Common traversal methods of Map sets in Java and Applications of hashmap

There are roughly three types of map traversal:

1. Traverse map. entryset (): each of its elements is a map. Entry object. In this object,

There is a key-value pair in the map;

2. Traverse map. keyset (): It is a set of key values in map. We can traverse this set

Read the elements in the map;

3. Traverse map. Values (): It is a set of values in map. We can traverse through this set directly.

The value in the map, but cannot read the key.

 

Package COM. sort; import Java. util. hashmap; import Java. util. iterator; import Java. util. map; import Java. util. set; import Java. util. map. entry; /*** the map set is the same as the set. * several common methods of map set traversal * @ author owner **/public class maptest5 {public static void main (string [] ARGs) {Map <string, string> map = new hashmap <string, string> (); map. put ("A", "zhangsan"); map. put ("B", "Lisi"); map. put ("C", "wangwu"); // system. out. println (MAP);/*** method 1 traverse map */set <string> keyset = map. keyset (); For (iterator <string> iterator = keyset. iterator (); iterator. hasnext ();) {string key = iterator. next (); string value = map. get (key); system. out. println (Key + "=" + value);} system. out. println ("**********************"); /*** method 2 traverse map */For (string key: map. keyset () {system. out. println (Key + "=" + map. get (key);} system. out. println ("**********************");/***** method 3 traverses the map, we recommend that you use this method to traverse Map sets, especially when the capacity is large */For (map. entry <string, string> entry: map. entryset () {system. out. println (entry. getkey () + "=" + entry. getvalue ();} system. out. println ("**********************"); /*** Method 4 traverse map */set <entry <string, string> entryset = map. entryset (); For (iterator <map. entry <string, string> iterator = entryset. iterator (); iterator. hasnext ();) {map. entry <string, string> entry = iterator. next (); system. out. println (entry. getkey () + "=" + entry. getvalue ();} system. out. println ("**********************");/***** Method 5, this method traverses all values */For (string value: map. values () {system. out. println (value );}}}

 

Elements in map are stored unordered.

Hashmap considerations:

1. An array is maintained at the bottom layer of hashmap. The objects we put into hashmap are actually stored in this array;

2. When a key value is put to hashmap, it calculates a location based on the hashcode value of the key, which is the location where the object is to be stored in the array.

Example of a hashmap application: enter an English sentence on the console to calculate the number of times each word appears.

 

Package COM. sort; import Java. util. hashmap; import Java. util. iterator; import Java. util. map; import Java. util. imports; import Java. util. set;/*** count the number of times each word appears in a simple English sentence ** @ author owner **/public class maptest3 {public static void main (string [] ARGs) {pipeline SC = new pipeline (system. in); system. out. println ("enter an English sentence separated by spaces:"); string sentence = SC. nextline (); string [] arr = sentence. split (""); // The key represents the word, Value Indicates the number of times Map <string, integer> map = new hashmap <string, integer> (); For (INT I = 0; I <arr. length; I ++) {If (! Map. containskey (ARR [I]) {map. put (ARR [I], 1);} else {// indicates that the element int num = map exists in the map. get (ARR [I]); map. put (ARR [I], ++ num) ;}} system. out. println ("count the number of words, the result is as follows:"); set <string> set = map. keyset (); For (iterator <string> iterator = set. iterator (); iterator. hasnext ();) {string key = iterator. next (); integer value = map. get (key); system. out. println (Key + "=" + value );}}}
 

 

Output result:

Enter an English sentence separated by spaces:
I love u China I am pround to be a Chinese
Count the number of words that appear. The result is as follows:
To = 1
U = 1
Love = 1
AM = 1
A = 1
Pround = 1
Chinese = 1
I = 2
China = 1
Be = 1