Lambda expression to convert array/List of String

Question

Since Java 8 comes with powerful lambda expressions,

I would like to write a function to convert a List/array of Strings to array/List of Integers, Floats, Doubles etc..

In normal Java, it would be as simple as

for(String str : strList){
   intList.add(Integer.valueOf(str));
}

But how do I achieve the same with a lambda, given an array of Strings to be converted to an array of Integers.

Answer 1

You can also use,

List<String> list = new ArrayList<>();
    list.add("1");
    list.add("2");
    list.add("3");

    Integer[] array = list.stream()
        .map( v -> Integer.valueOf(v))
        .toArray(Integer[]::new);

Answer 2

List<Integer> intList = strList.stream()
                               .map(Integer::valueOf)
                               .collect(Collectors.toList());

Answer 3

In addition - control when string array doesn't have elements:

Arrays.stream(from).filter(t -> (t != null)&&!("".equals(t))).map(func).toArray(generator) 

Answer 4

For List :

List<Integer> intList 
 = stringList.stream().map(Integer::valueOf).collect(Collectors.toList());

For Array :

int[] intArray = Arrays.stream(stringArray).mapToInt(Integer::valueOf).toArray();

Answer 5

Arrays.toString(int []) works for me.

Answer 6

EDIT: As pointed out in the comments, this is a much simpler version: Arrays.stream(stringArray).mapToInt(Integer::parseInt).toArray() This way we can skip the whole conversion to and from a list.

---

I found another one line solution, but it's still pretty slow (takes about 100 times longer than a for cycle - tested on an array of 6000 0's)

String[] stringArray = ...
int[] out= Arrays.asList(stringArray).stream().map(Integer::parseInt).mapToInt(i->i).toArray();

What this does:

1. Arrays.asList() converts the array to a List
2. .stream converts it to a Stream (needed to perform a map)
3. .map(Integer::parseInt) converts all the elements in the stream to Integers
4. .mapToInt(i->i) converts all the Integers to ints (you don't have to do this if you only want Integers)
5. .toArray() converts the Stream back to an array