Discover The mysteries of Set set = Hashmap.keyset ()

There are still a lot of things to look out for between set and HashMap:

hashmap<string,string> map = new hashmap<string,string> ();
Map.put ("1", "AA");
Map.put ("2", "BB");
Map.put ("3", "AA");

set<string> set = Map.keyset ();

1. The keyset () method of map only returns a set instance, so when you delete an object from Key1, the other will also be affected.

2. For set cannot be add operation, otherwise will report "Java.lang.UnsupportedOperationException". The reasons are as follows:

Public set<k> Keyset () returns the Set view of the keys contained in this map. The collection is supported by mappings, so changes to the mappings are also reflected in the collection and vice versa. The collection supports the removal of elements by removing the corresponding mapping from the mapping through the Iterator.remove, Set.remove, RemoveAll, Retainall, and clear operations. It does not support add or addall operations.

3.

Set<string> set= Map.keyset ();

for (String Key:set) {

Map.Remove (key);

}

This code reports this exception: Exception in thread "main" java.util.ConcurrentModificationException
At Java.util.hashmap$hashiterator.nextentry (Unknown Source)
At Java.util.hashmap$keyiterator.next (Unknown Source)
At Com.nan.an.jing.TestSet.main (testset.java:23)

The reason: Because after deleting a entry, because the Ketset is incorrect, the problem occurs in the For loop.

The correct method is:

string[] keyset = Map.keyset (). ToArray (new string[0]);

for (String Key:keyset) {

Map.Remove (key);

} or

object[] keyset = Map.keyset (). ToArray ();
for (Object Key:keyset) {
Map.Remove ((String) key);
}

NOTES: Code instances

Package com.nan.an.jing;
Import java.util.ArrayList;
Import Java.util.HashMap;
Import Java.util.Iterator;
Import java.util.List;
Import Java.util.Map;
Import Java.util.Map.Entry;

Import Java.util.Set; public class Testset {public static void main (string[] args) {hashmap<string,string> map = new Hashmap<str
	 Ing,string> ();
	 Map.put ("1", "AA");
	 Map.put ("2", "BB");
	 
	 Map.put ("3", "AA");
	 
	 Map.Remove ("1");
	 Must be converted to array, or string[] keyset = Map.keyset (). ToArray (new string[0]);
	 object[] keyset = Map.keyset (). ToArray ();
	 for (Object key:keyset) {map.remove ((String) key);
	 
	 } System.out.println (map);
	 Set only one instance, set Delete, map also follow delete set<string> Set = Map.keyset ();
	 Set.remove ("1");
	 SYSTEM.OUT.PRINTLN (set);
 
	 SYSTEM.OUT.PRINTLN (map);
     The iterator method deletes the map for (iterator<string> iterator = Set.iterator (); Iterator.hasnext ();)
         {Iterator.next ();
     Iterator.remove ();
	 
	 } System.out.println (map); Adopt itErator method obtains map information set<entry<string, string>> set0 = Map.entryset (); For (iterator<map.entry<string, string>> iterator = Set0.iterator (); Iterator.hasnext ();) {Map.Entry<
		 String, string> mm = Iterator.next ();
		 String key1 = Mm.getkey ();
	 String value = Mm.getvalue ();
	 
	  } System.out.println (SET0);
	 Use list to add element hashmap<string,string> Map1 = new hashmap<string,string> ();
	 Map1.put ("1", "AA");
		
	 Map1.put ("2", "BB");
	 set<string> Set1 = Map1.keyset ();	 
	 list<string> ll = new arraylist<string> ();
	 for (String Ss:set1) {ll.add (ss);
	 } ll.add ("CC");
	 System.out.println (LL);
System.out.println (MAP1);
 }
}