Wednesday, September 22, 2010

Iterating over map in java

There is no direct iteration over Maps. To iterate over map, get Set of keys or key-value pairs from Map. Maps do not provide an iterator() method as do Lists and Sets. A Set of either keys (keySet()) or key-value Map.Entry elements (entrySet()) can be obtained from the Map, and one can iterate over that.

Order of map elements in iteration

The order of the elements obtained from a Map depends on the type of Map they came from.
The elements are ordered by key.
The elements will be in an unpredictable, "chaotic", order.
The elements will be ordered by entry order or last reference order, depending on the type of LinkedHashMap.

Example - Iterating over the "entry set"

This example utility method prints the key-value pairs in a map. This utility method is designed to work for all Maps, so it isn't able to take advantage of generics.
//=====utility method dumpMap
public static void dumpMap(Map mp) {
  Iterator it = mp.entrySet().iterator();
  while (it.hasNext()) {
    Map.Entry pairs = (Map.Entry);
    System.out.println(pairs.getKey() +  
                         " = " + pairs.getValue());

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