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