adding 2 BigDecimal values

Perhaps this is what you prefer:

BigDecimal z = new BigDecimal(5).add(x);

Every operation of BigDecimal returns a new BigDecimal but not change the current instance.

BigDecimal is immutable. Every operation returns a new instance containing the result of the operation:

 BigDecimal sum = x.add(y);

If you want x to change, you thus have to do

x = x.add(y);

Reading the javadoc really helps understanding how a class and its methods work.