Questions tagged [java-8]
Use this tag for questions specific to Java 8 which is version 8 (internal number 1.8) of the Java platform, released on 18 March 2014. In most cases, you should also specify the java tag.
23,216
questions
1286
votes
34
answers
1.4m
views
How to install Java 8 on Mac
Editors note: This question was asked in 2014, and the answers may be outdated.
I want to do some programming with the latest JavaFX, which requires Java 8. I'm using IntelliJ 13 CE and Mac OS X 9 ...
- 13.2k
1145
votes
17
answers
496k
views
:: (double colon) operator in Java 8
I was exploring the Java 8 source and found this particular part of code very surprising:
// Defined in IntPipeline.java
@Override
public final OptionalInt reduce(IntBinaryOperator op) {
return ...
- 41.7k
1087
votes
10
answers
710k
views
How to convert a Java 8 Stream to an Array?
What is the easiest/shortest way to convert a Java 8 Stream into an array?
user972946
1058
votes
23
answers
763k
views
Java 8 List<V> into Map<K, V>
I want to translate a List of objects into a Map using Java 8's streams and lambdas.
This is how I would write it in Java 7 and below.
private Map<String, Choice> nameMap(List<Choice> ...
- 16k
975
votes
21
answers
634k
views
What's the difference between map() and flatMap() methods in Java 8?
In Java 8, what's the difference between Stream.map() and Stream.flatMap() methods?
- 128k
965
votes
25
answers
518k
views
Error:java: javacTask: source release 8 requires target release 1.8
Using IntelliJ IDE can't compile any projects. Screenshots of settings below:
Used JDK:
Project SDK and Language level:
Language Level:
Anybody have any ideas?
- 16.1k
843
votes
12
answers
695k
views
How can I turn a List of Lists into a List in Java 8?
If I have a List<List<Object>>, how can I turn that into a List<Object> that contains all the objects in the same iteration order by using the features of Java 8?
- 10.6k
791
votes
1
answer
75k
views
Why does array[idx++]+="a" increase idx once in Java 8 but twice in Java 9 and 10?
For a challenge, a fellow code golfer wrote the following code:
import java.util.*;
public class Main {
public static void main(String[] args) {
int size = 3;
String[] array = new String[...
- 34.9k
704
votes
8
answers
625k
views
Find first element by predicate
I've just started playing with Java 8 lambdas and I'm trying to implement some of the things that I'm used to in functional languages.
For example, most functional languages have some kind of find ...
- 9,327
684
votes
35
answers
582k
views
Java 8 Distinct by property
In Java 8 how can I filter a collection using the Stream API by checking the distinctness of a property of each object?
For example I have a list of Person object and I want to remove people with the ...
- 11.6k
656
votes
6
answers
242k
views
Should I always use a parallel stream when possible?
With Java 8 and lambdas it's easy to iterate over collections as streams, and just as easy to use a parallel stream. Two examples from the docs, the second one using parallelStream:
...
- 21.4k
656
votes
26
answers
320k
views
Why should Java 8's Optional not be used in arguments
I've read on many Web sites Optional should be used as a return type only, and not used in method arguments. I'm struggling to find a logical reason why. For example I have a piece of logic which ...
- 6,985
640
votes
28
answers
516k
views
Java 8 Lambda function that throws exception?
I know how to create a reference to a method that has a String parameter and returns an int, it's:
Function<String, Integer>
However, this doesn't work if the function throws an exception, say ...
- 23k
635
votes
28
answers
297k
views
Is it possible to use Java 8 for Android development?
Searching the web, it is not clear if Java 8 is supported for Android development or not.
Before I download/setup Java 8, can some one point me at any "official" documentation that says Java 8 is or ...
- 16.5k
634
votes
16
answers
166k
views
When to use: Java 8+ interface default method, vs. abstract method
Java 8 allows for default implementation of methods in interfaces called Default Methods.
I am confused between when would I use that sort of interface default method, instead of an abstract class (...
- 41.7k
625
votes
12
answers
346k
views
How to convert an Iterator to a Stream?
I am looking for a concise way to convert an Iterator to a Stream or more specifically to "view" the iterator as a stream.
For performance reason, I would like to avoid a copy of the iterator in a ...
- 29.1k
624
votes
9
answers
669k
views
Converting between java.time.LocalDateTime and java.util.Date
Java 8 has a completely new API for date and time. One of the most useful classes in this API is LocalDateTime, for holding a timezone-independent date-with-time value.
