Generic array creation error [duplicate]

问题:

This question already has an answer here:

• How to create a generic array in Java? 29 answers

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.)

回答1:

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 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); 

回答2:

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>();
}

回答3:

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\")

回答4:

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...

回答5:

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.