yourTypes.stream()
         .collect(Collectors.groupingBy(
               ObjectA::getType,
               Collectors.mapping(
                    ObjectA::getId, Collectors.toList()
)))

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)

subjects.stream()
        .map(subjectName -> {
            List<User> users = .... create the users;
            return new SimpleEntry<>(subjectName, users);
        })
        .collect(Collectors.toMap(Entry::getKey, Entry::getValue))

This would be one way if you really wanted to do it with java-8 and streams. One improvement would be to have a method that takes a String subjectName and create that Entry for example:

private static Entry<String, List<User>> createEntry(String subjectName) {
    List<User> users = .... create the user;
    return new SimpleEntry<>(subjectName, users);
}

And use it with:

subjects.stream()
        .map(YourClass::createEntry)
        .collect(Collectors.toMap(Entry::getKey, Entry::getValue))

Just notice that your loop is the cleanest way to do it IMO

Based on Collectors documentation it's as simple as:

Map<String, Choice> result =
    choices.stream().collect(Collectors.toMap(Choice::getName,
                                              Function.identity()));