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());
    }
  }
}

这是我为这个目的写的一个小方法。它使用来自Apache Commons的Validate。

请随意使用。

/**
 * Converts a <code>List</code> to a map. One of the methods of the list is called to retrive
 * the value of the key to be used and the object itself from the list entry is used as the
 * objct. An empty <code>Map</code> is returned upon null input.
 * Reflection is used to retrieve the key from the object instance and method name passed in.
 *
 * @param <K> The type of the key to be used in the map
 * @param <V> The type of value to be used in the map and the type of the elements in the
 *            collection
 * @param coll The collection to be converted.
 * @param keyType The class of key
 * @param valueType The class of the value
 * @param keyMethodName The method name to call on each instance in the collection to retrieve
 *            the key
 * @return A map of key to value instances
 * @throws IllegalArgumentException if any of the other paremeters are invalid.
 */
public static <K, V> Map<K, V> asMap(final java.util.Collection<V> coll,
        final Class<K> keyType,
        final Class<V> valueType,
        final String keyMethodName) {

    final HashMap<K, V> map = new HashMap<K, V>();
    Method method = null;

    if (isEmpty(coll)) return map;
    notNull(keyType, Messages.getString(KEY_TYPE_NOT_NULL));
    notNull(valueType, Messages.getString(VALUE_TYPE_NOT_NULL));
    notEmpty(keyMethodName, Messages.getString(KEY_METHOD_NAME_NOT_NULL));

    try {
        // return the Method to invoke to get the key for the map
        method = valueType.getMethod(keyMethodName);
    }
    catch (final NoSuchMethodException e) {
        final String message =
            String.format(
                    Messages.getString(METHOD_NOT_FOUND),
                    keyMethodName,
                    valueType);
        e.fillInStackTrace();
        logger.error(message, e);
        throw new IllegalArgumentException(message, e);
    }
    try {
        for (final V value : coll) {

            Object object;
            object = method.invoke(value);
            @SuppressWarnings("unchecked")
            final K key = (K) object;
            map.put(key, value);
        }
    }
    catch (final Exception e) {
        final String message =
            String.format(
                    Messages.getString(METHOD_CALL_FAILED),
                    method,
                    valueType);
        e.fillInStackTrace();
        logger.error(message, e);
        throw new IllegalArgumentException(message, e);
    }
    return map;
}

在java-8中，您可以使用流和collections类在一行中完成此操作。

Map<String, Item> map = 
    list.stream().collect(Collectors.toMap(Item::getKey, item -> item));

简短的演示:

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 + "]";
    }
}

输出:

Key-1 => Item [i=1]
Key-2 => Item [i=2]
Key-3 => Item [i=3]
Key-4 => Item [i=4]

如注释中所述，您可以使用Function.identity()而不是item -> item，尽管我发现I -> I相当显式。

为了完整起见，如果你的函数不是双射的，你可以使用二元算子。例如，让我们考虑这个List和映射函数，对于一个int值，计算它的模3的结果:

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));

当运行这段代码时，您将得到一个错误，说java.lang.IllegalStateException: Duplicate key 1。这是因为1% 3与4% 3相同，因此在给定键映射函数时具有相同的键值。在这种情况下，您可以提供一个合并操作符。

这是两个值的总和;(i1, i2) -> i1 + i2;可以用引用Integer::sum方法替换。

Map<String, Integer> map = 
    intList.stream().collect(toMap(i -> String.valueOf(i % 3), 
                                   i -> i, 
                                   Integer::sum));

现在输出:

0 => 9 (i.e 3 + 6)
1 => 5 (i.e 1 + 4)
2 => 7 (i.e 2 + 5)

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。

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);
        }
    });
}};