Modifying local variable from inside lambda

Modifying a local variable in forEach gives a compile error:

Normal

    int ordinal = 0;
    for (Example s : list) {
        s.setOrdinal(ordinal);
        ordinal++;
    }

With Lambda

    int ordinal = 0;
    list.forEach(s -> {
        s.setOrdinal(ordinal);
        ordinal++;
    });

Any idea how to resolve this?

Solution 1:

Use a wrapper

Any kind of wrapper is good.

With Java 10+, use this construct as it's very easy to setup:

var wrapper = new Object(){ int ordinal = 0; };
list.forEach(s -> {
  s.setOrdinal(wrapper.ordinal++);
});

With Java 8+, use either an AtomicInteger:

AtomicInteger ordinal = new AtomicInteger(0);
list.forEach(s -> {
  s.setOrdinal(ordinal.getAndIncrement());
});

... or an array:

int[] ordinal = { 0 };
list.forEach(s -> {
  s.setOrdinal(ordinal[0]++);
});

Note: be very careful if you use a parallel stream. You might not end up with the expected result. Other solutions like Stuart's might be more adapted for those cases.

For types other than int

Of course, this is still valid for types other than int.

For instance, with Java 10+:

var wrapper = new Object(){ String value = ""; };
list.forEach(s->{
  wrapper.value += "blah";
});

Or if you're stuck with Java 8 or 9, use the same kind of construct as we did above, but with an AtomicReference...

AtomicReference<String> value = new AtomicReference<>("");
list.forEach(s -> {
  value.set(value.get() + s);
});

... or an array:

String[] value = { "" };
list.forEach(s-> {
  value[0] += s;
});

Solution 2:

This is fairly close to an XY problem. That is, the question being asked is essentially how to mutate a captured local variable from a lambda. But the actual task at hand is how to number the elements of a list.

In my experience, upward of 80% of the time there is a question of how to mutate a captured local from within a lambda, there's a better way to proceed. Usually this involves reduction, but in this case the technique of running a stream over the list indexes applies well:

IntStream.range(0, list.size())
         .forEach(i -> list.get(i).setOrdinal(i));

Solution 3:

If you only need to pass the value from the outside into the lambda, and not get it out, you can do it with a regular anonymous class instead of a lambda:

list.forEach(new Consumer<Example>() {
    int ordinal = 0;
    public void accept(Example s) {
        s.setOrdinal(ordinal);
        ordinal++;
    }
});