With java-8, you'll be able to do this in one line using streams, and the Collectors class.
Map<String, Item> map =
list.stream().collect(Collectors.toMap(Item::getKey, item -> item));
Short demo:
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Test{
public static void main (String [] args){
List<Item> list = IntStream.rangeClosed(1, 4)
.mapToObj(Item::new)
.collect(Collectors.toList()); //[Item [i=1], Item [i=2], Item [i=3], Item [i=4]]
Map<String, Item> map =
list.stream().collect(Collectors.toMap(Item::getKey, item -> item));
map.forEach((k, v) -> System.out.println(k + " => " + v));
}
}
class Item {
private final int i;
public Item(int i){
this.i = i;
}
public String getKey(){
return "Key-"+i;
}
@Override
public String toString() {
return "Item [i=" + i + "]";
}
}
Output:
Key-1 => Item [i=1]
Key-2 => Item [i=2]
Key-3 => Item [i=3]
Key-4 => Item [i=4]
As noted in comments, you can use Function.identity() instead of item -> item, although I find i -> i rather explicit.
And to be complete note that you can use a binary operator if your function is not bijective. For example let's consider this List and the mapping function that for an int value, compute the result of it modulo 3:
List<Integer> intList = Arrays.asList(1, 2, 3, 4, 5, 6);
Map<String, Integer> map =
intList.stream().collect(toMap(i -> String.valueOf(i % 3), i -> i));
When running this code, you'll get an error saying java.lang.IllegalStateException: Duplicate key 1. This is because 1 % 3 is the same as 4 % 3 and hence have the same key value given the key mapping function. In this case you can provide a merge operator.
Here's one that sum the values; (i1, i2) -> i1 + i2; that can be replaced with the method reference Integer::sum.
Map<String, Integer> map =
intList.stream().collect(toMap(i -> String.valueOf(i % 3),
i -> i,
Integer::sum));
which now outputs:
0 => 9 (i.e 3 + 6)
1 => 5 (i.e 1 + 4)
2 => 7 (i.e 2 + 5)
Hope it helps! :)