java-8 – Make Me Engineer

How can I parse/format dates with LocalDateTime? (Java 8)

Parsing date and time To create a LocalDateTime object from a string you can use the static LocalDateTime.parse() method. It takes a string and a DateTimeFormatter as parameter. The DateTimeFormatter is used to specify the date/time pattern. String str = “1986-04-08 12:30”; DateTimeFormatter formatter = DateTimeFormatter.ofPattern(“yyyy-MM-dd HH:mm”); LocalDateTime dateTime = LocalDateTime.parse(str, formatter); Formatting date and … Read more

Collectors.summingInt() vs mapToInt().sum()

You are looking at the intersection of two otherwise distinct use cases. Using mapToInt(…) allows you to chain other IntStream operations before the terminal operation. In contrast, Collectors.summingInt(…) can be combined with other collectors, e.g. used as downstream collector in a groupingBy collector. For these use cases, there is no question about which to use. … Read more

Why does this java 8 stream operation evaluate to Object instead of List or just List?

The stream(), map(), collect(), and all the other stream methods rely on generic typing for things to work well. If you use a raw type as input, then all the generics are erased, and everything turns into a raw type. For example, if you have a List<String>, calling the stream() method gives you an object … Read more

Iterate two Java-8-Streams together [duplicate]

static <A, B> Stream<Pair<A, B>> zip(Stream<A> as, Stream<B> bs) { Iterator<A> i=as.iterator(); return bs.filter(x->i.hasNext()).map(b->new Pair<>(i.next(), b)); } This does not offer parallel execution but neither did the original zip implementation. And as F. Böller has pointed out it doesn’t work if bs is infinite and as is not. For a solution which works for all … Read more

JavaFX : Pass parameters while instantiating controller class

The two easiest ways of doing it for small applications are : Do not specify the fx:controller in the fxml. Create a controller instance by passing data to it and then pass it to the FXMLLoader. Specify the fx:controller in the fxml. Fetch the controller instance from the FXMLLoader and pass the data to the … Read more

Local variable log defined in an enclosing scope must be final or effectively final

The message says exactly what the problem is: your variable log must be final (that is: carry the keyword final) or be effectively final (that is: you only assign a value to it once outside of the lambda). Otherwise, you can’t use that variable within your lambda statement. But of course, that conflicts with your … Read more

How to persist JSR-310 types with Spring Data JPA?

UPDATE: The answer below is valid if you need to stay on a Hibernate version < 5.0. Hibernate 5.0 supports persisting JSR-310 date/time types out of the box. I.e. if you are on Hibernate 5.0 or newer, Adam’s answer is the way to go. Everyone else, read on. The root cause for this at none … Read more

Recursive use of Stream.flatMap()

Well, I used the same pattern with a generic Tree class and didn’t have a wrong feeling with it. The only difference is, that the Tree class itself offered a children() and allDescendants() methods, both returning a Stream and the latter building on the former. This is related to “Should I return a Collection or … Read more

Why can’t I throw an exception in a Java 8 lambda expression? [duplicate]

You are not allowed to throw checked exceptions because the accept(T t, U u) method in the java.util.function.BiConsumer<T, U> interface doesn’t declare any exceptions in its throws clause. And, as you know, Map#forEach takes such a type. public interface Map<K, V> { default void forEach(BiConsumer<? super K, ? super V> action) { … } } … Read more

How to dynamically do filtering in Java 8?

Interesting problem. There are several things going on here. No doubt this could be solved in less than half a page of Haskell or Lisp, but this is Java, so here we go…. One issue is that we have a variable number of filters, whereas most of the examples that have been shown illustrate fixed … Read more

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