How to debug stream().map(...) with lambda expressions?

I usually have no problem debugging lambda expressions while using Eclipse or IntelliJ IDEA. Just set a breakpoint and be sure not to inspect the whole lambda expression (inspect only the lambda body).

Another approach is to use peek to inspect the elements of the stream:

List<Integer> naturals = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13);
naturals.stream()
    .map(n -> n * 2)
    .peek(System.out::println)
    .collect(Collectors.toList());

UPDATE:

I think you're getting confused because map is an intermediate operation - in other words: it is a lazy operation which will be executed only after a terminal operation was executed. So when you call stream.map(n -> n * 2) the lambda body isn't being executed at the moment. You need to set a breakpoint and inspect it after a terminal operation was called (collect, in this case).

Check Stream Operations for further explanations.

UPDATE 2:

Quoting Holger's comment:

What makes it tricky here is that the call to map and the lambda expression are in one line so a line breakpoint will stop on two completely unrelated actions.

Inserting a line break right after map( would allow you to set a break point for the lambda expression only. And it’s not unusual that debuggers don’t show intermediate values of a return statement. Changing the lambda to n -> { int result=n * 2; return result; } would allow you to inspect result. Again, insert line breaks appropriately when stepping line by line…

IntelliJ has such a nice plugin for this case as a Java Stream Debugger plugin. You should check it out: https://plugins.jetbrains.com/plugin/9696-java-stream-debugger?platform=hootsuite

It extends the IDEA Debugger tool window by adding the Trace Current Stream Chain button, which becomes active when debugger stops inside of a chain of Stream API calls.

It has nice interface for working with separate streams operations and gives you opportunity to follow some values that u should debug.

You can launch it manually from the Debug window by clicking here: