private void doStuff(Long inLong) {
    long theNumber = inLong;

    /* ... */
}

private void doStuff(Long inLong) {
  assert inLong != null;
  long theNumber = inLong;
  /* ... */
}

public void doStuff(Long inLong) {
  if (inLong == null) { throw new IllegalArgumentException(); }
  long theNumber = inLong;
  /* ... */
}

private void doStuff(Long inLong) {
  if (inLong == null) {
    handleUndef();
  } else {
    long theNumber = inLong;
    /* ... */
  }
}

theNumber = 0; // or Long.MIN_VALUE or Long.MAX_VALUE or whatever you prefer
if (inLong != null) {
    theNumber = inLong;
}
// ...

long theNumber = inLong;

long theNumber = inLong.longValue();