IntelliJ IDEA underlines variables when using += in JAVA

If you know what is the side effect in programming then it will be easy for you. To protect your variable from the side effect, the IDE shows the underline as a warning to you.

Hope this screenshot would help.

It's a new feature of IntelliJ IDEA 2018.2:

Underlining reassigned local variables and reassigned parameters

IntelliJ IDEA now underlines reassigned local variables and reassigned parameters, by default. The attributes for all the languages supporting this feature, which for now include Java and Groovy, can be changed in Preferences/Settings | Editor | Color Scheme | Language Defaults | Identifiers | Reassigned.

Why it may be useful?

If the variable/parameter is underlined, you know that you can't use it in lambda/anonymous class directly.

When reading a very long method code, if the parameter is not underlined, you know for sure that its value is not reassigned anywhere in this method and it contains exactly the same value that was passed to this method at any point.

Some code guidelines are against reassigned variables and you may want to avoid them where possible to keep the code clean and make it easier to read/debug.

Nowadays many developers prefer to avoid mutable state, and reassign variables only in rare cases when it is really necessary. We don't want to manually enforce immutability, we suppose that everything is immutable by default and want to bring additional attention to the cases when something is not. If you use final to mark non-mutable variables it means that you need to write more code for regular cases and less code in exceptional cases. (BTW in modern languages declaring immutable variables doesn't require writing additional code, but unfortunately not in Java).

Brian Goetz, Java Language Architect, also likes the way IntelliJ IDEA highlights reassigned variables (see his tweet).