Cautious use of keyset: Comparison of 2 traversal modes for HASHMAP

The HashMap store is a key-value pair, so its traversal with the list and set should be different in general.

But Java cleverly handles HashMap's key-value pairs as a monolithic object (Java.util.Map.Entry), which optimizes the traversal of the hashmap, making it no different from the list and set.

The first type:

Java code

1. Map map = new HashMap ();
2. Iterator iter = Map.entryset (). Iterator ();
3. while (Iter.hasnext ()) {
4. Java.util.Map.Entry Entry = (map.entry) iter.next ();
5. Object key = Entry.getkey ();
6. Object val = Entry.getvalue ();
7. }

The second type:

Java code

1. Map map = new HashMap ();
2. Iterator iter = Map.keyset (). Iterator ();
3. while (Iter.hasnext ()) {
4. Object key = Iter.next ();
5. Object val = map.get (key);
6. }

For example:HASHMAP traversal has two common methods, that is to use keyset and entryset to traverse, but the two traverse speed is different, see the example: Java code

2. public class Hashmaptest {
6. public static void Main (string[] args) ... {
10. HashMap HashMap = new HashMap ();
12. for (int i = 0; i <; i) ... {
14. Hashmap.put ("I," that's All ");
16. }
18. Long num = Calendar.getinstance (). Gettimeinmillis ();
20. Iterator Iterator = Hashmap.keyset (). Iterator ();
24. while (Iterator.hasnext ()) ... {
26. System.out.print (Hashmap.get (Iterator.next ()));
28. }
30. System.out.println ();
32. System.out.println (Calendar.getinstance (). Gettimeinmillis ()-num);
34. Listhashmap ();
36. }
38. public static void Listhashmap () ... {
40. Java.util.HashMap HashMap = new Java.util.HashMap ();
44. for (int i = 0; i <; i) ... {
46. Hashmap.put ("I," that's All ");
48. }
50. Long num = Calendar.getinstance (). Gettimeinmillis ();
52. Java.util.Iterator it = Hashmap.entryset (). Iterator ();
54. while (It.hasnext ()) ... {
56. Java.util.Map.Entry Entry = (java.util.Map.Entry) it.next ();
58. Entry.getkey () returns the key corresponding to this key
60. Entry.getvalue () returns the value corresponding to this key
62. System.out.print (Entry.getvalue ());
64. }
66. System.out.println ();
68. System.out.println (Calendar.getinstance (). Gettimeinmillis ()-num);
70. }
72. }

  

Look at the JDK source code, compare two ways of access:

First look at keyset access mode:

Java code

2. Public set<k> KeySet () {
4. if (KeySet = = null) {
6. KeySet = new Abstractset<k> () {
8. Public iterator<k> Iterator () {
10. return new iterator<k> () {
12. Private Iterator<entry<k,v>> i = EntrySet (). Iterator ();
16. public Boolean hasnext () {
18. return I.hasnext ();
20. }
24. Public K Next () {
26. Return I.next (). GetKey ();
28. }
32. public void Remove () {
34. I.remove ();
36. }
38. };
40. }
44. public int size () {
46. return AbstractMap.this.size ();
48. }
52. Public Boolean contains (Object k) {
54. Return AbstractMap.this.containsKey (k);
56. }
58. };
60. }
62. return keySet;
64. }

Conclusion:Through the above test we can see that the use of entryset-way traversal efficiency is better than keyset, so in the development to use EntrySet, try to avoid less use keyset.

Cautious use of keyset: Comparison of 2 traversal modes for HASHMAP