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<>();
Map<String, Integer> items = new LinkedHashMap<>();

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

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

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. (An even better alternative is to use replace the array with a possibly-empty collection.)

import java.util.*;

public final class StockPortfolio {

  //..elided

  /**
   A field is implemented internally as a List,
   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 stocks, then
    toArray will allocate a new array, using NO_STOCKS simply as an indicator
    of the proper type to use. If stocks has no elements at all, then toArray
    will simply return the NO_STOCKS array itself.
    */
    return stocks.toArray(NO_STOCKS);
  }

  // PRIVATE

  /** The underlying collection. */
  private List<String> stocks;

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