Error: Generic Array Creation [duplicate]

Question

I don't understand the error of Generic Array Creation.
First I tried the following:

  public PCB[] getAll() {
       PCB[] res = new PCB[list.size()];
           for (int i = 0; i < res.length; i++) {
               res[i] = list.get(i);
            }
       list.clear();
       return res;
}

Then I tried doing this:

PCB[] res = new PCB[100];

I must be missing something cause that seems right. I tried looking it up I really did. And nothing is clicking.

My question is: What can I do to fix this?

the error is :

.\Queue.java:26: generic array creation
PCB[] res = new PCB[200];
            ^
Note: U:\Senior Year\CS451- file      
uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error

Tool completed with exit code 1

Answer 1

You can't create arrays with a generic component type.

Create an array of an explicit type, like Object[], instead. You can then cast this to PCB[] if you want, but I don't recommend it in most cases.

PCB[] res = (PCB[]) new Object[list.size()]; /* Not type-safe. */

If you want type safety, use a collection like java.util.List<PCB> instead of an array.

By the way, if list is already a java.util.List, you should use one of its toArray() methods, instead of duplicating them in your code. This doesn't get your around the type-safety problem though.

Answer 2

The following will give you an array of the type you want while preserving type safety.

PCB[] getAll(Class<PCB[]> arrayType) {  
    PCB[] res = arrayType.cast(java.lang.reflect.Array.newInstance(arrayType.getComponentType(), list.size()));  
    for (int i = 0; i < res.length; i++)  {  
        res[i] = list.get(i);  
    }  
    list.clear();  
    return res;  
}

How this works is explained in depth in my answer to the question that Kirk Woll linked as a duplicate.

Answer 3

Besides the way suggested in the "possible duplicate", the other main way of getting around this problem is for the array itself (or at least a template of one) to be supplied by the caller, who will hopefully know the concrete type and can thus safely create the array.

This is the way methods like ArrayList.toArray(T[]) are implemented. I'd suggest you take a look at that method for inspiration. Better yet, you should probably be using that method anyway as others have noted.