CIS 212 Test 1

1. Which is the access modifier which makes an element accessible only from the same package or from subclasses

Public

Protected

Package (aka default)

Private

The access modifier "protected" makes an element accessible only from the same package or from subclasses. This means that any class within the same package can access the protected element, as well as any subclass of the class that contains the protected element, regardless of whether the subclass is in the same package or not. This access modifier provides a level of encapsulation and restricts access to the protected element from classes outside the package or unrelated subclasses.

Explanation

The access modifier "protected" makes an element accessible only from the same package or from subclasses. This means that any class within the same package can access the protected element, as well as any subclass of the class that contains the protected element, regardless of whether the subclass is in the same package or not. This access modifier provides a level of encapsulation and restricts access to the protected element from classes outside the package or unrelated subclasses.

2. Why do we have different levels of access modifiers (public, protected, package, private)?

For making composition possible

For data abstraction

For encapsulation and information hiding

For being able to use inheritance

None of the above

Some elements of a class need to be part of the public interface, however you might want some of the elements to be hidden from certain classes, and not hidden from others. So the programmer has fine control on what is visible to whom

Explanation

Some elements of a class need to be part of the public interface, however you might want some of the elements to be hidden from certain classes, and not hidden from others. So the programmer has fine control on what is visible to whom

3. An abstract class is a class which

Doesn't have any implementation of methods

Has abstract methods but possibly some defined methods as well

Is only accessed from the same package

Has only implemented methods

None of the above

An abstract class is a class that cannot be instantiated and is meant to be extended by other classes. It may have abstract methods, which are methods without any implementation that must be implemented by the subclasses, and it may also have defined methods, which are methods with a complete implementation. Therefore, an abstract class can have a combination of abstract and defined methods.

Explanation

An abstract class is a class that cannot be instantiated and is meant to be extended by other classes. It may have abstract methods, which are methods without any implementation that must be implemented by the subclasses, and it may also have defined methods, which are methods with a complete implementation. Therefore, an abstract class can have a combination of abstract and defined methods.

4. In Java how many abstract classes can I extend?

One

Two

Infinite

You cannot

In Java you can only extend (inherit) one class, including abstract classes.

Explanation

In Java you can only extend (inherit) one class, including abstract classes.

5. What is composition?

When you extend a class

When you implement an interface

When you extend an abstract class

When a class calls the super constructors

Composition refers to a design pattern in object-oriented programming where one object, A, has a reference to another object, B. In this pattern, only object A has a reference to object B, and it also determines the lifetime of object B. This means that if object A is destroyed or goes out of scope, object B will also be destroyed. Composition is commonly used to create complex objects by combining simpler objects, allowing for code reuse and modularity.

Explanation

Composition refers to a design pattern in object-oriented programming where one object, A, has a reference to another object, B. In this pattern, only object A has a reference to object B, and it also determines the lifetime of object B. This means that if object A is destroyed or goes out of scope, object B will also be destroyed. Composition is commonly used to create complex objects by combining simpler objects, allowing for code reuse and modularity.