Chapter 14 - Inheritance

Question 1

What class is a superclass for all other classes?

Answer 1

The Object class

Question 2

What is unique about the Object class compared to other superclasses?

Answer 2

You don’t have to specifiy it as a superclass, it is automatically a superclass.

Question 3

What are the two most important Object methods?

Answer 3

equals and toString

Question 4

If a class doesn’t have it’s own equals method, but an object from that class calls the equals method what happens?

Answer 4

It inherits and uses the Object class’s equals method.

Question 5

What does the Object class’s equal method return if two reference variables that are being compared point to the same object?

Answer 5

The method returns true.

Question 6

What operator works the same as the Object class’s equals method?

Answer 6

==

Question 7

Why is the Object class’s equals method usually not good enough?

Answer 7

Because you usually want to compare the contents of two objects rather than just whether two reference variables point to the same object.

Question 8

How do you compare the contents of two objects using the Object class’s equals method?

Answer 8

You need to have an equals method in the object’s class definition that compares the contents of the two objects.

Question 9

The equals method is built into a lot of Java’s API classes?

T or F

Answer 9

True

Question 10

What method in the Object class returns a string that’s a concatenation of the calling object’s class name, an @sign, and a sequence of digits and letters (called a hashcode)?

Answer 10

The toString method

Question 11

What does toString prefix if a clas is stored in a package?

Answer 11

The class name with the class’s package.

Question 12

Since retrieving the class name, an @ sign, and a hashcode is usually worthless, we want to almost always avoid calling the Object class’s toString method. What do we call instead?

Answer 12

An overriding toString method

Question 13

In general, what should a toString method return?

Answer 13

A string that descirbes the calling object’s contents.

Question 14

Since retrieving the contents of an object is a common need, what habit should a programmer get into?

Answer 14

Proving a toString method for most programmer-defined classes.

Question 15

What should a programmer’s toString method typically do?

Answer 15

Simply concatenate the calling object’s store data and return the resulting string.

Question 16

What should a toString method NOT do?

Answer 16

It should NOT print the concatenated string value, it should just RETURN it.

Question 17

When does the JVM automatically call the toString method?

Answer 17

When a reference variable is an argument in a System.out.print, println, or printf call.

Example:

System.out.println(car);

Question 18

What is a second circumstance that JVM will automatically call the toString method?

Answer 18

When a reference variable is concatenated (+ operator) to a string

Example:

String carInfo = “Car data:\n” + car;

Question 19

What is the standard method-call syntax for the toString method?

Answer 19

car.toString();

Question 20

What does the toString method do in regards to the primitive wrapper classes?

Answer 20

Returns a string representation of the given primitive value.

Example:

Integer.toString(22) : evaluates to string “22”

Double.toString(123.45) : evaluates to string “123.45”

Question 21

What is polymorphism?

Answer 21

When different types of objects respond differently to the same method call.

Question 22

How do you implement polymorphic behavior?

Answer 22

Declare a general type of reference variable that is able to refer to objects of different types.

Question 23

How do you declare a “general type of reference variable”?

Answer 23

Use a superclass (like the Object superclass)

Question 24

Since polymorphism is a concept, what is dynamic binding?

Answer 24

A description of how that concept is implemented.

Question 25

How does JVM use dynamic binding?

Answer 25

It uses it in order to match up a polymorphic method call with a particular method.

Question 26

What does the JVM do right before it execute’s a method call?

Answer 26

It looks at the method call’s calling object. More specifically, it looks at the type of the object that’s been assigned into the calling object’s reference variable.

Question 27

After the JVM looks at a method’s object what occurs through dynamic binding?

Answer 27

If the assigned object is from class X, the JVM binds class X’s method to the method call. If the assigned object is from class Y, the JVM bind class Y’s method to the method call. After the JVM bind the appropriate method to the method call, the JVM execute the bound method.

Question 28

What happens when a compiler sees a method call, “reference-variable”.”method-name”()?

Answer 28

It checks to see if the reference variable’s class contains a method definition for the called method.

Question 29

Normally, when an object is assigned into a reference variable, the object’s class and the reference variable’s class are the same. How can a different assignment work?

Answer 29

Such assignments only work if the right side’s class is a subclass of the left side’s class.

Question 30

When is polymorphism the most useful?

Answer 30

When you have an array of generic reference variables and assign different types of objects to different elements in the array.

Question 31

What does polymorphism with Arrays allow?

Answer 31

The ability to step through the array and for each array element, you call a polymorphic method.

Question 32

How does the JVM pick out the particular methods that apply to different types of objects that are in the array?

Answer 32

At runtime, the JVM use dynamic binding.

Question 33

When do you declare a method to be abstract?

Answer 33

If the method’s class is a superclass and the method is merely a “dummy” method for an overriding method(s) in a subclass(es).

Question 34

What does Java require when you define a method to be abstract?

Answer 34

• Use an abstract method heading instead of a method definition. An abstract method heading is the same as a standard method heading except that it includes the abstract modifier and a trailing semicolon.
• Define an overriding version of that method in each of the superclass’s subclasses.
• Define the superclass to be abstract by using the abstract modifier.

Question 35

What are you telling the computer by defining a class to be abstract?

Answer 35

You’re telling the compiler to not allow the class to be instantiated

Example; if a program attempts to instantiate an abstract class, a compiliation error will be generated.

Question 36

Where does the protected access modifier lie in terms of accessibility?

Answer 36

It lies in between public and private in terms of how accessible something is.

Question 37

When can protected members (instance variables, class variables and methods) be accessed?

Answer 37

Only by classes within the subtree associated with the member’s class.

Question 38

When do you use the protected access modifier?

Answer 38

• When you want a member to be directly accessible from any clas within its subtree of classes, but you don’t wan it to be accessible elsewhere.

Question 39

Protected example

Answer 39