Java8, convert for loop with a condition to lambda

Question

Java 8.

Converting a forEach loop into a lambda expression is understood, at least for me. Converting a for loop based with a condition, in the other hand, isn't.

If it's possible ,my question would be: how can you convert the following for loop into a lambda expression

List<Field> fields = new LinkedList<>();
        for (Class<?> c = this.getClass(); c != null; c = c.getSuperclass())
            Collections.addAll(fields, c.getDeclaredFields());

Many thanks in advance,

~Ben.

Lambda isn't going to help you here. If you want to get all fields of class including inherited, try this solution: stackoverflow.com/questions/1042798/… — esin88
– esin88, Commented Mar 1, 2017 at 9:46
I disagree with the accepted answer that recursion is the better approach, second my inductive code works better. — Benma
– Benma, Commented Mar 1, 2017 at 9:50
Stream s direct replacement for for-each style. But here, you can't either flatten the stream, what you need is iterate over the stream by yourself. Which means, internal iteration will not help you. — Jude Niroshan
– Jude Niroshan, Commented Mar 1, 2017 at 9:51

Eugene · Accepted Answer · 2017-03-01 10:26:27Z

5

Well, there is a way, but it needs takeWhile that it is in jdk-9.

I'm doing a mapping here to get the names of the fields. You would have to add a @SuppressWarnings("null") to the method.

System.out.println(Stream.iterate(this.getClass(), (Class<?> x) -> x.getSuperclass())
            .takeWhile(x -> x != null)
            .flatMap(c -> Arrays.stream(c.getDeclaredFields()))
            .map(c -> c.getName())
            .collect(Collectors.toList()));

jdk-9 also introduces a Stream.iterate that acts like an Iterator with seed, hasNext, next that is far more suited for your case.

You could use StreamEx library for this btw:

 StreamEx.of(Stream.iterate(this.getClass(), (Class<?> x) -> x.getSuperclass()))
            .takeWhile(x -> x != null)
            .flatMap(c -> Arrays.stream(c.getDeclaredFields()))
            .map(c -> c.getName())
            .collect(Collectors.toList());

And with new iterate method:

 Stream.iterate(this.getClass(), c -> c != null, (Class<?> c) -> c.getSuperclass())
            .flatMap(c -> Arrays.stream(c.getDeclaredFields()))
            .map(c -> c.getName())
            .collect(Collectors.toList())

edited Mar 1, 2017 at 10:26

answered Mar 1, 2017 at 10:05

Eugene

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4 Comments

Benma Over a year ago

Stream.iterate(this.getClass(), c -> c != null, (Class<?> c) -> c.getSuperclass()) .flatMap(c -> Arrays.stream(c.getDeclaredFields())) .map(c -> c.getName()) .collect(Collectors.toList())

Benma Over a year ago

is it only possible with java9 ?

Eugene Over a year ago

@Benma you need takeWhile for it to work in jdk-8. This is either in StreamEx library or you can add it yourself until available from here: stackoverflow.com/questions/20746429/…

Eugene Over a year ago

@Roland indeed, a Stream out of the Iterator.

Koer · Accepted Answer · 2017-03-01 09:47:24Z

0

You maybe want to work with streams. Open a stream on your list and simply use filter. Filter takes a FunctionalInterface/Lambda-expression.

answered Mar 1, 2017 at 9:47

Koer

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1 Comment

Benma Over a year ago

I'm well familiar with streams and filters, but to implement the following loop condition with these methods isn't straightforward to me for some reason, is there any chance for a code snippet?

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