If you want to instantiate an object in Java you need to pass the full qualified name like ‘MyFirstModule.Customer’ You can not instantiate with a Decimal.
Probably you have created an entity that should contain the values. Like ‘MyFirstModule.DecimalParameter’ with two attributes of type Decimal ‘UnderStatusParam’ and ‘OverStatusParam’.
Use the proxy is easier.
DecimalParameter decimalParameter = new DecimalParameter(getContext());
decimalParameter.setunderStatusParam(underStatusParam);
decimalParameter.setoverStatusParam(overStatusParam);
returnList.add(decimalParameter.getMendixObject());
Please note: this is pseudocode, not checked.
BTW This could be easier achieved in a microflow.