Java 8 Collections – How to Turn a List of Lists into a List in Java 8

collectionsjavajava-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?

Best Answer

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());

Related Solutions

Java 8 – How to Convert List with Properties to Another List

you will have to use something like below :

List<ProductManager> B = A.stream()
        .map(developer -> new ProductManager(developer.getName(), developer.getAge()))
        .collect(Collectors.toList());

// for large # of properties assuming the attributes have similar names //other wise with different names refer this

List<ProductManager> B = A.stream().map(developer -> {
            ProductManager productManager = new ProductManager();
            try {
                PropertyUtils.copyProperties(productManager, developer);
            } catch (Exception ex) {
                ex.printStackTrace();
            }
            return productManager;
        }).collect(Collectors.toList());

        B.forEach(System.out::println);

Java 8 – How to Convert a List into a List of Lists Using Lambda

Try this approach:

static <T> List<List<T>> listSplitter(List<T> incoming, int size) {
    // add validation if needed
    return incoming.stream()
            .collect(Collector.of(
                    ArrayList::new,
                    (accumulator, item) -> {
                        if(accumulator.isEmpty()) {
                            accumulator.add(new ArrayList<>(singletonList(item)));
                        } else {
                            List<T> last = accumulator.get(accumulator.size() - 1);
                            if(last.size() == size) {
                                accumulator.add(new ArrayList<>(singletonList(item)));
                            } else {
                                last.add(item);
                            }
                        }
                    },
                    (li1, li2) -> {
                        li1.addAll(li2);
                        return li1;
                    }
            ));
}
System.out.println(
        listSplitter(
                Arrays.asList(0, 1, 2, 3, 4, 5, 6, 7, 8, 9),
                4
        )
);

Also note that this code could be optimized, instead of:

new ArrayList<>(Collections.singletonList(item))

use this one:

List<List<T>> newList = new ArrayList<>(size);
newList.add(item);
return newList;

Best Answer

Related Solutions

Java 8 – How to Convert List with Properties to Another List

Java 8 – How to Convert a List into a List of Lists Using Lambda

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