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

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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).

Debugging Lambdas

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…

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