Of course, people will do what they want. But we did have a clear intention when adding this feature, and it was not to be a general purpose Maybe type, as much as many people would have liked us to do so. Our intention was to provide a limited mechanism for library method return types … Read more
You can use flatMap to flatten the internal lists (after converting them to Streams) into a single Stream, and then collect the result into a list: List<List<Object>> list = … List<Object> flat = list.stream() .flatMap(List::stream) .collect(Collectors.toList());
A parallel stream has a much higher overhead compared to a sequential one. Coordinating the threads takes a significant amount of time. I would use sequential streams by default and only consider parallel ones if I have a massive amount of items to process (or the processing of each item takes time and is parallelizable) … Read more
Based on Collectors documentation it’s as simple as: Map<String, Choice> result = choices.stream().collect(Collectors.toMap(Choice::getName, Function.identity()));
You’ll need to do one of the following. If it’s your code, then define your own functional interface that declares the checked exception: @FunctionalInterface public interface CheckedFunction<T, R> { R apply(T t) throws IOException; } and use it: void foo (CheckedFunction f) { … } Otherwise, wrap Integer myMethod(String s) in a method that doesn’t … Read more
It is equivalent but not identical. Simply said, if a lambda expression does not capture values, it will be a singleton that is re-used on every invocation. The behavior is not exactly specified. The JVM is given big freedom on how to implement it. Currently, Oracle’s JVM creates (at least) one instance per lambda expression … Read more
The cleanest way is to start from a stream of indices: String[] names = {“Sam”, “Pamela”, “Dave”, “Pascal”, “Erik”}; IntStream.range(0, names.length) .filter(i -> names[i].length() <= i) .mapToObj(i -> names[i]) .collect(Collectors.toList()); The resulting list contains “Erik” only. One alternative which looks more familiar when you are used to for loops would be to maintain an ad … Read more
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
tl;dr Instant and LocalDateTime are two entirely different animals: One represents a moment, the other does not. Instant represents a moment, a specific point in the timeline. LocalDateTime represents a date and a time-of-day. But lacking a time zone or offset-from-UTC, this class cannot represent a moment. It represents potential moments along a range of … Read more
… starting in Java SE 8, a local class can access local variables and parameters of the enclosing block that are final or effectively final. A variable or parameter whose value is never changed after it is initialized is effectively final. For example, suppose that the variable numberLength is not declared final, and you add … Read more