How to convert object[] to specific type array

The other answers show what to do if you really need to convert an Object[] - but there's a better approach. Change your code to start with:

List<B> c = new ArrayList<B>();
c.add(***);
B[] b = c.toArray(new B[c.size()]);

Or:

List<B> c = new ArrayList<B>();
c.add(***);
B[] b = c.toArray(new B[0]);

If every element of a is of type B, you have two options (if not, you need to explain what's going on first):

B[] bArray;
if(a instanceof B[]){
    // a is actually of type B[], so we'll cast it
    bArray = (B[]) a;
}else{
    // a is of type Object[], so we'll create a new array and copy the values
    bArray = Array.newInstance(B.class, a.length);
    System.arraycopy(a, 0, bArray, 0, a.length);
}

Also, this will only work if B is a real type, not a generic parameter!