Key existence check in HashMap

Question

Is checking for key existence in HashMap always necessary   I have a HashMap with say a 1000 entries and I am looking at improving the efficiency  If the HashMap is being accessed very frequently  then checking for the key existence at every access will lead to a large overhead  Instead if the key is not present and hence an exception occurs  I can catch the exception   when I know that this will happen rarely   This will reduce accesses to the HashMap by half   This might not be a good programming practice  but it will help me reduce the number of accesses  Or am I missing something here    Update  I do not have null values in the HashMap

User · Answer

Better way is to use containsKey method of HashMap  Tomorrow somebody will add null to the Map  You should differentiate between key presence and key has null value

User · Answer

The Jon Skeet answer addresses well the two scenarios  map with null value and not null value  in an efficient way   About the number entries and the efficiency concern  I would like add something      I have a HashMap with say a 1 000 entries and I am looking at improving   the efficiency  If the HashMap is being accessed very frequently  then   checking for the key existence at every access will lead to a large   overhead    A map with 1 000 entries is not a huge map  As well as a map with 5 000 or 10 000 entries  Map are designed to make fast retrieval with such dimensions   Now  it assumes that hashCode   of the map keys provides a good distribution   If you may use an Integer as key type  do it  Its hashCode   method is  very efficient since the collisions are not possible for unique int values    public final class Integer extends Number implements Comparable lt Integer gt                 Override     public int hashCode             return Integer hashCode value              public static int hashCode int value            return value                    If for the key  you have to use another built-in type as String for example that is often used in Map  you may have some collisions but from 1 thousand to some thousands of objects in the Map  you should have very few of it as the String hashCode   method provides a good distribution   If you use a custom type  override hashCode   and equals   correctly and ensure overall that hashCode   provides a fair distribution  You may refer to the item 9 of Java Effective refers it  Here s a post that details the way

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I usually use the idiom  Object value   map get key   if  value    null        value   createValue key       map put key  value       This means you only hit the map twice if the key is missing

User · Answer

Do you mean that you ve got code like      if map containsKey key   doSomethingWith map get key     all over the place   Then you should simply check whether map get key  returned null and that s it  By the way  HashMap doesn t throw exceptions for missing keys  it returns null instead  The only case where containsKey is needed is when you re storing null values  to distinguish between a null value and a missing value  but this is usually considered bad practice

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You won t gain anything by checking that the key exists  This is the code of HashMap    Override public boolean containsKey Object key        Entry lt K  V gt  m   getEntry key       return m    null      Override public V get Object key        Entry lt K  V gt  m   getEntry key       if  m    null            return m value            return null      Just check if the return value for get   is different from null   This is the HashMap source code     Resources     HashMap source code Bad one HashMap source code Good one

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Do you ever store a null value  If not  you can just do   Foo value   map get key   if  value    null              else          No such key     Otherwise  you could just check for existence if you get a null value returned   Foo value   map get key   if  value    null              else          Key might be present        if  map containsKey key               Okay  there s a key but the value is null       else             Definitely no such key

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You can also use the computeIfAbsent   method in the HashMap class    In the following example  map stores a list of transactions  integers  that are applied to the key  the name of the bank account   To add 2 transactions of 100 and 200 to checking account you can write   HashMap lt String  ArrayList lt Integer gt  gt  map   new HashMap lt  gt     map computeIfAbsent  checking account   key - gt  new ArrayList lt  gt         add 100      add 200     This way you don t have to check to see if the key checking account exists or not    If it does not exist  one will be created and returned by the lambda expression   If it exists  then the value for the key will be returned by computeIfAbsent       Really elegant

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Just use containsKey   for clarity  It s fast and keeps the code clean and readable  The whole point of HashMaps is that the key lookup is fast  just make sure the hashCode   and equals   are properly implemented

User · Answer

If key class is your s make sure the hashCode   and equals   methods implemented  Basically the access to HashMap should be O 1  but with wrong hashCode method implementation it s become O n   because value with same hash key will stored as Linked list

User · Answer

if map get key     null     map get key     null  amp  amp  map containsKey key

[java] Key existence check in HashMap

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