Lambda Expression to Convert Array/List of String to Array/List of Integers

Lambda expression to convert array/List of String to array/List of Integers

You could create helper methods that would convert a list (array) of type T to a list (array) of type U using the map operation on stream.

//for lists
public static <T, U> List<U> convertList(List<T> from, Function<T, U> func) {
    return from.stream().map(func).collect(Collectors.toList());
}

//for arrays
public static <T, U> U[] convertArray(T[] from, 
                                      Function<T, U> func, 
                                      IntFunction<U[]> generator) {
    return Arrays.stream(from).map(func).toArray(generator);
}

And use it like this:

//for lists
List<String> stringList = Arrays.asList("1","2","3");
List<Integer> integerList = convertList(stringList, s -> Integer.parseInt(s));

//for arrays
String[] stringArr = {"1","2","3"};
Double[] doubleArr = convertArray(stringArr, Double::parseDouble, Double[]::new);

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Note that s -> Integer.parseInt(s) could be replaced with Integer::parseInt (see Method references)

Convert array of Strings to list of Integers?

You only need to stream once.

Instead of using int Integer::parseInt(String s), you should use Integer Integer::valueOf(String s), so you don't have to call boxed() or rely on auto-boxing.

Then use collect(Collectors.toList()) directly, instead of creating intermediate array first.

List<Integer> allAnswerList = Arrays.stream(allAnswers)    // stream of String
                                    .map(Integer::valueOf) // stream of Integer
                                    .collect(Collectors.toList());

How to simplify a lambda method that receives a list of integers and returns a string by adding a letter before the number?

You can simply map to String as:

return list.stream()
            .map(x -> x % 2 == 0 ? "e" + x : "o" + x)
            .collect(Collectors.joining(", "));

Converting a List of Strings to a List of custom made objects in Java using Lambda Expression

If you add a constructor to your Person class that takes a string, you can just use:

List<Person> people = persons.stream()
    .map(Person::new)
    .collect(Collectors.toList());

If you don't have a constructor, then you will need a lambda:

List<Person> people = persons.stream()
    .map(s -> { Person p = new Person(); p.setName(s); return p; })
    .collect(Collectors.toList());

Convert ListString to ListInteger directly

No, you need to loop over the array

for(String s : strList) intList.add(Integer.valueOf(s));

Java 8 lambda create list of Strings from list of objects

You need to collect your Stream into a List:

List<String> adresses = users.stream()
    .map(User::getAdress)
    .collect(Collectors.toList());

For more information on the different Collectors visit the documentation.

User::getAdress is just another form of writing (User user) -> user.getAdress() which could as well be written as user -> user.getAdress() (because the type User will be inferred by the compiler)

ListString to Map using Lambda expressions

You need to iterate. Here's one-line using an int stream:

IntStream.range(0, fillMyList.size()).forEach(i -> fillMyList.set(i, ""));

Using lambda expression of Java 8, convert ListByte to primitive byte[], without external libraries

You can try doing like this:

listByte.stream().collect(ByteArrayOutputStream::new, (baos, i) -> baos.write((byte) i),
        (baos1, baos2) -> baos1.write(baos2.toByteArray(), 0, baos2.size()))
        .toByteArray()

Error trying to convert a ListString to ListInteger

You should split the lines of String using the delimeter , and flatMap them to collect as a List.

lineas = bf.lines()
        .flatMap(s -> Arrays.stream(s.split(",")))
        .collect(Collectors.toList());

then return the List<Integer> mapped as:

return lineas.stream()
        .map(Integer::parseInt)
        .collect(Collectors.toList());

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