How do I join two lists in Java?

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旧巷少年郎
旧巷少年郎 2020-11-22 14:36

Conditions: do not modifiy the original lists; JDK only, no external libraries. Bonus points for a one-liner or a JDK 1.3 version.

Is there a simpler way than:

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  • 2020-11-22 15:33
    public static <T> List<T> merge(List<T>... args) {
        final List<T> result = new ArrayList<>();
    
        for (List<T> list : args) {
            result.addAll(list);
        }
    
        return result;
    }
    
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  • 2020-11-22 15:33

    We can join 2 lists using java8 with 2 approaches.

        List<String> list1 = Arrays.asList("S", "T");
        List<String> list2 = Arrays.asList("U", "V");
    

    1) Using concat :

        List<String> collect2 = Stream.concat(list1.stream(), list2.stream()).collect(toList());
        System.out.println("collect2 = " + collect2); // collect2 = [S, T, U, V]
    

    2) Using flatMap :

        List<String> collect3 = Stream.of(list1, list2).flatMap(Collection::stream).collect(toList());
        System.out.println("collect3 = " + collect3); // collect3 = [S, T, U, V]
    
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  • 2020-11-22 15:34

    The smartest in my opinion:

    /**
     * @param smallLists
     * @return one big list containing all elements of the small ones, in the same order.
     */
    public static <E> List<E> concatenate (final List<E> ... smallLists)
    {
        final ArrayList<E> bigList = new ArrayList<E>();
        for (final List<E> list: smallLists)
        {
            bigList.addAll(list);
        }
        return bigList;
    }
    
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  • 2020-11-22 15:36

    another one liner solution using Java8 stream, since flatMap solution is already posted, here is a solution without flatMap

    List<E> li = lol.stream().collect(ArrayList::new, List::addAll, List::addAll);
    

    or

    List<E> ints = Stream.of(list1, list2).collect(ArrayList::new, List::addAll, List::addAll);
    

    code

        List<List<Integer>> lol = Arrays.asList(Arrays.asList(1, 2, 3), Arrays.asList(4, 5, 6));
        List<Integer> li = lol.stream().collect(ArrayList::new, List::addAll, List::addAll);
        System.out.println(lol);
        System.out.println(li);
    

    output

    [[1, 2, 3], [4, 5, 6]]
    [1, 2, 3, 4, 5, 6]
    
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  • 2020-11-22 15:37

    Another Java 8 one-liner:

    List<String> newList = Stream.of(listOne, listTwo)
                                .flatMap(Collection::stream)
                                .collect(Collectors.toList());
    

    As a bonus, since Stream.of() is variadic, you may concatenate as many lists as you like.

    List<String> newList = Stream.of(listOne, listTwo, listThree)
                                .flatMap(Collection::stream)
                                .collect(Collectors.toList());
    
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  • 2020-11-22 15:38

    Not simpler, but without resizing overhead:

    List<String> newList = new ArrayList<>(listOne.size() + listTwo.size());
    newList.addAll(listOne);
    newList.addAll(listTwo);
    
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