How to force max to return ALL maximum values in a Java Stream?

Question

I\'ve tested a bit the max function on Java 8 lambdas and streams, and it seems that in case max is executed, even if more than one object compares

Answer 1

If I understood well, you want the frequency of the max value in the Stream.

One way to achieve that would be to store the results in a TreeMap when you collect elements from the Stream. Then you grab the last key (or first depending on the comparator you give) to get the value which will contains the list of max values.

List maxValues = st.collect(toMap(i -> i,
                     Arrays::asList,
                     (l1, l2) -> Stream.concat(l1.stream(), l2.stream()).collect(toList()),
                     TreeMap::new))
             .lastEntry()
             .getValue();

Collecting it from the Stream(4, 5, -2, 5, 5) will give you a List [5, 5, 5].

Another approach in the same spirit would be to use a group by operation combined with the counting() collector:

Entry maxValues = st.collect(groupingBy(i -> i,
                TreeMap::new,
                counting())).lastEntry(); //5=3 -> 5 appears 3 times

Basically you firstly get a Map>. Then the downstream counting() collector will return the number of elements in each list mapped by its key resulting in a Map. From there you grab the max entry.

The first approaches require to store all the elements from the stream. The second one is better (see Holger's comment) as the intermediate List is not built. In both approached, the result is computed in a single pass.

If you get the source from a collection, you may want to use Collections.max one time to find the maximum value followed by Collections.frequency to find how many times this value appears.

It requires two passes but uses less memory as you don't have to build the data-structure.

The stream equivalent would be coll.stream().max(...).get(...) followed by coll.stream().filter(...).count().