Why cant I cast Object[] to StringBuilder[] in Java?

Question

ArrayList<StringBuilder> al = new ArrayList<>(
                              Arrays.asList(new StringBuilder[]{
                                  new StringBuilder("Oracle"), 
                                  new StringBuilder("Java"), 
                                  new StringBuilder("Sun"), 
                                  new StringBuilder("DataBase")}));

StringBuilder[] al2array = (StringBuilder[]) al.toArray();

If al.toArray() returns an Object[] which i know that its actually a StringBuilder[], then why cannot i cast it?

Answer 1

You most certainly can cast a variable typed as Object[] to one typed as StringBuilder[]. The compiler will not complain, and it will execute without error, IF the source reference does indeed reference a StringBuilder[].

public class ArrayCastTest {
    public static void main(String[] argv) {
        Object[] objArray;
        StringBuilder[] sbArray;
        objArray = getArray();
        sbArray = (StringBuilder[]) objArray;
        System.out.println(sbArray.toString());
    }

    public static Object[] getArray() {
        return new StringBuilder[5];
    }
}

This executes without error:

C:\JavaTools>javac ArrayCastTest.java

C:\JavaTools>java ArrayCastTest
[Ljava.lang.StringBuilder;@76f4da6d

C:\JavaTools>

The important thing to understand is that cast is "overloaded" -- it transforms simple values (like int to char) but it does not transform object references -- it only changes the declared type of the reference. The problem is that ArrayList.toArray() returns an Object[]. To get your Object[] into a StringBuilder[] use System.arraycopy.