Lecture 7 - OOP Foundations Flashcards
What is a publicized service made available to others?
Interface
TF: The implementation of an interface is usually visible to user of the interface
False. Implementation generally hidden
At what time is a collection of classes organized into packages (hint: static point of view)?
At compile-time
At what time is a program a collaborating community of objects that uses class features (data, method)?
At runtime
What are Basic (primitive) types?
- int
- long
- double
- void
- boolean
What are Reference types?
- Class
- Interface
- Array
TF: A stack cannot hold the value of a primitive type
False. It can
TF: A stack can hold the value NULL
True
TF: A stack cannot hold a reference (pointer) to an object allocated in Heap
False. It can
TF: A stack can hold Objects
False. It cannot
TF: A heap can only hold objects that were created via “new”
True
What are the two things defined by a class?
- A type
- An implementation
What is defined by an interface?
- A type
(without an implementation)
Note: interface has no logic (logic defined by class implementing interface)
What are the two types an expression can have?
- Compile-time (static) type
- Runtime (dynamic) type
The compile-time type is also known as…
The declared type
Animal a = new Cat();
What is the declared type of the provided code?
Animal
Animal a = new Cat();
What is the run-time type of the provided code?
Cat
Will the following code run?
Animal a = new Animal();
Cat c = (Cat) a;
No. Cannot cast downwards
Will the following result in a compile-time error or runtime error?
Animal a = new Animal();
Cat c = (Cat) a;
Runtime error. The compiler thinks Animal “a” could be a Cat
Will the following code run?
Cat c = new Cat();
Animal a = (Animal) c;
Yes. Casting upwards is allowed
What is the purpose of Casting? (2)
- Inform compiler of (assumed) subtype of an object
[ex: Animal a = new Cat()] - So compiler can perform better type checking
TF: Dynamic dispatching is when the runtime decides which implementation of a non-static method to call based on the calling object
True
Which sleep method gets called for the following:
Animal a = new Cat();
a.sleep();
Since Cat is the actual type, the Cat’s sleep method gets called
TF: Dynamic dispatching can occur on static methods
False. Only on non-static methods
TF: Dynamic dispatching happens for constructors
False. Object constructor is decided at compile-time
Given the following code,the compiler will check where for the existence of the method?
A a = new Cat();
a.m();
It will check the declared type (i.e. A) and all superclass of it for the method existence.
