Use Array.setShort and Array.setLong : Array « Reflection « Java Tutorial

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import java.lang.reflect.Array;

public class ArrayTroubleToo {
  public static void main(String... args) {
    Object o = new int[2];
    Array.setShort(o, 0, (short) 2); // widening, succeeds
    Array.setLong(o, 1, 2L); // narrowing, fails
  }
}