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

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

User · Accepted Answer

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);

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

User · Answer

Arrays toString int     works for me

User · Answer

I used maptoInt   with Lambda operation for converting string to Integer  int   arr   Arrays stream stringArray  mapToInt item - gt  Integer parseInt item   toArray

User · Answer

For List    List lt Integer gt  intList     stringList stream   map Integer  valueOf  collect Collectors toList       For Array    int   intArray   Arrays stream stringArray  mapToInt Integer  valueOf  toArray

User · Answer

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- gt i  toArray      What this does    Arrays asList   converts the array to a List  stream converts it to a Stream  needed to perform a map   map Integer  parseInt  converts all the elements in the stream to Integers  mapToInt i- i  converts all the Integers to ints  you don t have to do this if you only want Integers   toArray   converts the Stream back to an array

User · Answer

List lt Integer gt  intList   strList stream                                   map Integer  valueOf                                  collect Collectors toList

User · Answer

You can also use    List lt String gt  list   new ArrayList lt  gt         list add  1        list add  2        list add  3         Integer   array   list stream            map  v - gt  Integer valueOf v            toArray Integer    new

User · Answer

The helper methods from the accepted answer are not needed  Streams can be used with lambdas or usually shortened using Method References  Streams enable functional operations  map   converts the elements and collect      or toArray   wrap the stream back up into an array or collection   Venkat Subramaniam s talk  video  explains it better than me   1 Convert List lt String gt  to List lt Integer gt   List lt String gt  l1   Arrays asList  1    2    3    List lt Integer gt  r1   l1 stream   map Integer  parseInt  collect Collectors toList         the longer full lambda version  List lt Integer gt  r1   l1 stream   map s - gt  Integer parseInt s   collect Collectors toList       2 Convert List lt String gt  to int    int   r2   l1 stream   mapToInt Integer  parseInt  toArray      3 Convert String   to List lt Integer gt   String   a1     4    5    6    List lt Integer gt  r3   Stream of a1  map Integer  parseInt  collect Collectors toList       4 Convert String   to int    int   r4   Stream of a1  mapToInt Integer  parseInt  toArray      5 Convert String   to List lt Double gt   List lt Double gt  r5   Stream of a1  map Double  parseDouble  collect Collectors toList       6  bonus  Convert int   to String    int   a2    7  8  9   String   r6   Arrays stream a2  mapToObj Integer  toString  toArray String    new     Lots more variations are possible of course   Also see Ideone version of these examples  Can click fork and then run to run in the browser

User · Answer

In addition - control when string array doesn t have elements   Arrays stream from  filter t - gt   t    null  amp  amp      equals t    map func  toArray generator

User · Answer

I didn t find it in the previous answers  so  with Java 8 and streams    Convert String   to Integer     Arrays stream stringArray  map Integer  valueOf  toArray Integer    new

[java] Lambda expression to convert array/List of String to array/List of Integers

Examples related to java

Examples related to arrays

Examples related to collections

Examples related to lambda

Examples related to java-8