Item 25 - Prefer lists to arrays

Aug 7

From Effective Java 2/e by Joshua Bloch

Arrays v/s Generic type

Arrays (covariant) v/s Generic type (invariant)

If Sub is a subtype of Super, then the array type Sub[] is a subtype of Super[]
For any two distinct types Type1 and Type2, List<Type1> is neither a subtype nor a supertype of List<Type2>

// Fails at runtime! Object[] objectArray = new Long[1]; objectArray[0] = "I don't fit in"; // Throws ArrayStoreException

// Won't compile! List<Object> ol = new ArrayList<Long>(); // Incompatible types ol.add("I don't fit in");

Arrays (reification) v/s Generic type (erasure)

Arrays know and enforce their element types at runtime
Generic enforce their type constraints only at compile time and discard (or erase) their element type information at runtime

arrays and generics do not mix well

new List<E>[]
new List<String>[]
new E[]

// List-based generic reduction
static <E> E reduce(List<E> list, Function<E> f, E initVal) {
   List<E> snapshot;
   synchronized(list) {
      snapshot = new ArrayList<E>(list);
   }
   E result = initVal;
   for (E e : snapshot)
      result = f.apply(result, e);
   return result;
}