Map is a data storage method. In my personal opinion, it is an array in php or asp.net and can be directly used for Traversing arrays. However, there are many inherent methods for Traversing map in java, let's take a look at some instances.
The traversal Map method can be divided into two types:
One type is map-based Entry; map. entrySet ();
One is map-based key; map. keySet ()
Each category has two traversal methods:
A. Use the iterator;
B. Use the for-each loop;
The first method is to obtain the set of keys based on the map's keyset () method, and then traverse the map to get the value of the value.
| The Code is as follows: |
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Import java. util. HashMap;
Import java. util. Iterator;
Import java. util. Set; Public class HashMapTest2
{
Public static void main (String [] args)
{
HashMap map = new HashMap (); Map. put ("a", "aaaa ");
Map. put ("B", "bbbb ");
Map. put ("c", "cccc ");
Map. put ("d", "dddd "); Set set = map. keySet (); For (Iterator iter = set. iterator (); iter. hasNext ();)
{
String key = (String) iter. next ();
String value = (String) map. get (key );
System. out. println (key + "===" + value );
}
}
} |
The second method is to use Map. Entry for obtaining:
| The Code is as follows: |
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Import java. util. HashMap;
Import java. util. Iterator;
Import java. util. Map;
Import java. util. Set; Public class HashMapTest4
{
Public static void main (String [] args)
{
HashMap map = new HashMap (); Map. put ("a", "aa ");
Map. put ("B", "bb ");
Map. put ("c", "cc ");
Map. put ("d", "dd "); Set set = map. entrySet (); For (Iterator iter = set. iterator (); iter. hasNext ();)
{
Map. Entry entry = (Map. Entry) iter. next (); String key = (String) entry. getKey ();
String value = (String) entry. getValue ();
System. out. println (key + ":" + value );
}
}
} |
Traversal method 3 treemap keySet () Traversal
| The Code is as follows: |
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System. out. println ("method 4 ");
For (Object o: tempMap. keySet ()){
System. out. println ("key =" + o + "value =" + tempMap. get (o ));
}
System. out. println ("11111 "); // How does java traverse Map <String, ArrayList> map = new HashMap <String,
// ArrayList> ();
System. out
. Println ("java traversal Map <String, ArrayList> map = new HashMap <String, ArrayList> ();");
Map <String, ArrayList> map = new HashMap <String, ArrayList> ();
Set <String> keys = map. keySet ();
Iterator <String> iterator = keys. iterator ();
While (iterator. hasNext ()){
String key = iterator. next ();
ArrayList arrayList = map. get (key );
For (Object o: arrayList ){
System. out. println (o + "traversal Process ");
}
}
System. out. println ("2222 ");
Map <String, List> mapList = new HashMap <String, List> ();
For (Map. Entry entry: mapList. entrySet ()){
String key = entry. getKey (). toString ();
List <String> values = (List) entry. getValue ();
For (String value: values ){
System. out. println (key + "-->" + value );
}
}
}
} |
Deletes a specified element while traversing the Map.
| The Code is as follows: |
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Package net. nie. test; Import java. util. HashMap;
Import java. util. Iterator;
Import java. util. Map; Public class HashMapTest {
Private static Map <Integer, String> map = new HashMap <Integer, String> ();
/** 1. The HashMap class ing does not guarantee the sequence. Some mappings can clearly ensure the sequence: TreeMap class
* 2. During Map traversal, map. put (key, newVal) and map. remove (key) cannot be used to modify or delete elements,
* Concurrent modification exceptions may occur. You can use the iterator remove ():
* Remove the elements of the current iteration from the collection to which the iterator points
* To delete access elements.
**/
Public static void main (String [] args ){
Map. put (1, "one ");
Map. put (2, "two ");
Map. put (3, "three ");
Map. put (4, "four ");
Map. put (5, "five ");
Map. put (6, "six ");
Map. put (7, "seven ");
Map. put (8, "eight ");
Map. put (5, "five ");
Map. put (9, "nine ");
Map. put (10, "ten ");
Iterator <Map. Entry <Integer, String> it = map. entrySet (). iterator ();
While (it. hasNext ()){
Map. Entry <Integer, String> entry = it. next ();
Int key = entry. getKey ();
If (key % 2 = 1 ){
System. out. println ("delete this:" + key + "=" + key );
// Map. put (key, "odd"); // ConcurrentModificationException
// Map. remove (key); // ConcurrentModificationException
It. remove (); // OK
}
}
// Traverse the current map. This new for loop cannot modify the map content because it does not pass the iterator.
System. out. println ("------- nt final map element traversal :");
For (Map. Entry <Integer, String> entry: map. entrySet ()){
Int k = entry. getKey ();
String v = entry. getValue ();
System. out. println (k + "=" + v );
}
}
} |
