How can I quickly load a large txt file into BigInteger?

Question

I'm importing a large text file, 17 million digits long and I'm using this code:

BufferedReader reader = new BufferedReader(new FileReader("test2.txt"));
String line = reader.readLine();
System.out.println("Done");
BigInteger num = new BigInteger(line);
System.out.println("Done Again");

It loads the file pretty much instantly and prints out 'Done' but it takes a long time (about an hour) for the String to be converted into a BigInteger, is there anything I can do to speed this up and quickly load the number?

Answer 1

As an optimization, since BigInteger is Serializable, you could save it to a binary file once and speed up your loading considerably.

Loading a serialized object should be way faster than parsing a huge string everytime.

Use ObjectOutputStream to save your big integer and ObjectInputStream to read it back in.

Answer 2

It is slow because new BigInteger(String) is doing radix conversion from decimal to binary, which is O(N²). Nothing you can do about that.

You could save either the object itself, via Serialization, or the byte array it is stored in, via BigInteger.toByteArray(). Either will load essentially instanteously.