Is it possible to debug Lambdas in Java 8

Question

I just started playing with Java 8 Lambdas and I noticed that I can't debug them in the NetBeans IDE. If I try to attach a breakpoint to the following code I get a variable breakpoint which is definately not what I wanted:

private EventListener myListener (Event event) ->
{
  command1;
  command2; // Set Breakpoint here
  command3;
};

NetBeans attaches the debugger at the "myListener" variable but I can't step into the EventListener itself so I can't see what is happening inside it.

Is there debugging information missing, is this a missing feature in NetBeans or is it not at all possible to debug Lambdas in Java?

Just as confirmation, are you running with Netbeans 8? It might be that Netbeans 7.4 also supports Java 8. — skiwi, Mar 20 '14 at 12:08

Jon Skeet · Answer 1 · 2014-03-20T12:45:59.883

It works for me in Eclipse. For example:

public class Foo {

    private static final Runnable r1 = () -> {
        System.out.println("r1a");
        System.out.println("r1b");
    };

    public static void main(String[] args) {
        Runnable r2 = () -> {
            System.out.println("r2a");
            System.out.println("r2b");
        };

        r1.run();
        r2.run();
    }
}

I can add breakpoints to individual lines within both r1 and r2, and they get hit appropriately, with stepping etc.

If I put a breakpoint on just the run() calls, I can step into the relevant lambda expression too.

So it sounds like all the debug information at least can be available.

EDIT: Apparently the above works in Netbeans too - I suggest you try it to check that you can get that working.

@JeroenVannevel: Did you set the breakpoint with strg+f8 or by clicking at the line number? It seems to work with strg+f8 but not with the line number so it's either a bug or my setup is broken... — mw88, Mar 20 '14 at 13:48

score 3 · Answer 2 · answered Mar 20 '14 at 12:11

With the following sample code in Netbeans 8 Release:

private void init() {
    List<Map<Integer, String>> mapList = new ArrayList<>();

    Map<Integer, String> map1 = new HashMap<>();
    map1.put(1, "String1");
    mapList.add(map1);

    Map<Integer, String> map2 = new HashMap<>();
    map2.put(2, "String2");
    mapList.add(map2);

    Map<Integer, String> map3 = new HashMap<>();
    map3.put(1, "String3");
    mapList.add(map3);

    Map<Integer, String> map4 = new HashMap<>();
    map4.put(2, "String4");
    mapList.add(map4);

    Map<Integer, List<String>> response = mapList.stream()
            .flatMap(map -> map.entrySet().stream())
            .collect(
                    Collectors.groupingBy(
                            Map.Entry::getKey, 
                            Collectors.mapping(
                                    Map.Entry::getValue, 
                                    Collectors.toList()
                            )
                    )
            );
    response.forEach((i, l) -> {
        System.out.println("Integer: " + i + " / List: " + l);
    });
}

I can set a breakpoint on System.out.println("Integer: " + i + " / List: " + l); and inspect the values (i, l) perfectly fine.

So I'm inclined to say that it is working.

Works for me, thank you for the answer. But I have the same problem as with Jon Skeet's answer. — mw88, Mar 20 '14 at 13:48

score 2 · Answer 3 · answered Mar 20 '14 at 12:10

2

In IntelliJ it also works for me:

Stream.generate(() -> {
    return random.nextInt();
}).limit(10).count();

I can debug into the return random.nextInt() but only when I provide a terminal operation like count().

answered Mar 20 '14 at 12:10

kavai77

6,282
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2

Just for your (and everyone elses) information, this code *could* be written as `new Random().ints(10)` to generate an `IntStream`, so total code could be `new Random().ints(10).count()`, I am aware though that then no lambda expression to capture would be present anymore. – skiwi Mar 20 '14 at 12:13
Works for me, thank you for the answer. But I have the same problem as with Jon Skeet's answer. – mw88 Mar 20 '14 at 13:49

Is it possible to debug Lambdas in Java 8

3 Answers3

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