There are probably millions ...
- 15.6k
614
votes
14
answers
630k
views
Convert java.util.Date to java.time.LocalDate
What is the best way to convert a java.util.Date object to the new JDK 8/JSR-310 java.time.LocalDate?
Date input = new Date();
LocalDate date = ???
- 62.3k
559
votes
5
answers
264k
views
What's the difference between Instant and LocalDateTime?
I know that:
Instant is rather a "technical" timestamp representation (nanoseconds) for computing.
LocalDateTime is rather date/clock representation including time-zones for humans.
Still ...
- 16.1k
555
votes
8
answers
508k
views
Java 8 Iterable.forEach() vs foreach loop
Which of the following is better practice in Java 8?
Java 8:
joins.forEach(join -> mIrc.join(mSession, join));
Java 7:
for (String join : joins) {
mIrc.join(mSession, join);
}
I have lots ...
- 8,515
530
votes
28
answers
447k
views
Is there a concise way to iterate over a stream with indices in Java 8?
Is there a concise way to iterate over a stream whilst having access to the index in the stream?
String[] names = {"Sam","Pamela", "Dave", "Pascal", "Erik"};
List<String> nameList;
Stream<...
- 8,213
514
votes
8
answers
162k
views
Convert Iterable to Stream using Java 8 JDK
I have an interface which returns java.lang.Iterable<T>.
I would like to manipulate that result using the Java 8 Stream API.
However Iterable can't "stream".
Any idea how to use the Iterable ...
- 21.2k
502
votes
15
answers
448k
views
Retrieving a List from a java.util.stream.Stream in Java 8
I was playing around with Java 8 lambdas to easily filter collections. But I did not find a concise way to retrieve the result as a new list within the same statement. Here is my most concise approach ...
- 5,847
495
votes
12
answers
965k
views
How can I parse/format dates with LocalDateTime? (Java 8)
Java 8 added a new java.time API for working with dates and times (JSR 310).
I have date and time as string (e.g., "2014-04-08 12:30"). How can I obtain a LocalDateTime instance from the ...
- 48.9k
492
votes
16
answers
405k
views
How do I define a method which takes a lambda as a parameter in Java 8?
In Java 8, methods can be created as Lambda expressions and can be passed by reference (with a little work under the hood). There are plenty of examples online with lambdas being created and used with ...
- 58.4k
491
votes
14
answers
252k
views
How to negate a method reference predicate
In Java 8, you can use a method reference to filter a stream, for example:
Stream<String> s = ...;
long emptyStrings = s.filter(String::isEmpty).count();
Is there a way to create a method ...
- 325k
489
votes
16
answers
290k
views
Custom thread pool in Java 8 parallel stream
Is it possible to specify a custom thread pool for Java 8 parallel stream? I can not find it anywhere.
Imagine that I have a server application and I would like to use parallel streams. But the ...
- 14k
487
votes
13
answers
581k
views
How to sum a list of integers with java streams?
I want to sum a list of Integers. It works as follows, but the syntax does not feel right. Could the code be optimized?
Map<String, Integer> integers;
integers.values().stream().mapToInt(i -> ...
- 84.2k
464
votes
13
answers
588k
views
Convert java.time.LocalDate into java.util.Date type
I want to convert java.time.LocalDate into java.util.Date type. Because I want to set the date into JDateChooser. Or is there any date chooser that supports java.time dates?
- 4,778
454
votes
13
answers
229k
views
Why should one use Objects.requireNonNull()?
I have noted that many Java 8 methods in Oracle JDK use Objects.requireNonNull(), which internally throws NullPointerException if the given object (argument) is null.
public static <T> T ...
- 4,631
431
votes
15
answers
600k
views
How to get milliseconds from LocalDateTime in Java 8
I am wondering if there is a way to get current milliseconds since 1-1-1970 (epoch) using the new LocalDate, LocalTime or LocalDateTime classes of Java 8.
The known way is below:
long ...
- 19.4k
430
votes
15
answers
497k
views
Break or return from Java 8 stream forEach?
When using external iteration over an Iterable we use break or return from enhanced for-each loop as:
for (SomeObject obj : someObjects) {
if (some_condition_met) {
break; // or return obj
...
- 29.3k
407
votes
5
answers
163k
views
How do I convert a Java 8 IntStream to a List?
I'm looking at the docs for the IntStream, and I see an toArray method, but no way to go directly to a List<Integer>
Surely there is a way to convert a Stream to a List?
