java warning: Varargs method could cause heap pollution from non-reifiable varargs parameter

Question

I am using IntelliJ IDEA with javac on JDK 1.8. I have the following code:

class Test
{
    @SafeVarargs
    final void varargsMe         


        

Answer 1

In fact you should not write your code in this way. Consider the following example:

import java.util.*;

class Test
{
    @SafeVarargs
    @SuppressWarnings("varargs")
    final void varargsMethod( Collection... varargs )
    {
        arrayMethod( varargs );
    }

    void arrayMethod( Collection[] args )
    {
        Object[] array = args;
        array[1] = new Integer(1);
        //
        //ArrayList list = new ArrayList<>();
        //list.add(new Integer(1));
        //array[1] = list;
    }

    public static void main(String[] args)
    {
        ArrayList list1 = new ArrayList<>();
        ArrayList list2 = new ArrayList<>();
        (new Test()).varargsMethod(list1, list2);
    }
}

If you run the code, you will see an ArrayStoreException because you put an Integer into a Collection array.

However, if you replace array[1] = new Integer(1); with the three comment lines (i.e. to put an ArrayList into the array), due to type erasure, no exception is thrown and no compilation error occurs.

You want to have a Collection array, but now it contains a ArrayList. This is quite dangerous as you won't realise there is a problem.