uint64_t_bigint.java

import java.math.BigInteger;

public class Main {

    static void func(long value) {
        BigInteger val = new BigInteger(Long.toHexString(value), 16);
        System.out.println("BigInteger: " + val);
        BigInteger byteVal = val.shiftRight(56);
        System.out.println("BigInteger 2byte: " + byteVal);
    }

    public static void main(String[] args) {
        long decLong = 0xFEDCBA9876543210L;
        System.out.println("long: " + decLong);
        System.out.println("long 2byte: " + (decLong >> 56));
        func(decLong);
    }
}