Java8：使用流将一张地图转换为另一张地图

Question

我需要将 Java HashMap转换为TreeMap的实例（包括地图内容）

HashMap<String, Object> src = ...;
TreeMap<String, Object> dest = src.entrySet().stream()
        .filter( ... )
        .collect(Collectors.toMap( ???, ???, ???, TreeMap::new));

我应该用什么代替??? 使这段代码可编译？

Answer 1

从Collectors.toMap(...) javadoc ：

 * @param keyMapper a mapping function to produce keys
 * @param valueMapper a mapping function to produce values
 * @param mergeFunction a merge function, used to resolve collisions between
 *                      values associated with the same key, as supplied
 *                      to {@link Map#merge(Object, Object, BiFunction)}
 * @param mapSupplier a function which returns a new, empty {@code Map} into
 *                    which the results will be inserted

例如：

HashMap<String, Object> src = ...;
TreeMap<String, Object> dest = src.entrySet().stream()
      .filter( ... )
      .collect(Collectors.toMap(Map.Entry::getKey , Map.Entry::getValue, (a,b) -> a, TreeMap::new));

Answer 2

将地图转换为流的另一种方法：

Stream.of(t) 的使用

HashMap<String, Object> src = ...;
TreeMap<String, Object> dest = Stream.of(src)
      .filter( ... )
      .collect(Collectors.toMap(Map.Entry::getKey , Map.Entry::getValue, (a,b) -> a, TreeMap::new));