class jj<T> {
T ob;
jj(T ob) {
this.ob = ob;
}
void show() {
System.out.println(ob);
}}
class demo {
public static void main(String[] args) {
// jj<Integer>[] lol=new jj<Integer>[10];// Not Allowed
jj<Integer>[] lol = new jj[10];//But this is allowed why?
for (int i = 0; i < 10; ++i)
lol[i] = new jj<>(i * 10);
for (jj<Integer> x : lol)
x.show();
}
Why the above code is allowed if generics arrays are not allowed in java? plz help!
The code is allowed because new jj[10] is not a generic array (e.g. new T[10]), but is instead an array of raw jj instances - jj with its generic arguments omitted. new jj[10] creates a new array of raw jj, not a new array of jj<Integer>. Java allows one to force instances of a raw type to be assigned to a reference of type with generic arguments as the code does with jj<Integer>[] lol = new jj[10];. I believe this is there for backwards compatibility with pre-Java 5 code.
jj<Object> which should be used instead (if one truly wants to allow all objects).
Generic arrays are allowed. The compiler is going to give you a warning because the generics implement type erasure meaning that they do not exist at run time. The compiler will give you a warning when creating a generic reference, and for the object's type you need to have a object array that is casted.
CAN'T do this:
//SYNTAX ERROR
jj = new T[10];
But CAN do this:
//This suppresses the unchecked-cast warning that the compiler will give
@SuppressWarnings("unchecked")
//Can't create a generic array but can create an Object array and cast it to be a //compatible type
T[] jj = (T[])new Object[10];
However in the code above is not creating a generic array, instead it is creating an array of the class jj that has a generic in it.
E[] instead of T[] for a collection of items.
Note: GenericArrays.java uses unchecked or unsafe operations.Note: Recompile with -Xlint:unchecked for details.. Compiling with-Xlint:uncheckedgivesGenericArrays.java:6: warning: [unchecked] unchecked conversionjj<Integer>[] lol = new jj[10];// But this is allowed why?withnewunderlined. As does my Eclipse Neon.1 install using the same JDK:Type safety: The expression of type jj[] needs unchecked conversion to conform to jj<Integer>[]. Perhaps some warnings are turned off in your compiler (+ possible IDE) setup?