Java Practices -> Prefer empty items to null ones

Prefer empty items to null ones

There are many cases in which an "empty" object can be used instead of a null object reference. This is usually preferable, since it helps eliminate one of the most common problems encountered during development -- the NullPointerException.

The following qualify as "empty" objects :

the empty String
zero-length arrays
collections containing 0 items
maps containing 0 items

Example declarations of empty items :

String item = "";
BigDecimal[] items = new BigDecimal[0];
List<Integer> items = new ArrayList<Integer>();
Map<String, Integer> items = new LinkedHashMap<String, Integer>();

The Collections class contains several type safe methods which return empty items (which are also immutable and Serializable) :

Collections.emptyList()
Collections.emptySet()
Collections.emptyMap()

The above methods are occasionally useful.

Example

Here's an example of using an empty, zero-length array instead of a null reference. This simplifies the caller, since they never have to check for null values. (Another alternative is to use replace the array with a possibly-empty collection.)


import java.util.*;

public class StockPortfolio {

  //..elided

  /**
  * A field is implemented internally as a Collection,
  * but offered to the user of this class as an Array.
  * This array is never null, even if the underlying Collection is
  * empty.
  */
  public String[] getStocks() {
    /*
    NO_STOCKS plays a double role here. If there are elements in fStocks, then
    toArray will allocate a new array, using NO_STOCKS simply as an indicator
    of the proper type to use. If fStocks has no elements at all, then toArray
    will simply return the NO_STOCKS array itself.
    */
    return (String[]) fStocks.toArray(NO_STOCKS);
  }

  // PRIVATE //

  /** The underlying collection.  */
  private List fStocks;

  /** A constant, empty array, to be used instead of a null array.  */
  private static final String[]  NO_STOCKS = new String[0];
}