GENERICS - Introduction to Java Programming: Comprehensive Version - page 751

Although ArrayList<String> and ArrayList<Integer> are two types at compile time,
only one ArrayList class is loaded into the JVM at runtime. list1 and list2 are both
instances of ArrayList , so the following statements display true :

System.out.println(list1 instanceof ArrayList);
System.out.println(list2 instanceof ArrayList);

However, the expression list1 instanceof ArrayList<String> is wrong. Since
ArrayList<String> is not stored as a separate class in the JVM, using it at runtime makes
no sense.
Because generic types are erased at runtime, there are certain restrictions on how generic
types can be used. Here are some of the restrictions:

Restriction 1: Cannot Use new E()

You cannot create an instance using a generic type parameter. For example, the following
statement is wrong:

E object = new E(); no new E()

The reason is that new E() is executed at runtime, but the generic type E is not available
at runtime.

Restriction 2: Cannot Use new E[]

You cannot create an array using a generic type parameter. For example, the following
statement is wrong:

E[] elements = new E[capacity]; no new E[capacity]

You can circumvent this limitation by creating an array of the Object type and then cast-
ing it to E[] , as follows:

E[] elements = (E[]) new Object[capacity];

However, casting to (E[]) causes an unchecked compile warning. The warning occurs
because the compiler is not certain that casting will succeed at runtime. For example, if
E is String and new Object[] is an array of Integer objects, (String[])(new
Object[]) will cause a ClassCastException . This type of compile warning is a limi-
tation of Java generics and is unavoidable.
Generic array creation using a generic class is not allowed, either. For example, the fol-
lowing code is wrong:

unavoidable compile warning

ArrayList<String>[] list = new ArrayList<String>[ 10 ];

You can use the following code to circumvent this restriction:

ArrayList<String>[] list = (ArrayList<String>[]) new
ArrayList[ 10 ];

However, you will still get a compile warning.

Restriction 3: A Generic Type Parameter of a Class Is Not Allowed in a Static
Context

Since all instances of a generic class have the same runtime class, the static variables and
methods of a generic class are shared by all its instances. Therefore, it is illegal to refer to