I have a list of integers [1,2,3,4,5] and I want to convert it into a map after applying a multiplication function (*5) like this:
{1 = 5, 2 = 10, 3 = 15, 4 = 20, 5 = 25}
I was able to use stream and use map function to perform the multiplication but I'm confused about how to convert the result into a map.
myList.stream().map(n -> n * 5).collect( ... )
Can someone please help.
Your current stream pipeline converts the original values to new values, so you can't collect it into a Map that contains both the original values and the new values.
You can achieve it if instead of map you use .collect(Collectors.toMap()) and perform the multiplication in toMap():
Map<Integer,Integer> map =
myList.stream()
.collect(Collectors.toMap(Function.identity(),
n -> n * 5));
In you still want to use map, you can retain the original values by converting each of them to a Map.Entry:
Map<Integer,Integer> map =
myList.stream()
.map (n -> new SimpleEntry<> (n, n * 5))
.collect(Collectors.toMap(Map.Entry::getKey,
Map.Entry::getValue));
myList.stream() .collect(Collectors.toMap(Function.identity(), someFunction()));
n -> someFunction(n){1={value=5}, 2={value=10), 3={value=15), 4={value=20), 5={value=25)} i.e. Map<key, object> One way to do it is I make my someFunction(n) return a map i.e. {value = n*5} Is there another way where I don't have to change my function someFunction(n)someFunction(n) return a Map? It should just return the value that should be mapped to n.{1={"value"=5}, 2={"value"=10), 3={"value"=15), 4={"value"=20), 5={"value"=25)} If someFunction(n) just returns a value called output then we how can we create n = {"value" = output} in Collectors.toMapYou can use Collectors#toMap and pass any function for the value e.g. by using a UnaryOperator as shown below:
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.function.UnaryOperator;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
UnaryOperator<Integer> add = x -> x + 5;
UnaryOperator<Integer> mutiply = x -> x * 5;
UnaryOperator<Integer> factorial = x -> factorial(x);
// Test
List<Integer> list = List.of(1, 2, 3, 4, 5);
Map<Integer, Integer> map1 = list.stream().collect(Collectors.toMap(Function.identity(), add));
Map<Integer, Integer> map2 = list.stream().collect(Collectors.toMap(Function.identity(), mutiply));
Map<Integer, Integer> map3 = list.stream().collect(Collectors.toMap(Function.identity(), factorial));
System.out.println(map1);
System.out.println(map2);
System.out.println(map3);
}
static int getValue(int x, UnaryOperator<Integer> function) {
return function.apply(x);
}
static int factorial(int x) {
if (x <= 0) {
return 1;
}
return x * factorial(x - 1);
}
}
Output:
{1=6, 2=7, 3=8, 4=9, 5=10}
{1=5, 2=10, 3=15, 4=20, 5=25}
{1=1, 2=2, 3=6, 4=24, 5=120}
The .collect(Collectors.toMap()) is what you need. Without the .map.
myList.stream().collect(Collectors.toMap(n-> n, n -> n * 5))
Map<Integer, Long> map = Arrays
.stream(nums)
.boxed() // this
.collect(Collectors.groupingBy(e -> e, Collectors.counting()));
Collectors.toMap( v -> v, v -> v * 5).