Question

I have the following nested hashmap:

HashMap<Foo1, HashMap<Foo2, TObjectDoubleHashMap<Foo1>>> my_map;

There are over a million entries in the TObjectDoubleHashmap and takes about a minute to generate. What are my options to save it to file so it won’t need to be generated every time?

So far I’ve saved the actual keys and values to a text file then load it into the hashmap again, but that is not saving that much time. Is there a faster way such as saving it as an object using serialization? If so, what do I have to do to make my keys serializable?

Answer 1

Is there a faster way such as saving it as an object using serialization?

Serialisation of the map is merely writing out the keys and the matching values. It won’t be faster to use serialisation than doing it yourself long-hand. The HashMap’s description of its serialised data is:

The capacity of the HashMap (the length of the
bucket array) is emitted (int), followed by the
size (an int, the number of key-value
mappings), followed by the key (Object) and value (Object)
for each key-value mapping. The key-value mappings are
emitted in no particular order.

You can see the code here.

What do I have to do to make my keys serializable?

To make an object serialisable, implement the Serializable interface. You probably also want to specify the serialVersionUID. Add a field:

private static final long serialVersionUID = 1L;