byte[] to unsigned BigInteger?

Examining the documentation for the relevant BigInteger constructor, we see:

The individual bytes in the value array should be in little-endian order, from lowest-order byte to highest-order byte

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The constructor expects positive values in the byte array to use sign-and-magnitude representation, and negative values to use two's complement representation. In other words, if the highest-order bit of the highest-order byte in value is set, the resulting BigInteger value is negative. Depending on the source of the byte array, this may cause a positive value to be misinterpreted as a negative value.

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To prevent positive values from being misinterpreted as negative values, you can add a zero-byte value to the end of the array.

The remarks for the BigInteger constructor state that you can make sure any BigInteger created from a byte[] is unsigned if you append a 00 byte to the end of the array before calling the constructor.

Note: the BigInteger constructor expects the array to be in little-endian order. Keep that in mind if you expect the resulting BigInteger to have a particular value.