Handy New Map Default Methods in JDK 8

The Map interface provides some handy new methods in JDK 8. Because the Map methods I cover in this post are implemented as default methods, all existing implementations of the Map interface enjoy the default behaviors defined in the default methods without any new code. The JDK 8 introduced Map methods covered in this post are getOrDefault(Object, V), putIfAbsent(K, V), remove(Object, Object), remove(Object, Object), replace(K, V), and replace(K, V, V).

Example Map for Demonstrations

I will be using the Map declared and initialized as shown in the following code throughout the examples in this blog post. The statesAndCapitals field is a class-level static field. I intentionally have only included a small subset of the fifty states in the United States for reading clarity and to allow easier demonstration of some of the new JDK 8 Map default methods.

   private final static Map<string, string=""> statesAndCapitals;

   static
   {
      statesAndCapitals = new HashMap<>();
      statesAndCapitals.put("Alaska", "Anchorage");
      statesAndCapitals.put("California", "Sacramento");
      statesAndCapitals.put("Colorado", "Denver");
      statesAndCapitals.put("Florida", "Tallahassee");
      statesAndCapitals.put("Nevada", "Las Vegas");
      statesAndCapitals.put("New Mexico", "Sante Fe");
      statesAndCapitals.put("Utah", "Salt Lake City");
      statesAndCapitals.put("Wyoming", "Cheyenne");
   }

Map.getOrDefault(Object, V)

Map's new method getOrDefault(Object, V) allows the caller to specify in a single statement to get the value of the map that corresponds to the provided key or else return a provided "default value" if no match is found for the provided key.

The next code listing compares how checking for a value matching a provided key in a map or else using a default if no match is found was implemented before JDK 8 and how it can now be implemented with JDK 8.

/*
 * Demonstrate Map.getOrDefault and compare to pre-JDK 8 approach. The JDK 8
 * addition of Map.getOrDefault requires fewer lines of code than the
 * traditional approach and allows the returned value to be assigned to a
 * "final" variable.
 */

// pre-JDK 8 approach
String capitalGeorgia = statesAndCapitals.get("Georgia");
if (capitalGeorgia == null)
{
   capitalGeorgia = "Unknown";
}

// JDK 8 approach
final String capitalWisconsin = statesAndCapitals.getOrDefault("Wisconsin", "Unknown");

The Apache Commons class DefaultedMap provides functionality similar to the newMap.getOrDefault(Object, V) method. The Groovy GDK includes a similar method for Groovy,Map.get(Object, Object), but that one's behavior is a bit different because it not only returns the provided default if the "key" is not found, but also adds the key with the default value to the underlying map.

Map.putIfAbsent(K, V)

Map's new method putIfAbsent(K, V) has Javadoc advertising its default implementation equivalent:

The default implementation is equivalent to, for this map:
 
 V v = map.get(key);
 if (v == null)
     v = map.put(key, value);

 return v;

This is illustrated with another code sample that compares the pre-JDK 8 approach to the JDK 8 approach.

/*
 * Demonstrate Map.putIfAbsent and compare to pre-JDK 8 approach. The JDK 8
 * addition of Map.putIfAbsent requires fewer lines of code than the
 * traditional approach and allows the returned value to be assigned to a
 * "final" variable.
 */

// pre-JDK 8 approach
String capitalMississippi = statesAndCapitals.get("Mississippi");
if (capitalMississippi == null)
{
   capitalMississippi = statesAndCapitals.put("Mississippi", "Jackson");
}

// JDK 8 approach
final String capitalNewYork = statesAndCapitals.putIfAbsent("New York", "Albany");

Alternate solutions in the Java space before the addition of this putIfAbsent method are discussed in theStackOverflow thread Java map.get(key) - automatically do put(key) and return if key doesn't exist?. It's worth noting that before JDK 8, the ConcurrentMap interface (extends Map) already provided a putIfAbsent(K, V)method.

Map.remove(Object, Object)

Map's new remove(Object, Object) method goes beyond the long-available Map.remove(Object) method to remove a map entry only if both the provided key and provided value match an entry in the map (the previously available version only looked for a "key" match to remove).

The Javadoc comment for this method explains the how the default method's implementation works in terms of equivalent pre-JDK 8 Java code:

The default implementation is equivalent to, for this map:

if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     map.remove(key);
     return true;
 } else
     return false;

A concrete comparison of the new approach to the pre-JDK 8 approach is shown in the next code listing.

/*
 * Demonstrate Map.remove(Object, Object) and compare to pre-JDK 8 approach.
 * The JDK 8 addition of Map.remove(Object, Object) requires fewer lines of
 * code than the traditional approach and allows the returned value to be
 * assigned to a "final" variable.
 */

// pre-JDK 8 approach
boolean removed = false;
if (   statesAndCapitals.containsKey("New Mexico")
    && Objects.equals(statesAndCapitals.get("New Mexico"), "Sante Fe"))
{
   statesAndCapitals.remove("New Mexico", "Sante Fe");
   removed = true;
}

// JDK 8 approach
final boolean removedJdk8 = statesAndCapitals.remove("California", "Sacramento");

Map.replace(K, V)

The first of the two new Map "replace" methods sets the specified value to be mapped to the specified key only if the specified key already exists with some mapped value. The Javadoc comment explains the Java equivalent of this default method implementation:

The default implementation is equivalent to, for this map:

 if (map.containsKey(key)) {
     return map.put(key, value);
 } else
     return null;

The comparison of this new approach to the pre-JDK 8 approach is shown next.

/*
 * Demonstrate Map.replace(K, V) and compare to pre-JDK 8 approach. The JDK 8
 * addition of replace(K, V) requires fewer lines of code than the traditional
 * approach and allows the returned value to be assigned to a "final" 
 * variable.
 */

// pre-JDK 8 approach
String replacedCapitalCity;
if (statesAndCapitals.containsKey("Alaska"))
{
   replacedCapitalCity = statesAndCapitals.put("Alaska", "Juneau");
}

// JDK 8 approach
final String replacedJdk8City = statesAndCapitals.replace("Alaska", "Juneau");

Map.replace(K, V, V)

The second newly added Map "replace" method is more narrow in its interpretation of which existing values are replaced. While the method just covered replaces any value in a value available for the specified key in the mapping, this "replace" method that accepts an additional (third) argument will only replace the value of a mapped entry that has both a matching key and a matching value. The Javadoc comment shows the default method's implementation:

The default implementation is equivalent to, for this map:
 
 if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     map.put(key, newValue);
     return true;
 } else
     return false;

My comparison of this approach to the pre-JDK 8 approach is shown in the next code listing.

/*
 * Demonstrate Map.replace(K, V, V) and compare to pre-JDK 8 approach. The
 * JDK 8 addition of replace(K, V, V) requires fewer lines of code than the
 * traditional approach and allows the returned value to be assigned to a
 * "final" variable.
 */

// pre-JDK 8 approach
boolean replaced = false;
if (   statesAndCapitals.containsKey("Nevada")
    && Objects.equals(statesAndCapitals.get("Nevada"), "Las Vegas"))
{
   statesAndCapitals.put("Nevada", "Carson City");
   replaced = true;
}

// JDK 8 approach
final boolean replacedJdk8 = statesAndCapitals.replace("Nevada", "Las Vegas", "Carson City");