Use READ-ONLY sparingly

Many modern programming languages, especially Java, recommend making class members read-only wherever appropriate to prevent accidental side effects.

While ABAP does offer the READ-ONLY addition for data declarations, we recommend to use it sparingly.

First, the addition is only available in the PUBLIC SECTION, reducing its applicability drastically. You can neither add it to protected or private members nor to local variables in a method.

Second, the addition works subtly different from what people might expect from other programming languages: READ-ONLY data can still be modified freely from any method within the class itself, its friends, and its sub-classes. This contradicts the more widespread write-once-modify-never behavior found in other languages. The difference may lead to bad surprises.

To avoid misunderstandings: Protecting variables against accidental modification is a good practice. We would recommend applying it to ABAP as well if there was an appropriate statement.