Iterate through a HashMap

Question

What s the best way to iterate over the items in a HashMap

User · Answer

Smarter   for  String key   hashMap keySet          System out println  Key      key      Value      map get key

User · Answer

Depends  If you know you re going to need both the key and the value of every entry  then go through the entrySet  If you just need the values  then there s the values   method  And if you just need the keys  then use keyset     A bad practice would be to iterate through all of the keys  and then within the loop  always do map get key  to get the value  If you re doing that  then the first option I wrote is for you

User · Answer

Extracted from the reference How to Iterate Over a Map in Java   There are several ways of iterating over a Map in Java  Let s go over the most common methods and review their advantages and disadvantages  Since all maps in Java implement the Map interface  the following techniques will work for any map implementation  HashMap  TreeMap  LinkedHashMap  Hashtable  etc    Method  1  Iterating over entries using a For-Each loop   This is the most common method and is preferable in most cases  It should be used if you need both map keys and values in the loop   Map lt Integer  Integer gt  map   new HashMap lt Integer  Integer gt     for  Map Entry lt Integer  Integer gt  entry   map entrySet          System out println  Key       entry getKey        Value       entry getValue         Note that the For-Each loop was introduced in Java 5  so this method is working only in newer versions of the language  Also a For-Each loop will throw NullPointerException if you try to iterate over a map that is null  so before iterating you should always check for null references   Method  2  Iterating over keys or values using a For-Each loop   If you need only keys or values from the map  you can iterate over keySet or values instead of entrySet   Map lt Integer  Integer gt  map   new HashMap lt Integer  Integer gt         Iterating over keys only for  Integer key   map keySet          System out println  Key       key         Iterating over values only for  Integer value   map values          System out println  Value       value       This method gives a slight performance advantage over entrySet iteration  about 10  faster  and is more clean   Method  3  Iterating using Iterator   Using Generics   Map lt Integer  Integer gt  map   new HashMap lt Integer  Integer gt     Iterator lt Map Entry lt Integer  Integer gt  gt  entries   map entrySet   iterator    while  entries hasNext          Map Entry lt Integer  Integer gt  entry   entries next        System out println  Key       entry getKey        Value       entry getValue         Without Generics   Map map   new HashMap    Iterator entries   map entrySet   iterator    while  entries hasNext          Map Entry entry    Map Entry  entries next        Integer key    Integer entry getKey        Integer value    Integer entry getValue        System out println  Key       key      Value       value       You can also use same technique to iterate over keySet or values   This method might look redundant  but it has its own advantages  First of all  it is the only way to iterate over a map in older versions of Java  The other important feature is that it is the only method that allows you to remove entries from the map during iteration by calling iterator remove    If you try to do this during For-Each iteration you will get  unpredictable results  according to Javadoc   From a performance point of view this method is equal to a For-Each iteration   Method  4  Iterating over keys and searching for values  inefficient    Map lt Integer  Integer gt  map   new HashMap lt Integer  Integer gt     for  Integer key   map keySet          Integer value   map get key       System out println  Key       key      Value       value       This might look like a cleaner alternative for method  1  but in practice it is pretty slow and inefficient as getting values by a key might be time-consuming  this method in different Map implementations is 20 -200  slower than method  1   If you have FindBugs installed  it will detect this and warn you about inefficient iteration  This method should be avoided   Conclusion   If you need only keys or values from the map  use method  2  If you are stuck with older version of Java  less than 5  or planning to remove entries during iteration  you have to use method  3  Otherwise use method  1

User · Answer

Iterate through the entrySet   like so   public static void printMap Map mp        Iterator it   mp entrySet   iterator        while  it hasNext              Map Entry pair    Map Entry it next            System out println pair getKey             pair getValue             it remove       avoids a ConcurrentModificationException           Read more about Map

User · Answer

for  Map Entry lt String  String gt  item   hashMap entrySet          String key   item getKey        String value   item getValue

User · Answer

You can iterate through the entries in a Map in several ways  Get each key and value like this   Map lt     gt  map   new HashMap lt Object  Object gt     for Entry lt      gt  e  map entrySet         System out println  Key     e getKey         System out println  Value     e getValue         Or you can get the list of keys with  Collection lt   gt  keys   map keySet    for Object key  keys       System out println  Key     key       System out println  Value     map get key        If you just want to get all of the values and aren t concerned with the keys  you can use   Collection lt   gt  values   map values

User · Answer

If you re only interested in the keys  you can iterate through the keySet   of the map   Map lt String  Object gt  map         for  String key   map keySet                     If you only need the values  use values     for  Object value   map values                     Finally  if you want both the key and value  use entrySet     for  Map Entry lt String  Object gt  entry   map entrySet          String key   entry getKey        Object value   entry getValue                   One caveat  if you want to remove items mid-iteration  you ll need to do so via an Iterator  see karim79 s answer   However  changing item values is OK  see Map Entry

[java] Iterate through a HashMap

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