Java8: convert one map to another map using stream

Question

I need to convert a Java HashMap to an instance of TreeMap (including map contents)

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

What should I put in place of ??? to make this code compilable?

Answer 1

From 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 

For example:

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));