This whole potential problem is one of the reasons the particular example isn't that great. It's overhyped in Java-land where some people treat naked strings like lepers but in places like Clojure-land you'd get a lot of people saying "Just use a keyword" for both places. The keyword's name is its value. It's immutable. An IDE should be able to find all uses of it, or there's always 'ag'. If you really want that coupling behavior, change the interface of the thing you're calling to require conformance to a schema. The simplest way in Java is probably just by taking an enum instead of a string, but there are richer ways (especially outside of Java) worth looking into if you're needing a string value eventually in the implementation since you'll run into the other Java-ism of making a complicated enum class instead of just using enums like keywords with their string value being their actual name value.

I'd bet dollars to donuts the situations where you want it to change outnumber the ones where you don't by more than 10:1.