Using an underscore to mark unused parameters required changes in the Java language.

Until Java 8, the underscore was used as a valid identifier. The first step was to deny permission to use an underscore as a regular identifier. So, Java 8 issued a compiler warning for the usage of _ as the name of formal parameters in lambdas. This was simple, and had no backward-compatibility issues since lambdas were introduced in Java 8. Moving forward, Java 9 replaced the compiler warning message with compilation errors for using underscores as parameter names.

With JEP 302, developers can use _ to mark an unused method parameter for the following:

• Lambdas
• Methods
• Catch handlers

In the next section, you'll see how (in the future) your lambda parameters will be able to shadow the variables with the same name in their enclosing scopes.