Inheritance - Data Structures and Problem Solving Using Java - page 155

primitive types

Primitive types cannot be used for a type parameter. Thus ArrayList<int> is
illegal. You must use wrapper classes.

Primitive types can-
not be used for a
type parameter.

instanceof tests

instanceof tests and type casts work only with the raw type. Thus, if instanceof tests

and type casts
work only with the
raw type.

ArrayList<Integer> list1 = new ArrayList<Integer>( );
list1.add( 4 );
Object list = list1;
ArrayList<String> list2 = (ArrayList<String>) list;
String s = list2.get( 0 );

was legal. then at runtime the typecast would succeed since all types are
ArrayList . Eventually, a runtime error would result at the last line because the
call to get would try to return a String but could not.

static contexts

In a generic class, static methods and fields cannot refer to the class's type
variables since after erasure, there are no type variables. Further, since there is
really only one raw class, static fields are shared among the class's generic
instantiations.

Static methods and
fields cannot refer
to the class's type
variables. Static
fields are shared
among the
class's generic
instantiations.

instantiation of generic types
It is illegal to create an instance of a generic type. If T is a type variable, the
statement

T obj = new T( ); // Right-hand side is illegal

is illegal. T is replaced by its bounds, which could be Object (or even an
abstract class), so the call to new cannot make sense.

It is illegal to
create an instance
of a generic type.

generic array objects

It is illegal to create an array of a generic type. If T is a type variable, the statement

It is illegal to cre-
ate an array of a
generic type.

T [ ] arr = new T[ 10 ]; // Right-hand side is illegal

is illegal. T would be replaced by its bounds, which would probably be Object ,
and then the cast (generated by type erasure) to T[] would fail because
Object[] IS-NOT-A T[] . Figure 4.37 shows a generic version of SimpleArrayL-
ist previously seen in Figure 4.24. The only tricky part is the code at line 38.
Because we cannot create arrays of generic objects, we must create an array
of Object and then use a typecast. This typecast will generate a compiler warn-
ing about an unchecked type conversion. It is impossible to implement the
generic collection classes with arrays without getting this warning. If clients
want their code to compile without warnings, they should use only generic
collection types, not generic array types.