how do I join multiple list with varargs in java?

user_13497272 0
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I want to join several list using Stream API.

I found a useful example at stackoverflow:

List<Integer>list1=Arrays.asList(1,2,3);
List<Integer>list2=Arrays.asList(4,5,6);
List<Integer>list3=Arrays.asList(7,8,9);
List<Integer>list4=Arrays.asList(10,0,-1);

List<Integer> newList = 
    Stream.of(list1, list2, list3,list4)
          .flatMap(Collection::stream)
          .collect(Collectors.toList());       
System.out.println(newList); // prints [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, -1]

Nice. But in my case, I can`t determine how many lists to use this Stream.of().

I thought about using varargs at jdk8. like this

public List<Integer> getNewList(List... list) {
    List<Integer> newList = 
         Stream.of(list)
               .flatMap(Collection::stream)
               .collect(Collectors.toList());
    return newList;
}

getNewList(list1, list2, list3, list...)

Unfortunately, it didn`t work well. Is there a way to make this work?

java-stream java-8 collections java
3 Answers

You should specify the type of List parameters you are passing as var args, and you can also use Arrays.stream instead of Stream.of

 public static List<Integer> getNewList(List<Integer>... list){
    return Arrays.stream(list)
            .flatMap(List::stream)
            .collect(Collectors.toList());
}
user_99591520

Arrays.stream should do the trick:

public static <T> List<T> merge(List<T>... lists) {
    return Arrays.stream(lists)
                 .flatMap(List::stream)
                 .collect(Collectors.toList());

}

BTW, I've just noticed that your solution is almost fine. The only thing you need to add is <Integer>. So Stream.of should be also fine.

public List<Integer> getNewList(List<Integer>... list) {
    List<Integer> newList = 
         Stream.of(list)
               .flatMap(Collection::stream)
               .collect(Collectors.toList());
    return newList;
}

In fact, Stream::of calls Arrays::stream under the hood. So you can use any of them for your particular use case.

user_105846050

Try it like this. Use Arrays.stream() to stream the array of lists via the varargs syntax.

public static <T> List<T> getNewList(List<T>... list) {
    return  Arrays.stream(list).flatMap(Collection::stream)
                    .collect(Collectors.toList());
}
user_15525340
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