// strip the sign bit, a modififed form of abs that never returns a negative number.
final long n = someValue();

final long p = n & 0x7fffffffffffffffL;

// or easier to proofread
final long p = n & (-1L >>> 1);

// takes two cycles, but simple to understand
final long p = (n << 1) >>> 1;

// or more simply
final long p = n & Long.MAX_VALUE;