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

56
Q

What access modifiers can constructors use?

A

Any

57
Q

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

A

Yes, be careful of this in the exam

58
Q

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

A

A no-arg constructor

59
Q

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

A

You must type it in yourself

60
Q

What does every constructor inherently contain?

A

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

61
Q

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

A

The static variable NAME

62
Q

Can an interface have a constructor?

A

No

63
Q

How is a constructor invoked?

A

Only through the call by another constructor

64
Q

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

A

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

65
Q

What can be called from the constructor?

A

Only static methods and static variables

66
Q

What is happening here?

Animal(){
this(makeRandomName());
}

A

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

67
Q

What is this and when does it run?

static { x= 8; }

A

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

68
Q

What is this and when does it run?

{ o = 0 }

A

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

69
Q

What is the order of init blocks?

A

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

70
Q

Do static variables need to be initialised?

A

No, they get initialised automatically, like instance variables.

71
Q

Can static methods be overridden in a subclass?

A

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

72
Q

What are the two features of encapsulation?

A
  • Instance variables are kept protected, usually with the private modifier
  • Getter and setter methods
73
Q

How is an IS-A relationship expressed?

A

Through extends or implements

74
Q

How is a HAS-A relationship expressed?

A

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

75
Q

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

A

The reference variable type.

76
Q

What is a covarient return with regards to overridden methods?

A

Return a subtype of the return type

77
Q

What determines what overridden method is used at runtime?

A

The object type

78
Q

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

A

The reference type

79
Q

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

A

Object o = “a String”;

String s = (String) s;

80
Q

How do you properly implement an interface?

A

By implementing all of it’s methods

81
Q

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

A

Simple issue a return;

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