I am trying do something like this:-
public static ArrayList<myObject>[] a = new ArrayList<myObject>[2];
myObject is a class. I am getting this error:- Generic array creation (arrow is pointing to new.)
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4public static ArrayList<MyObject>[] a = new ArrayList[10];houssam– houssam2014-03-16 00:58:04 +00:00Commented Mar 16, 2014 at 0:58
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12This is NOT a duplication! That question is about "new E[n]" while this one is "new Generic<E>[n]". Totally different!Cruise Liu– Cruise Liu2016-09-07 17:28:52 +00:00Commented Sep 7, 2016 at 17:28
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5 Answers
You can't have arrays of generic classes. Java simply doesn't support it.
You should consider using a collection instead of an array. For instance,
public static ArrayList<List<MyObject>> a = new ArrayList<List<MyObject>();
Another "workaround" is to create an auxilliary class like this
class MyObjectArrayList extends ArrayList<MyObject> { }
and then create an array of MyObjectArrayList.
Here is a good article (now archived) on why this is not allowed in the language. The article gives the following example of what could happen if it was allowed:
List<String>[] lsa = new List<String>[10]; // illegal
Object[] oa = lsa; // OK because List<String> is a subtype of Object
List<Integer> li = new ArrayList<Integer>();
li.add(new Integer(3));
oa[0] = li;
String s = lsa[0].get(0);
5 Comments
aioobe
If it would compile you would be able to create inconsistent collections. (Arrays doesn't work like this, and instead rely on throwing an ArrayStoreException at runtime.) I'll dig up a link for you about it...
aioobe
Here's an article. ibm.com/developerworks/java/library/j-jtp01255/index.html
aalku
If you try to convert the example from the article so arrays are ArrayList then you get an error here:
List<Object> oa = lsa; // Type mismatch: cannot convert from List<List<String>> to List<Object> Why don't allow generics arrays doing the same checks? You could activate it with an annotation losing some backward compatibility in that method.aalku
The same problem here without generics ` String[] x = new String[10]; Object[] y = x; y[1] = 10; //boxing String z = x[1];`
Antani
I can't see any difference with this: Integer [] integerArray=new Integer[20]; Object [] objectArray = integerArray; objectArray[0]= "Hello world!"; integerArray[0].floatValue(); This code compiles and at runtime throws this Exception: java.lang.ArrayStoreException: java.lang.String Why?
There is a easier way to create generic arrays than using List.
First, let
public static ArrayList<myObject>[] a = new ArrayList[2];
Then initialize
for(int i = 0; i < a.length; i++) {
a[i] = new ArrayList<myObject>();
}
2 Comments
newacct
or, to avoid raw types,
ArrayList<myObject>[] a = (ArrayList<myObject>[])new ArrayList<?>[2];
AndrewF
Using the raw type
ArrayList like this will harm type safety. For example, you could write a[0] = new ArrayList<unrelatedObject>(); and it would compile just fine. Anyway, the question does not concern the contents of the array, so I don't see initialization as relevant.You can do
public static ArrayList<myObject>[] a = (ArrayList<myObject>[])new ArrayList<?>[2];
or
public static ArrayList<myObject>[] a = (ArrayList<myObject>[])new ArrayList[2];
(The former is probably better.) Both will cause unchecked warnings, which you can pretty much ignore or suppress by using: @SuppressWarnings("unchecked")
Comments
if you are trying to declare an arraylist of your generic class you can try:
public static ArrayList<MyObject> a = new ArrayList<MyObject>();
this will give you an arraylist of myobject (size 10), or if u only need an arraylist of size 2 you can do:
public static ArrayList<MyObject> a = new ArrayList<MyObject>(2);
or you may be trying to make an arraylist of arraylists:
public static ArrayList<ArrayList<MyObject>> a = new ArrayList<ArrayList<MyObject>>();
although im not sure if the last this i said is correct...
Comments
It seems to me that you use the wrong type of parenthesis. The reason why you can't define an array of generic is type erasure.
Plus, declaration of you variable "a" is fragile, it should look this way:
List<myObject>[] a;
Do not use a concrete class when you can use an interface.
