why do we use entrySet() method and use the returned set to iterate a map?

Question

Usually we write this to get the keys and values from a map.

Map m=new HashMap();
Set s=map.entrySet();
Iterator i=s.iterator()
while(s.hasNext()){
    Map.Entry m= (map.Entry) s.next();
    System.out.println(""+m.getKey()+""+ m.getValue());
}

Why do we iterate using a set why not directly map?

Answer 1

This is as close to iterating over the map as we can because you have to say whether you want just the keys, just the values or the whole key/value entry. For Sets and Lists, there is only one option so, no need to have a separate method to do this.

BTW: This is how I would iterate over a Map. Note the use of generics, the for-each loop and the LinkedHashMap so the entries appear in some kind of logical order. TreeMap would be another good choice.

Map<K,V> m=new LinkedHashMap<K,V>();
for(Map.Entry<K,V> entry: m.entrySet())
    System.out.println(entry.getKey() + ": " + entry.getValue());

In Java 8 you can write

m.forEach((k, v) -> System.out.println(k + ": " + v));