toArray() method

[Vijay Chandran]

import java.util.LinkedList;
 
public class MyList
{
    public static void main(String[] args)
    {
        LinkedList<String> list = new LinkedList<String>();
        list.add("one");
        list.add("two");
        list.add("three");
        String[] sa = new String[3];
        sa = list.toArray(sa); // line 1
        sa = (String[]) list.toArray(); // line 2
        for(String s : sa)
            System.out.println(s);
    }
}

Dear friends,
In the above code, line 1 gives no runtime exception but line2 gives ClassCastException. The toArray() method returns an Object[] array which is type casted into String[] array and assigned to String[] reference sa.

I couldn't understand why it threw a ClassCastException.
Kindly provide me some explanation.
Regards,
Vijay

[marc weber]

Originally posted by vijaychandran rajagopalan:
... The toArray() method returns an Object[] array which is type casted into String[] array...

You can only downcast the reference to String[] if the object is, in fact, a String[]. But it is not a String[]. It is an Object[]. Yes, the objects it contains happen to be Strings, but the type of the array is not String[].

Consider the following code...

public class ArrayTypes {
  public static void main(String[] args) {
    //two Object[] references...
    Object[] a1 = new String[1]; //points to String[]
    Object[] a2 = new Object[1]; //points to Object[]
 
    //both can contain Strings (upcast to Objects)...
    a1[0] = "abc";
    a2[0] = "def";
    String s1 = (String)a1[0];
    String s2 = (String)a2[0];
    
    //but only one can be cast to type String[]...
    String[] sa1 = (String[])a1; //okay
    String[] sa2 = (String[])a2; //runtime error!!!
  }
}

[ January 06, 2008: Message edited by: marc weber ]

[Rob Spoor]

If you read the API, it tells you that the first way you used will use the array you passed to it, or it will create a new array of the same type if the array is not large enough. The second way you used just returns an Object[], since it cannot create an array of E[] (String is not available in the code, just the generic type E) and has no other options except Object[].