Load Factor in HashMapThe HashMap is one of the highperformance data structure in the Java collections framework. It gives a constant time performance for insertion and retrieval. There are two factors which affect the performance of the hashmap.
We have to choose these two factors very carefully while creating the HashMap object. Load Factor and initial capacity can be configured while we create a constructor of HashMap class, as shown below: Initial Capacity of HashMapThe initial capacity of the HashMap is the number of buckets in the hash table. It creates when we create the object of HashMap class. The initial capacity of the HashMap is 2^{4}, i.e., 16. The capacity of the HashMap is doubled each time it reaches the threshold. The capacity is increased to 2^{5}=32, 2^{6}=64, and so on. Suppose we have implemented the hashCode() method, which makes sure that keyvalue pair will be distributed among 16 buckets equally. Consider the following scenarios:
We can observe from the above scenarios that the number of items in HashMap is doubled. The maximum lookup time in each bucket is not increasing very high and remain almost constant. Alternatively, the hashmap grows in the power of 2^{n} and keep on growing when starting point it reached its limit. Load FactorThe Load factor is a measure that decides when to increase the HashMap capacity to maintain the get() and put() operation complexity of O(1). The default load factor of HashMap is 0.75f (75% of the map size). Problem The problem is, keeping the bucket size fixed (i.e., 16), we keep on increasing the total number of items in the map that disturbs time complexity. Solution When we increase the total number of buckets, total items in each bucket starts increasing. Now we are able to keep the constant number of items in each bucket and maintain the time complexity of O(1) for get() and put() operation. How Load Factor is calculatedLoad Factor decides "when to increase the number of buckets." We can find when to increase the hashmap size by using the following formula: The initial capacity of hashmap is=16 It represents that 12^{th} keyvalue pair of hashmap will keep its size to 16. As soon as 13^{th} element (keyvalue pair) will come into the Hashmap, it will increase its size from default 2^{4} = 16 buckets to 2^{5} = 32 buckets. Another way to calculate size: When the load factor ratio (m/n) reaches 0.75 at that time, hashmap increases its capacity. Where, m is the number of entries in a hashmap. n is the total size of hashmap. Example of Load FactorLet's understand the load factor through an example. We know that the default bucket size of the hashmap is 16. We insert the first element, now check that we need to increase the hashmap capacity or not. It can be determined by the formula: In this case, the size of the hashmap is 1, and the bucket size is 16. So, 1/16=0.0625. Now compare this value with the default load factor. 0.0625<0.75 So, no need to increase the hashmap size. We do not need to increase the size of hashmap up to 12^{th} element, because 12/16=0.75 This load factor is equal to the default load factor, i.e., 0.75. As soon as we insert the 13^{th} element in the hashmap, the size of hashmap is increased because: 13/16=0.8125 Which is greater than the default hashmap size. 0.8125>0.75 Now we need to increase the hashmap size. If you want to keep get and put complexity O(1), it is advisable to have a load factor around 0.75.
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