How Java collections are traversed

Traversal of Map

1. traverse key and value via Map.entryset

New Hashmap<>();        Map.put (1, ten);        Map.put (2, 8);  for (Map.entry<integer, integer> entry:map.entrySet ()) {    + "----" + Entry.getvalue ());}

If the map you traverse is an empty object (null), the For-each loop will throw a java.lang.NullPointerException exception

2. Traverse key through Map.keyset () to traverse value through Map.values (), for cases where only the key or value in the map is required

New Hashmap<>();        Map.put (1, ten);        Map.put (2, 8);          for (Integer i:map.keyset ()) {            System.out.println (i);        }                  for (Integer i:map.values ()) {            System.out.println (i);        }

3. Traverse the map via iterator

Using generics
map<integer,integer> map =NewHashmap<>(); Map.put (1, 10); Map.put (2, 8); Iterator<map.entry<integer, integer>> iterator =Map.entryset (). iterator ();  while(Iterator.hasnext ()) {Map.entry<integer, Integer> temp =Iterator.next (); System.out.println (Temp.getkey ()+ " ----- " +Temp.getvalue ()); }

Do not use generics
map<integer,integer> map = new hashmap<> ();
Map.put (1, 10);
Map.put (2, 8);
Iterator Iterator = Map.entryset (). Iterator ();
while (Iterator.hasnext ()) {
Map.entry Entry = (map.entry) iterator.next ();
Integer key = (integer) entry.getkey ();
Integer value = (integer) entry.getvalue ();
SYSTEM.OUT.PRINTLN (key + "-----" + value);
}

4. Find value by key ( low efficiency )

New Hashmap<>();        Map.put (1, ten);        Map.put (2, 8);          for (Integer key:map.keySet ()) {            = map.get (key);             + "-----" + value);        }

the traversal of the list

1. Mode One

New Arraylist<>();        List.add (1);        List.add (2);          for (Integer i:list) {            System.out.println (i);        }

2. Mode II

New Arraylist<>();        List.add (1);        List.add (2);         = list.iterator ();          while (Iterator.hasnext ()) {            System.out.println (Iterator.next ());        }

3. Mode Three

New Arraylist<>();        List.add (1);        List.add (2);          for int i = 0; I < list.size (); i++ ) {            System.out.println (List.get (i));        }

How Java collections are traversed