Chapter 16: Inheritance

Question 1

How do we delete an element from a partially filled array?

Answer 1

Replace unwanted item with end item. Decrease length by 1

Question 2

What is UML?

Answer 2

Unified modelling language. Set of diagrams showing relationships between entities

Question 3

What is a UML Class diagram?

Answer 3

Represents an inheritance hierarchy. Each class appears as a box with: Name, variables, methods, - means private, + means public, Italics means abstract

Question 4

What is inheritance

Answer 4

A class might inherit properties from another. A class classifies objects, sometime we want sub-classes that have something in common

Question 5

How does a subclass constructor start?

Answer 5

With a call to super class constructor. This can be an explicit or implicit invoke.

Question 6

Describe objects as the route of the inheritance hierarchy

Answer 6

All objects are instances of the object class. Unless it explicitly extends another class, it extends object directly. All classes reside in a single inheritance hierarchy with object at the route. Every class except object has a superclass.

Question 7

Describe how a method can be overridden

Answer 7

Sometimes a subclass needs to change the definition of an instance method. It redefines it and overrides the inherited definition

Question 8

What is polymorphism?

Answer 8

A thing can have more than one form: a thing can have more than one type. A bike is a vehicle and is an object

Question 9

What is dynamic method binding?

Answer 9

The process of determining at run time which actual method to invoke. When the compiler is producing the byte code for an instance method it doesn’t know which will get called. i.e. two sub-classes that both override the same method from the same superclass.

Some call can invoke different versions of method at different times depending on the run time value of object refrence

Question 10

What is an abstract class?

Answer 10

No instances can be made. We dont want any direct instances of a class. It can only be implemented through its sub-classes.

Question 11

What is an abstract method?

Answer 11

An instance method that has non-static modifiers, return type, name, method parameters, no method body.

We are saying “This method exists but we dont implement it here”. Implemented by subclass shown using ; Every subclass must implement it (rather than override)

Question 12

What is a final method or class?

Answer 12

No subclass may override a public instance method if we declare it to be final. A class that cannot have any sub-classes

Question 13

How can a subclass extend a superclass?

Answer 13

Add object state- adding instance variables

Add instance methods

Question 14

How do we test for an instance of a class?

Answer 14

x Instance of y

Question 15

How do we cast to a subclass?

Answer 15

An instance of a subclass is an instance of its superclass too
Vehicle v = new Bicycle()
Bicycle b = (Bicycle) v
The compiler accepts this, any errors through a class cast exception at run time.
This doesn't create a new object it just checks the format

Question 16

Descibe is a and has a

Answer 16

Every instance of subclass is a superclass
If class c has instance variable d c has a d

Question 17

How do we use an overridden method?

Answer 17

super.method()

Question 18

What is constructor chaining?

Answer 18

When a constructor method is invoked, the first thing is either to call another constructor in the same class of to call constructor method in superclass

Question 19

Describe overload and override

Answer 19

Overload: two methods have the same name in the same class bu a different signature
Override: methods in the subclass has the same signature as one in the superclass- it overrides it

Question 20

How can an accidental overload occur?

Answer 20

We may think we have overridden a method but instead we have overloaded it. Dynamic method binding chooses current method implementation at run time

Question 21

How do we show multiple constructors?

Answer 21

To explicitly show the relationship alternative constructors could call the original using this(10, true) ie filling in a default value for one of the parameters

Question 22

What happens if you do not write a constructor?

Answer 22

There is a default which calls super()

Question 23

Describe to string and print ln

Answer 23

Println is an overloaded method for every primitive type of method argument. There is also an instance for obejcts. When we write a toString method, we override this.