假设我有一个不可变的DecimalNumber类:publicfinalclassDecimalNumber{publicfinalStringstr;publicDecimalNumber(Stringstr){this.str=str;}publicDecimalNumber(DecimalNumbercopy){this(copy.str);}publicbooleanisZero(){...}publicDecimalNumberadd(DecimalNumberother){...}...}我决定像这样实现add:publicDecimalNumberadd(DecimalN