- 58.6k
401
votes
13
answers
262k
views
Ignore duplicates when producing map using streams
Map<String, String> phoneBook = people.stream()
.collect(toMap(Person::getName,
Person::getAddress));
...
- 17.5k
395
votes
12
answers
496k
views
Java 8: Difference between two LocalDateTime in multiple units
I am trying to calculate the difference between two LocalDateTime.
The output needs to be of the format y years m months d days h hours m minutes s seconds. Here is what I have written:
import java....
- 29.3k
390
votes
15
answers
207k
views
Difference between final and effectively final
I'm playing with lambdas in Java 8 and I came across warning local variables referenced from a lambda expression must be final or effectively final. I know that when I use variables inside anonymous ...
- 11.2k
378
votes
5
answers
49k
views
Why is "final" not allowed in Java 8 interface methods?
One of the most useful features of Java 8 are the new default methods on interfaces. There are essentially two reasons (there may be others) why they have been introduced:
Providing actual default ...
- 216k
376
votes
14
answers
426k
views
Calculate days between two Dates in Java 8
I know there are lots of questions on SO about how to get Dates in Java, but I want an example using new Java 8 Date API. I also know about the JodaTime library, but I want a method without relying on ...
- 4,967
376
votes
13
answers
594k
views
Functional style of Java 8's Optional.ifPresent and if-not-Present?
In Java 8, I want to do something to an Optional object if it is present, and do another thing if it is not present.
if (opt.isPresent()) {
System.out.println("found");
} else {
System.out....
- 11.7k
374
votes
18
answers
163k
views
How can I throw checked exceptions from inside Java 8 lambdas/streams?
How can I throw checked exceptions from inside Java 8 lambda, used in a stream for example?
In other words, I want to make code like this compile:
public List<Class> getClasses() throws ...
- 30.1k
372
votes
7
answers
104k
views
Should Java 8 getters return optional type?
Optional type introduced in Java 8 is a new thing for many developers.
Is a getter method returning Optional<Foo> type in place of the classic Foo a good practice? Assume that the value can be ...
- 4,748
371
votes
3
answers
243k
views
Java 8 lambdas, Function.identity() or t->t
I have a question regarding the usage of the Function.identity() method.
Imagine the following code:
Arrays.asList("a", "b", "c")
.stream()
.map(Function.identity()) // <- ...
user4464654
369
votes
8
answers
353k
views
UnsupportedTemporalTypeException when formatting Instant to String
I'm trying to format an Instant to a String using the new Java 8 Date and Time API and the following pattern:
Instant instant = ...;
String out = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss&...
- 10.3k
360
votes
14
answers
161k
views
Uses for Optional
Having been using Java 8 now for 6+ months or so, I'm pretty happy with the new API changes. One area I'm still not confident in is when to use Optional. I seem to swing between wanting to use it ...
- 6,611
354
votes
7
answers
108k
views
Explicitly calling a default method in Java
Java 8 introduces default methods to provide the ability to extend interfaces without the need to modify existing implementations.
I wonder if it's possible to explicitly invoke the default ...
- 45.2k
352
votes
10
answers
300k
views
Java 8 lambda Void argument
Let's say I have the following functional interface in Java 8:
interface Action<T, U> {
U execute(T t);
}
And for some cases I need an action without arguments or return type. So I write
...
- 7,043
346
votes
19
answers
228k
views
Maven is not working in Java 8 when Javadoc tags are incomplete
Since I use Maven I have been able to build and install in my local repository projects that have incomplete Javadoc tags (for example, a missing parameter).
However, since I migrated to Java 8 (1.8....
- 8,592
346
votes
5
answers
106k
views
What is difference between Collection.stream().forEach() and Collection.forEach()?
I understand that with .stream(), I can use chain operations like .filter() or use parallel stream. But what is difference between them if I need to execute small operations (for example, printing the ...
- 4,216
340
votes
10
answers
149k
views
Default interface methods are only supported starting with Android 7.0 (Nougat)
I upgraded to Android Studio 3.1 and I'm getting the following error:
Default interface methods are only supported starting with Android N (--min-api 24): void android.arch.lifecycle....
- 61.8k
338
votes
6
answers
224k
views
Why use Optional.of over Optional.ofNullable?
When using the Java 8 Optional class, there are two ways in which a value can be wrapped in an optional.
String foobar = <value or null>;
Optional.of(foobar); // May throw ...
- 16.3k