The number converter in JSF2 is used to convert a component data into java.lang.Number format. Additionally it have lot of attributes for customizing number format. The sample usage is given here.
The important one is, the numberConverter binding component managed bean property should be a primitive type or has a type of java.lang.Number only. Here the "number" type is java.lang.Number. This is another usage of number converter.
THe result of the above code is something like $22.50. The sample application is available in Github.