In UnitPool.java changed ArmyUnit to MovableUnit.  Make UnitPool more generic.  If you need an ArmyUnit do the following:

void someMeathod (ArmyUnit mUnit){
  unit.setLocation("0534")
  if( unit instanceof Shark){
    ArmyUnit aUnit = (ArmyUnit)mUnit;
    aUnit.attack(); // Now you can call attack.
 }
}
In UnitPool.java changed ArmyUnit to MovableUnit.  Make UnitPool more generic.  If you need an ArmyUnit do the following:

void someMeathod (ArmyUnit mUnit){
  unit.setLocation("0534")
  if( unit instanceof Shark){
    ArmyUnit aUnit = (ArmyUnit)mUnit;
    aUnit.attack(); // Now you can call attack.
 }
}