Generics

Generics allow type flexibility in code.
Allows for behavior independent of type. (like a LIFO “stack”)
Generic type indicated with <T> or <K, V> usually
Compiler replaces generic types with given type (eg: ArrayList<String>())

Generics - a FIFO Queue

queue: a data structure that allows for FIRST IN, FIRST OUT.
this is a thought experiment, just imagine having this thing…
Queue s = new Queue(); give you a “queue for waiting objects”
s.append(object1); s.append(object2);
when you s.next(); you get object1,
if you do it again - s.next(); you get object2
if you do a s.next(); again, you maybe get an error or a the null object
imagine a Queue of Persons at DMV. “standing in line”

Hmm.

BUT how to we make this generic enough to use for anything: Cars at a CarWash
We don’t want to have PersonQueue, CarQueue, etc.
think of all the copied code, duplicates, a different class for every kind of object…
if you’re thinking UGH, what’s that code smell?
- you’re starting to understand

Queue

this is an imaginary example…

// Pretend Queue has a add() method and a next() method.
Queue personQueue = new Queue<Person>(); // this queue is for persons

personQueue.add(joe); // joe starts standing in line
currentPerson = personQueue.next();  // "Next!" person being serviced

Queue carWashQueue = new Queue<Vehicle>(); // this queue tracks cars to be washed
carWashQueue.add(teslaS);

Terms

Parameterized Type
Generic Type
Type Inference

Examples of generic Types

List (ArrayList, LinkedList, etc)

List carWashQueue = new ArrayList<Vehicle>();

Map (HashMap, TreeMap, etc)

Map carsWeSell = new HashMap<String,Vehicle>();
carsWeSell.put("Tesla S", teslaS)

Primitive types cannot be type parameters

Example:

// This will not compile!!!!

public class Main{
  public static void main(String[] args){
    ArrayList<int> primitiveArrayList = new ArrayList<>();
  }
}

The solution? Wrapper classes!


public class Main{
  public static void main(String[] args){
    ArrayList<Integer> pg = new ArrayList<>();
  }
}

Primitives and autoboxing

public static void main(String[] args){
  int x, y;
  x = 5;
  ArrayList<Integer> box = new ArrayList<>();
  box.add(x);
  y = box.get(0);
  System.out.println(y);