Top 9 questions about Java Maps

In general, Map is a data structure consisting of a set of key-value pairs, and each key can only appears once in the map. This post summarizes Top 9 FAQ of how to use Java Map and its implemented classes. For sake of simplicity, I will use generics in examples. Therefore, I will just write Map instead of specific Map. But you can always assume that both the K and V are comparable, which means K extends Comparable and V extends Comparable.

0. Convert a Map to a List

In Java, Map interface provides three collection views: key set, value set, and key-value set. All of them can be converted to List by using a constructor or addAll() method. The following snippet of code shows how to construct an ArrayList from a map.

// key list
List keyList = new ArrayList(map.keySet());
// value list
List valueList = new ArrayList(map.values());
// key-value list
List entryList = new ArrayList(map.entrySet());

1. Iterate over a Map

Iterating over every pair of key-value is the most basic operation to traverse a map. In Java, such pair is stored in the map entry called Map.Entry. Map.entrySet() returns a key-value set, therefore the most efficient way of going through every entry of a map is

for(Entry entry: map.entrySet()) {
  // get key
  K key = entry.getKey();
  // get value
  V value = entry.getValue();
}

Iterator can also be used, especially before JDK 1.5

Iterator itr = map.entrySet().iterator();
while(itr.hasNext()) {
  Entry entry = itr.next();
  // get key
  K key = entry.getKey();
  // get value
  V value = entry.getValue();
}

2. Sort a Map on the keys

Sorting a map on the keys is another frequent operation. One way is to put Map.Entry into a list, and sort it using a comparator that sorts the value.

List list = new ArrayList(map.entrySet());
Collections.sort(list, new Comparator() {
 
  @Override
  public int compare(Entry e1, Entry e2) {
    return e1.getKey().compareTo(e2.getKey());
  }
 
});

The other way is to use SortedMap, which further provides a total ordering on its keys. Therefore all keys must either implement Comparable or be accepted by the comparator.

One implementing class of SortedMap is TreeMap. Its constructor can accept a comparator. The following code shows how to transform a general map to a sorted map.

SortedMap sortedMap = new TreeMap(new Comparator() {
 
  @Override
  public int compare(K k1, K k2) {
    return k1.compareTo(k2);
  }
 
});
sortedMap.putAll(map);

3. Sort a Map on the values

Putting the map into a list and sorting it works on this case too, but we need to compare Entry.getValue() this time. The code below is almost same as before.

List list = new ArrayList(map.entrySet());
Collections.sort(list, new Comparator() {
 
  @Override
  public int compare(Entry e1, Entry e2) {
    return e1.getValue().compareTo(e2.getValue());
  }
 
});

We can still use a sorted map for this question, but only if the values are unique too. Under such condition, you can reverse the key=value pair to value=key. This solution has very strong limitation therefore is not really recommended by me.

4. Initialize a static/immutable Map

When you expect a map to remain constant, it’s a good practice to copy it into an immutable map. Such defensive programming techniques will help you create not only safe for use but also safe for thread maps.

To initialize a static/immutable map, we can use a static initializer (like below). The problem of this code is that, although map is declared as static final, we can still operate it after initialization, like Test.map.put(3,"three");. Therefore it is not really immutable. To create an immutable map using a static initializer, we need an extra anonymous class and copy it into an unmodifiable map at the last step of initialization. Please see the second piece of code. Then, an UnsupportedOperationException will be thrown if you run Test.map.put(3,"three");.

public class Test {
 
  private static final Map map;
  static {
    map = new HashMap();
    map.put(1, "one");
    map.put(2, "two");
  }
}
public class Test {
 
  private static final Map map;
  static {
    Map aMap = new HashMap();
    aMap.put(1, "one");
    aMap.put(2, "two");
    map = Collections.unmodifiableMap(aMap);
  }
}

Guava libraries also support different ways of intilizaing a static and immutable collection. To learn more about the benefits of Guava’s immutable collection utilities, see Immutable Collections Explained in Guava User Guide.

5. Difference between HashMap, TreeMap, and Hashtable

There are three main implementations of Map interface in Java: HashMap, TreeMap, and Hashtable. The most important differences include:

1. The order of iteration. HashMap and Hashtable make no guarantees as to the order of the map; in particular, they do not guarantee that the order will remain constant over time. But TreeMap will iterate the whole entries according the “natural ordering” of the keys or by a comparator.
2. key-value permission. HashMap allows null key and null values (Only one null key is allowed because no two keys are allowed the same). Hashtable does not allow null key or null values. If TreeMap uses natural ordering or its comparator does not allow null keys, an exception will be thrown.
3. Synchronized. Only Hashtable is synchronized, others are not. Therefore, “if a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable.”

A more complete comparison is

                 | HashMap | Hashtable | TreeMap
-------------------------------------------------------
iteration order  | no      | no        | yes
null key-value   | yes-yes | no-no   | no-yes
synchronized     | no      | yes       | no
time performance | O(1)    | O(1)      | O(log n)
implementation   | buckets | buckets   | red-black tree

Read more about HashMap vs. TreeMap vs. Hashtable vs. LinkedHashMap.

6. A Map with reverse view/lookup

Sometimes, we need a set of key-key pairs, which means the map’s values are unique as well as keys (one-to-one map). This constraint enables to create an “inverse lookup/view” of a map. So we can look up a key by its value. Such data structure is called bidirectional map, which unfortunately is not supported by JDK.

Both Apache Common Collections and Guava provide implementation of bidirectional map, called BidiMap and BiMap, respectively. Both enforce the restriction that there is a 1:1 relation between keys and values.

7. Shallow copy of a Map

Most implementation of a map in java, if not all, provides a constructor of copy of another map. But the copy procedure is not synchronized. That means when one thread copies a map, another one may modify it structurally. To [prevent accidental unsynchronized copy, one should use Collections.synchronizedMap() in advance.

Map copiedMap = Collections.synchronizedMap(map);

Another interesting way of shallow copy is by using clone() method. However it is NOT even recommended by the designer of Java collection framework, Josh Bloch. In a conversation about “Copy constructor versus cloning“, he said

I often provide a public clone method on concrete classes because people expect it. … It’s a shame that Cloneable is broken, but it happens. … Cloneable is a weak spot, and I think people should be aware of its limitations.

For this reason, I will not even tell you how to use clone() method to copy a map.

8. Create an empty Map

If the map is immutable, use

map = Collections.emptyMap();

Otherwise, use whichever implementation. For example

map = new HashMap();

THE END