Generics and Comparators

Question 1

What are the 4 different types of access levels for class methods or member variables?

Answer 1

public

protected

no modifier

private

Question 2

Who can access public methods or member variables?

Answer 2

1. The Class
2. The SubClasses.
3. The package.
4. The World

Question 3

Who can access the protected methods or member variables?

Answer 3

1. The Class.
2. The Subclasses
3. The Package

Question 4

Who can access the private methods or member variables

Answer 4

1. The class

Question 5

What term refers to things such as public, private, and protected?

Answer 5

Modifier

Question 6

What is a modifier give 4 examples.

Answer 6

A modifier is a declaration of the access level.

Ex: public, private, protected, no modifier

Question 7

When no modifier has been declared who does the default modifier allow to access member variables or methods?

Answer 7

1. The class.

2. Classes within the package.

Question 8

Give an example of a common generic class that we use all the time.

Answer 8

ArrayList

Question 9

What is a generic placeholder?

Answer 9

During the class implementation you can use to create a generic placeholder that will be filled in during construction.

Question 10

How do generic types treat class inheritance?

Answer 10

They ignore it.

Ex. An ArrayList is NOT an ArrayList

Even though a Triangle is a Shape.

Question 11

How do method signatures apply to generic placeholders?

Answer 11

The method signature must specify a type and the method cannot be called on the derived classes of that type.

Ex:
public void doStuff(ArrayList x){….}
ArrayList triList ;
ArrayList shapeList;

doStuff(triList) is ILLEGAL
doStuff(shapeList) is Allowed.

Question 12

How can we create method signatures that will allow a generic type and any of its subclasses to be passed as parameters?

Answer 12

Use wildcards
public void doStuff(ArrayList x)
ArrayList triList ;
ArrayList shapeList;

doStuff(triList) is Allowed
doStuff(shapeList) is Allowed.

Question 13

What is an upper-bounded wildcard?

Answer 13

extends Shape>
Refers to Shape and any of it’s derived classes.
Ex: Shape, Circle, Triangle

Question 14

What is a lower-bounded wildcard?

Answer 14

super Triangle>
Refers to Triangle and any that is a superclass of it.
Ex: Triangle, and Shape.

Question 15

If List and List do not have List as their parent what is their parent?

Answer 15

List> is the parent.

Question 16

Why use generics instead of Objects?

Answer 16

Generics allow for type-checking at compile time before running. Objects can only be checked at runtime.

Generics allow you to detect type mismatches before your code runs.

Question 17

Give an example of the runtime error if an arrayList took only objects vs using generics.

Answer 17

Runtime crash.
ArrayListL; // implemented with Object in mind
L.add(new String (“hi”));
Shape i= (Shape)L.get(0); // crash!

Compile Time error
ArrayList L;
L.add(new String (“hi”));
Shape i= L.get(0); // compile error

Question 18

Why are compile time errors preferred over run time errors?

Answer 18

Easier to detect and fix. Won’t run without them fixed so you can’t get false positives.

Question 19

What data types do generics work with?

Answer 19

Only with reference types not with primitives,

Question 20

What are 4 important Java primitives and their associated wrapper classes?

Answer 20

int Integer
float Float
double Double
char Character

Question 21

What does a wrapper class do?

Answer 21

Converts a primitive into an object.

Question 22

Why is the wrapper classes function important?

Answer 22

1. The java.util package handles only objects contains Array, Random, LinkedList etc.
2. Data structures in the collection framework, such as ArrayList, handle only objects.
3. Objects are needed to modify the arguments passed into a method. Primitive types are only passed by value. Remember that this means a primitive type is copied for the method call thus it cannot be altered by a void method. Objects are passed by reference.

Question 23

What does Java do automatically for us with wrapper classes?

Answer 23

It converts between them automatically when dealing with list accesses.
L.add(5); is equivalent to...
L.add(new Integer(5));

int i = L.get(n); is equivalent to…
int i = L.get(n).intValue();

Question 24

How do you have to declare a generic static method?

Answer 24

You must declare the generic type before the return type.

public static boolean doWork(…)

Question 25

Give an example of a generic contains method that searches a generic array for a generic item and returns true if we find it.

Answer 25

public static boolean contains(T[] arr, T item){

for(i=0;i

Question 26

What are the two tools we can use to make decisions about sorting generic types?

Answer 26

1. Comparable interface - natural ordering a.compareTo(b)

2. Comparator interface - custom ordering mechanism

Question 27

What interface does compareTo belong to? What does a call to compareTo look like? What does compareTo Return?

Answer 27

x.compareTo(y) refers to the natural ordering or Comparable interface.
Returns a negative integer for xy.

Question 28

What interface does compare belong to? How is it different than compareTo? How is it the same?

Answer 28

Compare belongs to Comparator for custom ordering or objects that might not have obvious natural ordering.

The call is different as it takes both arguments. comparator.compare(x,y).

The return type is the same:
+int for x>y,
-int for x

Question 29

What is a functor?

Answer 29

A class with a single method.
Ex: Comparator.

Question 30

When should you use Comparator vs Comparable?

Answer 30

Comparator provides a way for custom comparison logic when you have no control over the types you’re comparing

Comparable provides a way for comparison logic for your own types. However, you must implement the logic in your type class.