How to Initialize a Java HashMap with Reasonable Values?

HashMap<String, Integer> mapJdK = new HashMap<String, Integer>();

is in principal valid Java code, but the hash map is not initialized with a reasonable values.

In this case the Java implementation has to resize the number of buckets with increasing number of data in the hash map. In Java the default load factor is 0.75 and the default initial capacity of a hash map is 16. This means that the threshold for the next resize equates like:

DEFAULT_INITIAL_CAPACITY = 16;

DEFAULT_LOAD_FACTOR = 0.75;

THRESHOLD = DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR;

The hash map will automatically rehash the contents of the hash map into a new array with a larger capacity. This happens when the number of data in the hash map exceeds the THRESHOLD. Each resize leads to a doubled capacity 16*0.75, 32*0.75, 64*0.75, etc. The following diagram depicts this:

Figure 1: Growth of HashMap

int capacity = (int) ((expected_maximal_number_of_data)/0.75+1);

HashMap<String, Integer> mapJdK = new HashMap<String, Integer>(capacity);