How to update a value given a key in a hashmap

Question

Suppose we have a HashMap lt String  Integer gt  in Java   How do I update  increment  the integer-value of the string-key for each existence of the string I find   One could remove and reenter the pair  but overhead would be a concern  Another way would be to just put the new pair and the old one would be replaced   In the latter case  what happens if there is a hashcode collision with a new key I am trying to insert  The correct behavior for a hashtable would be to assign a different place for it  or make a list out of it in the current bucket

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Java 8 way   You can use computeIfPresent method and supply it a mapping function  which will be called to compute a new value based on existing one   For example   Map lt String  Integer gt  words   new HashMap lt  gt     words put  hello   3   words put  world   4   words computeIfPresent  hello    k  v  - gt  v   1   System out println words get  hello       Alternatevely  you could use merge method  where 1 is the default value and function increments existing value by 1   words merge  hello   1  Integer  sum     In addition  there is a bunch of other useful methods  such as putIfAbsent  getOrDefault  forEach  etc

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The cleaner solution without NullPointerException is   map replace key  map get key    1

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Integer i   map get key   if i    null     i    aValue  map put key  i   1     or  Integer i   map get key   map put key  i    null   newValue   i   1     Integer is Primitive data types http   cs fit edu  ryan java language java-data html  so you need to take it out  make some process  then put it back  if you have a value which is not Primitive data types  you only need to take it out  process it  no need to put it back into the hashmap

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Matthew s solution is the simplest and will perform well enough in most cases   If you need high performance  AtomicInteger is a better solution ala  BalusC   However  a faster solution  provided thread safety is not an issue  is to use TObjectIntHashMap which provides a increment key  method and uses primitives and less objects than creating AtomicIntegers  e g   TObjectIntHashMap lt String gt  map   new TObjectIntHashMap lt String gt    map increment  aaa

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Use a for loop to increment the index   for  int i  0  i lt 5  i         HashMap lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  beer   100        int beer   map get  beer   i      System out println  beer     beer       System out

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There are misleading answers to this question here that imply Hashtable put method will replace the existing value if the key exists  this is not true for Hashtable but rather for HashMap  See Javadoc for HashMap http   docs oracle com javase 7 docs api java util HashMap html put 28K  20V 29

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hashmap put key  hashmap get key    1     The method put will replace the value of an existing key and will create it if doesn t exist

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One line solution   map put key  map containsKey key    map get key    1   1

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Does the hash exist  with 0 as the value  or is it  put  to the map on the first increment  If it is  put  on the first increment  the code should look like   if  hashmap containsKey key         hashmap put key  hashmap get key  1     else        hashmap put key 1

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The simplified Java 8 way   map put key  map getOrDefault key  0    1     This uses the method of HashMap that retrieves the value for a key  but if the key can t be retrieved it returns the specified default value  in this case a  0     This is supported within core Java  HashMap lt K V gt  getOrDefault  40 Object key  V defaultValue  41

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Try   HashMap hm new HashMap lt String  Double  gt       NOTE   String- gt give the new value    THIS IS THE KEY else Double- gt pass new value    THIS IS THE VALUE   You can change either the key or the value in your hashmap  but you can t change both at the same time

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Since I can t comment to a few answers due to less reputation  I will post a solution which I applied   for String key   someArray       if hashMap containsKey key   will check if a particular key exist or not             hashMap put hashMap get key  value 1     increment the value by 1 to an already existing key         else            hashMap put key value     make a new entry into the hashmap

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Use Java8 built in fuction  computeIfPresent   Example   public class ExampleToUpdateMapValue        public static void main String   args            Map lt String String gt  bookAuthors   new TreeMap lt  gt             bookAuthors put  Genesis   Moses            bookAuthors put  Joshua   Joshua            bookAuthors put  Judges   Samuel             System out println  ---------------------Before----------------------            bookAuthors entrySet   stream   forEach System out  println              To update the existing value using Java 8         bookAuthors computeIfPresent  Judges    k v  - gt  v    Samuel Nathan Gad             System out println  ---------------------After----------------------            bookAuthors entrySet   stream   forEach System out  println

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map put key  map get key    1     should be fine   It will update the value for the existing mapping   Note that this uses auto-boxing  With the help of map get key  we get the value of corresponding key  then you can update with your requirement  Here I am updating to increment value by 1

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It may be little late but here are my two cents   If you are using Java 8 then you can make use of computeIfPresent method  If the value for the specified key is present and non-null then it attempts to compute a new mapping given the key and its current mapped value   final Map lt String Integer gt  map1   new HashMap lt  gt     map1 put  A  0   map1 put  B  0   map1 computeIfPresent  B   k v - gt v 1       A 0  B 1    We can also make use of another method putIfAbsent to put a key  If the specified key is not already associated with a value  or is mapped to null  then this method associates it with the given value and returns null  else returns the current value   In case the map is shared across threads then we can make use of ConcurrentHashMap and AtomicInteger  From the doc      An AtomicInteger is an int value that may be updated atomically   An   AtomicInteger is used in applications such as atomically incremented   counters  and cannot be used as a replacement for an Integer  However    this class does extend Number to allow uniform access by tools and   utilities that deal with numerically-based classes    We can use them as shown   final Map lt String AtomicInteger gt  map2   new ConcurrentHashMap lt  gt     map2 putIfAbsent  A  new AtomicInteger 0    map2 putIfAbsent  B  new AtomicInteger 0       A 0  B 0  map2 get  B   incrementAndGet          A 0  B 1    One point to observe is we are invoking get to get the value for key B and then invoking incrementAndGet   on its value which is of course AtomicInteger  We can optimize it as the method putIfAbsent returns the value for the key if already present    map2 putIfAbsent  B  new AtomicInteger 0   incrementAndGet      A 0  B 2    On a side note if we plan to use AtomicLong then as per documentation under high contention expected throughput of LongAdder is significantly higher  at the expense of higher space consumption  Also check this question

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Replace Integer by AtomicInteger and call one of the incrementAndGet getAndIncrement methods on it   An alternative is to wrap an int in your own MutableInteger class which has an increment   method  you only have a threadsafety concern to solve yet

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You can increment like below but you need to check for existence so that a NullPointerException is not thrown  if  map containsKey key      p put key 1     else    p put key  map getKey   1

[java] How to update a value, given a key in a hashmap?

Java 8 way:

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