What is the right approach to concatenating a null String in Java?

The most concise solution this is:

System.out.println("s: " + (s == null ? "" : s));

or maybe create or use a static helper method to do the same; e.g.

System.out.println("s: " + denull(s));

However, this question has the “smell” of an application that is overusing / misusing null. It is better to only use / return a null if it has a specific meaning that is distinct (and needs to be distinct) from the meanings of non-null values.

For example:

• If these nulls are coming from String attributes that have been default initialized to null, consider explicitly initializing them to "" instead.
• Don’t use null to denote empty arrays or collections.
• Don’t return null when it would be better to throw an exception.
• Consider using the Null Object Pattern.

Now obviously there are counter-examples to all of these, and sometimes you have to deal with a pre-existing API that gives you nulls … for whatever reason. However, in my experience it is better to steer clear of using null … most of the time.

So, in your case, the better approach may be:

String s = "";  /* instead of null */
System.out.println("s: " + s);