### Question:
Concat extends ERE { public static ERE get(ERE left, ERE right) { Concat cat = new Concat(left, right); ERE ret = cat.simplify(); return ret; } private  Concat(ERE left, ERE right); static ERE get(ERE left, ERE right); @Override EREType getEREType(); @Override String toString(); @Override ERE copy(); @Override boolean containsEpsilon(); @Override ERE derive(Symbol s);  }### Answer:
@Test public void testEquality() { ERE one = Concat.get(a, b); ERE two = Concat.get(a, b); assertEquals(one, two); assertEquals(0, one.compareTo(two)); }
@Test public void testInequality() { ERE one = Concat.get(a, b); ERE two = Concat.get(a, c); assertFalse(one.equals(two)); assertFalse(0 == one.compareTo(two)); assertFalse(one.equals(a)); assertFalse(0 == one.compareTo(a)); }