Generics.md

Generics

1. Check the type in compile time
   a. Simple example of code

  ArrayList<Tv> tvList = new ArrayList<Tv>();
  tvList.add(new Tv());
  tvList.add(new Audio());   // compile error

2. To catch a runtime error in advance, applied Tv objecte name into ArrayList so that compiler will detect the error of code prior to run it.
   

  ArrayList<Tv> tvList = new ArrayList<Tv>();
  tvList.add(new Tv());
  Tv t = tvList.get(0);
  // TV t = (Tv) tvList.get(0);  not necessary to the type casting

3. The type of reference variable should be equal to its constructor. For example,

  ArrayList<Tv> list = new ArrayList<Tv>() ;  // OK
  ArrayList<Product> list = new ArrayList<Tv>() ;  // Error

4. Polymorphism of generics

  List<Tv> list = new ArrayList<Tv>() ;
  List<Tv> list = new LinkedList<tv>() ;

• List is the parent of ArrayList and LinkedList

5. Another features of Generics in Java

   a. Iterator

    public interface Iterator<E> {
      boolean hasNext();
      E next();
      void remove();
    }
        ...
    
    Iterator <Student> it = list.itrator() ;
    while(it.hasNext()){
      Student s = it.next() ;  // (Student)it.next() is not required to transfer the type
    }

b. HashMap

  HashMap<String, Student> map = new HashMap<String, Student>() ;   // new HashMap<> () is possible.

   public class HashMap extends AbstractMap {
     ...
     public Student get(Object key) { ... }
     public Student put(String key, Student value) { ... }
     public Student remove(Object kye) { ... }
     ...
   }

For HashMap <Key, Value> , it applies diferent types in <>

• By extends , java can assign the type from parent to children types.

• For example,

    class FruitBox< T extends Fruit > {
      ArrayList<T> list = new ArrayList<T> () ;
      ...
    }
  
    FruitBox<Apple> appleBox = new FruitBox<Apple>() ;    // Allowed
    FruitBox<Toy>  toyBox = new FruitBox<Toy>() ;    // Error
  
    interface Eatable {}
    class FruitBox<T extends Eatable> { ... }

Exception

• Exception is a run-time error. It consists of IOException, ClassNotFoundException, RuntimeException and so on.
• Inside RuntimeException, there are several errors such as ArithmeticException, ClassCastExceoption,NullPointerException and so on.

References

1. Java_Jaungsuk