Getting the set of all keys with the keySet() method: public Set keySet() : HashMap « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. HashMap

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class MainClass {
  public static void main(String[] a) {
    Map map = new HashMap();
    map.put("key1", "value1");
    map.put("key2", "value2");
    map.put("key3", "value3");
    map.put(null, null);

    Set set = map.keySet();

    Iterator iter = set.iterator();

    while (iter.hasNext()) {
      System.out.println(iter.next());
    }
  }
}

key1
key3
null
key2