Lambda expression to convert array/List of String to array/List of Integers
Solution 1
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)
Solution 2
List<Integer> intList = strList.stream()
.map(Integer::valueOf)
.collect(Collectors.toList());
Solution 3
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<String>
to List<Integer>
List<String> l1 = Arrays.asList("1", "2", "3");
List<Integer> r1 = l1.stream().map(Integer::parseInt).collect(Collectors.toList());
// the longer full lambda version:
List<Integer> r1 = l1.stream().map(s -> Integer.parseInt(s)).collect(Collectors.toList());
2 Convert List<String>
to int[]
int[] r2 = l1.stream().mapToInt(Integer::parseInt).toArray();
3 Convert String[]
to List<Integer>
String[] a1 = {"4", "5", "6"};
List<Integer> 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<Double>
List<Double> 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.
Solution 4
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:
- 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
Solution 5
For List :
List<Integer> intList
= stringList.stream().map(Integer::valueOf).collect(Collectors.toList());
For Array :
int[] intArray = Arrays.stream(stringArray).mapToInt(Integer::valueOf).toArray();
Related videos on Youtube
Comments
-
RaceBase about 4 years
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.
-
Unheilig over 9 yearsCan you explain this a little?
-
Alexandros over 8 yearsIf you're using Guava, you can also use
Lists.transform()
. -
xehpuk over 8 yearsYou don't need steps 1 and 4:
Arrays.stream(stringArray).mapToInt(Integer::parseInt).toArray()
-
hemanto over 6 yearsThis is converting a list of String into a stream then mapping/ converting every element of the list into Integer then collecting into a list.
-
Jonathan Benn almost 6 yearsThis should be the accepted answer, since the point of lambdas (the original question asks for a lambda expression) is to avoid defining a new function. On top of which, this answer elegantly uses the existing Integer::valueOf function
-
Sergei Sirik over 5 yearsI doesn't answer the question: "How to achieve this with lambdas".
-
Marcelo Rebouças almost 5 yearsVery good example. The function turns the code more versatile and powerful. Thks!!!