如果没有java-8，你可以在一行Commons集合和Closure类中完成这些

List<Item> list;
@SuppressWarnings("unchecked")
Map<Key, Item> map  = new HashMap<Key, Item>>(){{
    CollectionUtils.forAllDo(list, new Closure() {
        @Override
        public void execute(Object input) {
            Item item = (Item) input;
            put(i.getKey(), item);
        }
    });
}};

如果你使用Kotlin，这里有一个例子:

listOf("one", "two").mapIndexed { i, it -> i to it }.toMap()

普遍的方法

public static <K, V> Map<K, V> listAsMap(Collection<V> sourceList, ListToMapConverter<K, V> converter) {
    Map<K, V> newMap = new HashMap<K, V>();
    for (V item : sourceList) {
        newMap.put( converter.getKey(item), item );
    }
    return newMap;
}

public static interface ListToMapConverter<K, V> {
    public K getKey(V item);
}

Alexis已经在Java 8中使用toMap方法(keyMapper, valueMapper)发布了一个答案。根据这个方法实现的文档:

没有对类型、可变性、可序列化性或 返回Map的线程安全。

因此，如果我们对Map接口的特定实现感兴趣，例如HashMap，那么我们可以使用重载形式:

Map<String, Item> map2 =
                itemList.stream().collect(Collectors.toMap(Item::getKey, //key for map
                        Function.identity(),    // value for map
                        (o,n) -> o,             // merge function in case of conflict with keys
                        HashMap::new));         // map factory - we want HashMap and not any Map implementation

虽然使用Function.identity()或i->i都可以，但似乎Function.identity()而不是i->i可能会根据这个相关的答案节省一些内存。

public class EmployeeDetailsFetchListToMap {
  public static void main(String[] args) {
    List<EmployeeDetailsFetch> list = new ArrayList<>();
    list.add(new EmployeeDetailsFetch(1L, "vinay", 25000F));
    list.add(new EmployeeDetailsFetch(2L, "kohli", 5000000F));
    list.add(new EmployeeDetailsFetch(3L, "dhoni", 20000000F));

    //adding id as key and map of id and student name
    Map<Long, Map<Long, String>> map1 = list.stream()
        .collect(
            Collectors.groupingBy(
                EmployeeDetailsFetch::getEmpId,
                Collectors.toMap(
                    EmployeeDetailsFetch::getEmpId,
                    EmployeeDetailsFetch::getEmployeeName
                )
            )
        );
    System.out.println(map1);

    //converting list into map of Student
    //Adding id as Key and Value as Student into a map
    Map<Long, EmployeeDetailsFetch> map = list.stream()
        .collect(
            Collectors.toMap(
                EmployeeDetailsFetch::getEmpId, 
                EmployeeDetailsFetch -> EmployeeDetailsFetch
            )
        );

    for(Map.Entry<Long, EmployeeDetailsFetch> m : map.entrySet()) {
      System.out.println("key :" + m.getKey() + "  Value : " + m.getValue());
    }
  }
}

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

当然，假设每个Item都有一个getKey()方法，该方法返回一个正确类型的键。