### Question:
Numeric { public static BigInteger decodeQuantity(String value) { if (!isValidHexQuantity(value)) { throw new MessageDecodingException("Value must be in format 0x[1-9]+[0-9]* or 0x0"); } try { return new BigInteger(value.substring(2), 16); } catch (NumberFormatException e) { throw new MessageDecodingException("Negative ", e); } } private  Numeric(); static String encodeQuantity(BigInteger value); static BigInteger decodeQuantity(String value); static String cleanHexPrefix(String input); static String prependHexPrefix(String input); static boolean containsHexPrefix(String input); static BigInteger toBigInt(byte[] value, int offset, int length); static BigInteger toBigInt(byte[] value); static BigInteger toBigInt(String hexValue); static BigInteger toBigIntNoPrefix(String hexValue); static String toHexStringWithPrefix(BigInteger value); static String toHexStringNoPrefix(BigInteger value); static String toHexStringNoPrefix(byte[] input); static String toHexStringWithPrefixZeroPadded(BigInteger value, int size); static String toHexStringNoPrefixZeroPadded(BigInteger value, int size); static byte[] toBytesPadded(BigInteger value, int length); static String littleEndian(long number); static byte[] hexStringToByteArray(String input); static String toHexString(byte[] input, int offset, int length, boolean withPrefix); static String toHexString(byte[] input); static String toHexString(String input); static byte asByte(int m, int n); static boolean isIntegerValue(BigDecimal value); static boolean isIntegerValue(String value); static List<Byte> intToBytes(int value);  }### Answer:
@Test public void testQuantityDecodeMissingPrefix() { Assertions.assertThrows(MessageDecodingException.class, () -> Numeric.decodeQuantity("ff")); }
@Test public void testQuantityDecodeMissingValue() { Assertions.assertThrows(MessageDecodingException.class, () -> Numeric.decodeQuantity("0x")); }
@Test public void testQuantityDecode() { Assertions.assertEquals(Numeric.decodeQuantity("0x0"), BigInteger.valueOf(0L)); Assertions.assertEquals(Numeric.decodeQuantity("0x400"), BigInteger.valueOf((1024L))); Assertions.assertEquals(Numeric.decodeQuantity("0x0"), BigInteger.valueOf((0L))); Assertions.assertEquals( Numeric.decodeQuantity("0x7fffffffffffffff"), BigInteger.valueOf((Long.MAX_VALUE))); Assertions.assertEquals( Numeric.decodeQuantity("0x99dc848b94efc27edfad28def049810f"), new BigInteger("204516877000845695339750056077105398031")); }
@Test public void testQuantityDecodeLeadingZero() { Assertions.assertEquals(Numeric.decodeQuantity("0x0400"), BigInteger.valueOf(1024L)); Assertions.assertEquals(Numeric.decodeQuantity("0x001"), BigInteger.valueOf(1L)); }