How to convert List to Map?

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! :)

List<Item> list;Map<Key,Item> map = new HashMap<Key,Item>();for (Item i : list) map.put(i.getKey(),i);

Assuming of course that each Item has a getKey() method that returns a key of the proper type.

Just in case this question isn't closed as a duplicate, the right answer is to use Google Collections:

Map<String,Role> mappedRoles = Maps.uniqueIndex(yourList, new Function<Role,String>() {  public String apply(Role from) {    return from.getName(); // or something else  }});