Get the maximum value for a BigInteger with n bits

Question

I want to compare two multiplication methods implemented in Java which use shift operations on big numbers. Thus I need sufficiently large BigIntegers.

Since I want to compare them bit-wise, what would be the best approach to generate BigIntegers with n bits which are fully used in the multiplication operation.

My approach so far is this:

byte[] bits = new byte[bitLength]; BigInteger number = new BigInteger(bits).flipBit(bitLength);

Jon Skeet · Accepted Answer · 2011-11-10 21:37:29Z

How about this:

import java.math.BigInteger; public class Test { public static void main(String[] args) { int bits = 3; BigInteger value = BigInteger.ZERO .setBit(bits) .subtract(BigInteger.ONE); System.out.println(value); // Prints 7 == 111 in binary } }

In other words, set the bit which is one higher than you want, then subtract one to get a value which uses all the lower bits.

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