Class Design

Question 1

What is included when a class inherits from another?

Answer 1

The public and protected primitives, objects, or methods defined in the parent class

Question 2

Single Inheritance

Answer 2

Java only allows a class to have one direct parent class

Question 3

How can you keep a class from being extended?

Answer 3

Mark it with the final modifier

Question 4

What is the basic Object class?

Answer 4

java.lang.Object

Question 5

How do you call the parent class’s constructor?

Answer 5

super()

Question 6

What type of constructor must a parent class have if you don’t specify a certain constructor in the child class to make things work?

Answer 6

A default no-arguments constructor

Question 7

What are the rules for overriding a method in a child class?

Answer 7

1) The method in the child class must have the same exact signature (name and parameters) as the one in the parent class
2) The method in the child class must be at least as accessible as the one in the parent class
3) The method in the child class may not throw a checked exception that is new or broader than in the parent method
4) Return type of the child class method must be the same or a subclass of the return type in the parent class

Question 8

Hidden Method

Answer 8

When a child class defines a static method with the same name and signature as a static method defined in the parent class.

Question 9

Can you override or hide variables?

Answer 9

You can only hide them

Question 10

Abstract Class

Answer 10

A class that is marked with the abstract keyword and cannot be instantiated.

Question 11

Abstract Method

Answer 11

A method marked with the abstract keyword defined in an abstract class, for which no implementation is provided in the class in which it is declared.
Can only be declared in an abstract class.

public abstract String() getName();

Question 12

Concrete Class

Answer 12

The first non-abstract subclass that extends an abstract class. It is required to implement all inherited abstract methods, which have not been implemented in a parent class.

Question 13

Can an abstract class that extends an abstract class provide an implementation for methods inherited?

Answer 13

yes! but they don’t have to

Question 14

Abstract Class Definition Rules

Answer 14

1) Abstract classes cannot be instantiated directly.
2) Abstract classes may be defined with any number, including zero, of abstract and nonabstract methods.
3) Abstract classes may not be marked as private or final.
4) An abstract class that extends another abstract class inherits all of its abstract methods as its own abstract methods.
5) The first concrete class that extends an abstract class must provide an implementation for all of the inherited abstract methods.

Question 15

Abstract Method Definition Rules

Answer 15

1) Abstract methods may only be defined in abstract classes.
2) Abstract methods may not be declared private or final.
3) Abstract methods must not provide a method body/implementation in the abstract class for which is it declared.
4) Implementing an abstract method in a subclass follows the same rules for overriding a method. For example, the name and signature must be the same, and the visibility of the method in the subclass must be at least as accessible as the method in the parent class.

Question 16

Interface

Answer 16

An abstract data type that defines a list of abstract public methods that any class implementing the interface must provide. An interface can also include a list of constant variables and default methods.

Question 17

How to declare and interface

Answer 17

public abstract interface myInterface {

}

Question 18

How to say you want to use an interface

Answer 18

public class myClass implements myInterface { 
}

Question 19

Rules of interfaces

Answer 19

1) Interfaces cannot be instantiated directly.
2) An interface is not required to have any methods.
3) An interface may not be marked as final.
4) All top-level interfaces are assumed to have public or default access, and they must include the abstract modifier in their definition. Therefore, marking an interface as private, protected, or final will trigger a compiler error, since this is incompatible with these assumptions.
5) All nondefault methods in an interface are assumed to have the modifiers abstract and public in their definition. Therefore, marking a method as private, protected, or final will trigger compiler errors as these are incompatible with the abstract and public keywords.

Question 20

How many interfaces can an interface extend?

Answer 20

As many as it wants!

Question 21

How many interfaces can a class implement?

Answer 21

As many as it wants!

Question 22

What are interface variables assumed to be?

Answer 22

public, static, final

Question 23

Default Method

Answer 23

A method in an interface that has a default implementation specified rather than just being abstract. Note that it can still be overridden as if it were abstract.
Denoted with the “default” keyword

Question 24

Static Methods in Interfaces

Answer 24

Aren’t inherited, so must be referenced as interfaceName.methodName();

Question 25

Polymorphism

Answer 25

A java object can take the form of it’s default class, and also anything that class extends or implements

Question 26

Casting an object from a ______ to a _______ doesn’t require an explicit cast.
Casting an object from a _______ to a _______ requires an explicit cast.

Answer 26

subclass, superclass

superclass, subclass

Question 27

Virtual Method

Answer 27

A method whose precise implementation is not determined until runtime. All nonfinal, nonprivate methods are virtual methods in Java since they may be overridden at runtime.

Question 28

Polymorphic Parameters

Answer 28

The property of method parameters in Java used to accept sub- classes and subtypes of the method parameter type automatically.

Question 29

What is always run as the first line in a constructor?

Answer 29

If there isn’t an explicit call to a different constructor using a variation of this() or super(), the compiler will attempt to insert a no argument super() call. However, if the parent class doesn’t have a no argument constructor, you MUST explicitly write in a line that works with a variation of this() or super()

Question 30

Difference between hidden and overridden methods

Answer 30

When a method is hidden, if the method is called from the parent class then it uses the definition in the parent class, and if the method is called from the child class then it uses the definition from the child class

When a method is overridden, whether the method is called from the parent class or the child class, it will use the definition in the child class.