Creating an array to store generic types in Java [duplicate]

Answer 1

Question 1:

Basically, this is forbidden by Java language. This is covered in Java Language Specification for generics.

When you use

ArrayList<Integer>[] pl2 = new ArrayList[10];    // warning

you get the compiler warning, because the following example will compile (generating warning for every line of code):

ArrayList wrongRawArrayList = new ArrayList();      // warning
wrongRawArrayList.add("string1");                   // warning 
wrongRawArrayList.add("string2");                   // warning  

pl2[0] = wrongRawArrayList;                         // warning 

but now you array, that supposed to contain ArrayList of Integer, contains totally wrong ArrayList of String objects.

Question 2:

As it was already answered, declaration of p12 provides you with compile time checking and frees you from using casting when getting items from your ArrayList.

Slightly modified previous example:

ArrayList<Integer>[] pl2 = new ArrayList[10];                // warning 

ArrayList<String> wrongArrayList = new ArrayList<String>();  // OK!
wrongArrayList.add("string1");                               // OK! 
wrongArrayList.add("string2");                               // OK!

pl2[0] = wrongArrayList;                                     // ERROR

Now, since you are using generics, this won't compile. But if you use

ArrayList[] pl2 = new ArrayList[10]; 

you will get the same result as in the first example.

Answer 2

Question1

You cannot create arrays of parameterized types

Question 2

 ArrayList<Integer>[] pl2 = new ArrayList[10];

It means you are telling to compiler that you are going to create array which will store arraylist of integers. Your arraylist will only contain Integer objects. That's where generics comes in. Generics make your code more safer and reliable. If you are sure your list should only contain integer objects, you should always go ahead with this.

But when you say

 ArrayList[] pl3 = new ArrayList[10];

it means arraylist can store any object type like string, integer, custom objects, etc.

Answer 3

Arrays are covariant. That means they retain the type of their elements at runtime. Java's generics are not. They use type erasure to basically mask the implicit casting that is going on. It's important to understand that.

You need to use Array.newInstance()

In addition, arrays carry runtime type information about their component type, that is, about the type of the elements contained. The runtime type information regarding the component type is used when elements are stored in an array in order to ensure that no "alien" elements can be inserted.

For more details look here

Answer 4

Question 1.

Well, it's not the correct syntax. Hence that does not work.

Question 2.

ArrayList<Integer>[] pl2 = new ArrayList[10];
ArrayList[] pl3 = new ArrayList[10];

Since pl2 is defined with generic type <Integer> at compile time, the compiler will be know that pl2 is only allowed to have Integers and if you try to assign somthing other than Integers you will be alerted and compilation will fail.

In pl3 since there is no generic type you can assign any type of object to the list.

Answer 5

As far as I know, in Java there are no such things as generics. In terms of types, ArrayList<Integer> and ArrayList are the same things.

Java uses type erasure for generics. It means that all type information about the generic is erased at compile time. So ArrayList<Integer> become ArrayList.

So it's just a compile-time trick. I am guessing, to avoid any confusions or mistakes that the programmer might do, they allowed ArrayList<Integer>[] to be instantiated like this: new ArrayList[10].

So an ArrayList<Integer>[] and a ArrayList[] are the same thing because the information in brackets is erased at compile time.