Core Java BrushUp : Generics

Always keep two things in mind

1. Compile Type Safety
• ArrayList stockList = new ArrayList();
•  stockList.add(“coins”); //compiler error , String not allowed
2. Achieved by Type Erasure

What is Type Erasure
Below code:

List<String> list = new ArrayList<String>();
list.add("Hi");
String x = list.get(0);

is compiled into

List list = new ArrayList();
list.add("Hi");
String x = (String) list.get(0);

Also Note

• it uses Autoboxing
• cannot be applied to primitive type
• cannot mix old and new types as it will not guarantee type safety
• limit Types parameters, by using

Using old and new together
 

ArrayList<String>  myList2 = new ArrayList<String>(); // valid
// BELOW is mixing of OLD + NEW Generics in such code there is 

// no guarantee of type safety
ArrayList myList3 = new ArrayList<String>(); // valid
ArrayList<String> myList4 = new ArrayList(); // valid

What’s difference between E , T or ?

• type parameter (E or T).
• ? is used as a wildcard which is used when providing a type argument, e.g. List foo = ... means that foo refers to a list of some type, but we don't know what.

void point_1( ){
 Set<Object> SetOfObject = new HashSet<String>(); //compiler error - incompatible type
 Holder<int> numbers = new Holder<int>(10); //compiler error - unexpected type required: reference found:int
}

 void point_2( ){
  List<Parent> parent_List = new ArrayList<Parent>(3);
  List<Child> child_List = new ArrayList<Child>(3);
     
  parent_List.add(new Parent("father 1"));
  parent_List.add(new Parent("father 2"));
  parent_List.add(new Child("child 1")); //upcasting ok
     
  child_List.add(new Child("lil 1"));
  child_List.add(new Child("lil 2"));
  child_List.add((Child)new Parent("Father of 1"));//HAVE to CAST as usual  

     
     
  // List<Parent> myList = new ArrayList<Child>();
  // ** ERROR ** .. incompatible types ...
  // polymorphism applies here ONLY to List & ArrayList ... ie ... base_types .. not generic_types
  Parent[] myArray = new Child[3]; // but this works..
  // WHY .. it works for arrays[] and not for generics .. coz compiler & JVM behave differently for generic collec & arrays[]
   point_8(child_List);
 }