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.
java-8
23,232
questions
1290
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Editors note: This question was asked in 2014, and the answers may be outdated.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
1147
votes
17
answers
501k
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 ...
- 41.8k
1093
votes
10
answers
725k
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
1059
votes
23
answers
774k
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> ...
- 16.1k
981
votes
21
answers
642k
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?
- 129k
966
votes
26
answers
520k
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
854
votes
12
answers
712k
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?
- 10.7k
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[...
- 35k
710
votes
8
answers
638k
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 ...
- 9,387
693
votes
35
answers
593k
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 ...
- 11.8k
667
votes
28
answers
331k
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 ...
- 7,095
657
votes
6
answers
245k
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:
...
- 21.6k
640
votes
28
answers
525k
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 ...
- 23.1k
637
votes
16
answers
168k
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 (...
- 41.8k
635
votes
28
answers
298k
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 ...
- 16.6k
631
votes
9
answers
683k
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
630
votes
12
answers
353k
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 ...
- 29.2k
617
votes
14
answers
644k
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 = ???
- 62.7k
565
votes
5
answers
273k
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 ...
- 16.2k
557
votes
8
answers
513k
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 ...
- 8,535
535
votes
28
answers
456k
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<...
- 8,303
521
votes
8
answers
165k
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 ...
- 21.4k
502
votes
15
answers
455k
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
500
votes
12
answers
994k
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 ...
- 49.1k
498
votes
14
answers
257k
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 ...
- 326k
496
votes
16
answers
413k
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 ...
- 58.5k
495
votes
14
answers
596k
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 -> ...
- 85k
492
votes
16
answers
295k
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
465
votes
13
answers
604k
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?
- 4,788
460
votes
13
answers
237k
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 ...
- 4,691
434
votes
15
answers
611k
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 ...
- 19.6k
431
votes
15
answers
506k
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
...
- 29.5k
408
votes
5
answers
167k
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?
- 58.9k
404
votes
13
answers
268k
views
Ignore duplicates when producing map using streams
Map<String, String> phoneBook = people.stream()
.collect(toMap(Person::getName,
Person::getAddress));
...
- 17.7k
401
votes
12
answers
504k
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....
- 29.5k
392
votes
15
answers
209k
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 ...
- 11.3k
379
votes
14
answers
434k
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 ...
- 4,997
378
votes
7
answers
107k
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 ...
- 4,808
378
votes
13
answers
602k
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....
- 11.8k
378
votes
5
answers
50k
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 ...
- 218k
376
votes
18
answers
165k
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 ...
- 30.3k
374
votes
8
answers
356k
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&...
- 10.4k
373
votes
3
answers
251k
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
364
votes
14
answers
166k
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 ...
- 6,651
362
votes
10
answers
306k
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
...
- 7,153
356
votes
7
answers
110k
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 ...
- 46k
346
votes
5
answers
107k
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 ...
- 4,226
345
votes
19
answers
229k
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
341
votes
10
answers
150k
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....
- 62k
340
votes
6
answers
229k
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 ...
- 16.4k