generics - Cannot use Java 8 method with lambda arguments without specifying type arguments

Question

I made a method with type arguments, returning a generic type using these type arguments, and taking Function arguments which also depends on the type arguments. When I use lambdas as arguments, the compiler forces me to specify the type arguments of the method, which feels wrong.

I am designing a utility class with methods to use with Stream.flatMap. It maps every kind of collection entry to a FlatEntry which contains a key and value element, and can do this on multiple levels with a builder. The affected method is flatEntryMapperBuilder. Here is the code:

import java.util.function.Function;
import java.util.stream.Stream;

public class GdkStreams
{
    public static <T, K, V> Function<T, Stream<FlatEntry<K, V>>> flatEntryMapper(Function<T, K> keyMapper,
                                                                                 Function<T, Stream<V>> valueMapper)
    {
        return input -> {
            K key = keyMapper.apply(input);
            return valueMapper.apply(input).map(value -> new FlatEntry<>(key, value));
        };
    }

    public static <T, K, V> FlatEntryMapperBuilder<T, K, V> flatEntryMapperBuilder(Function<T, K> keyMapper,
                                                                                   Function<T, Stream<V>> valueMapper)
    {
        return new FlatEntryMapperBuilder<>(keyMapper, valueMapper);
    }

    public static class FlatEntryMapperBuilder<T, K, V>
    {
        private Function<T, K>         keyMapper;

        private Function<T, Stream<V>> valueMapper;

        private FlatEntryMapperBuilder (Function<T, K> keyMapper, Function<T, Stream<V>> valueMapper)
        {
            this.keyMapper = keyMapper;
            this.valueMapper = valueMapper;
        }

        public Function<T, Stream<FlatEntry<K, V>>> build()
        {
            return flatEntryMapper(keyMapper, valueMapper);
        }

        public <K2, V2> FlatEntryMapperBuilder<T, K, FlatEntry<K2, V2>> chain(Function<V, K2> keyMapper2,
                                                                              Function<V, Stream<V2>> valueMapper2)
        {
            return new FlatEntryMapperBuilder<>(keyMapper,
                                                valueMapper.andThen(stream -> stream.flatMap(flatEntryMapper(keyMapper2,
                                                                                                             valueMapper2))));
        }
    }

    public static class FlatEntry<K, V>
    {
        public final K key;

        public final V value;

        public FlatEntry (K key, V value)
        {
            this.key = key;
            this.value = value;
        }
    }
}

The problem comes with its usage. Say I have:

Map<String, Set<String>> level1Map;

I can map every element in the sub Sets to a FlatEntry by doing:

level1Map.entrySet().stream().flatMap(GdkStreams.flatEntryMapper(Entry::getKey, entry -> entry.getValue().stream()));

And it works just fine. But when I try to do this:

level1Map.entrySet()
         .stream()
         .flatMap(GdkStreams.flatEntryMapperBuilder(Entry::getKey, entry -> entry.getValue().stream()).build());

The eclipse (Mars 4.5.0) compiler breaks with:

- The type Map.Entry does not define getKey(Object) that is applicable here
- The method getValue() is undefined for the type Object
- Type mismatch: cannot convert from GdkStreams.FlatEntryMapperBuilder<Object,Object,Object> to 
 <unknown>

And javac (1.8.0_51) breaks with:

MainTest.java:50: error: incompatible types: cannot infer type-variable(s) T,K#1,V#1
                 .flatMap(GdkStreams.flatEntryMapperBuilder(Entry::getKey, entry -> entry.getValue().stream()).build());
                                                           ^
    (argument mismatch; invalid method reference
      method getKey in interface Entry<K#2,V#2> cannot be applied to given types
        required: no arguments
        found: Object
        reason: actual and formal argument lists differ in length)
  where T,K#1,V#1,K#2,V#2 are type-variables:
    T extends Object declared in method <T,K#1,V#1>flatEntryMapperBuilder(Function<T,K#1>,Function<T,Stream<V#1>>)
    K#1 extends Object declared in method <T,K#1,V#1>flatEntryMapperBuilder(Function<T,K#1>,Function<T,Stream<V#1>>)
    V#1 extends Object declared in method <T,K#1,V#1>flatEntryMapperBuilder(Function<T,K#1>,Function<T,Stream<V#1>>)
    K#2 extends Object declared in interface Entry
    V#2 extends Object declared in interface Entry
MainTest.java:50: error: invalid method reference
                 .flatMap(GdkStreams.flatEntryMapperBuilder(Entry::getKey, entry -> entry.getValue().stream()).build());
                                                            ^
  non-static method getKey() cannot be referenced from a static context
  where K is a type-variable:
    K extends Object declared in interface Entry
2 errors

If I replace Entry::getKey by entry -> entry.getKey(), javac changes its output drastically:

MainTest.java:51: error: cannot find symbol
                 .flatMap(GdkStreams.flatEntryMapperBuilder(entry -> entry.getKey(), entry -> entry.getValue().stream()).build());

                                                                          ^
  symbol:   method getKey()
  location: variable entry of type Object
MainTest.java:51: error: cannot find symbol
                 .flatMap(GdkStreams.flatEntryMapperBuilder(entry -> entry.getKey(), entry -> entry.getValue().stream()).build());

                                                                                                   ^
  symbol:   method getValue()
  location: variable entry of type Object
2 errors

It compiles fine by specifying type parameters, which is what I expected:

level1Map.entrySet()
         .stream()
         .flatMap(GdkStreams.<Entry<String, Set<String>>, String, String> flatEntryMapperBuilder(Entry::getKey,
                                                                                                 entry -> entry.getValue()
                                                                                                               .stream())
                            .build());

or specifying one of the arguments type parameters:

Function<Entry<String, Set<String>>, String> keyGetter = Entry::getKey;
level1Map.entrySet()
         .stream()
         .flatMap(GdkStreams.flatEntryMapperBuilder(keyGetter, entry -> entry.getValue().stream()).build());

But this is clumsy! Imagine now how clumsy it would be to write all type parameters with 2 levels in the map, using the chain method (which is my target usage):

Map<String, Map<String, Set<String>>> level2Map;

I have read many other questions about lambdas and generics type inference but none is answering my particular case.

Am I missing something? Can I correct my API so that its usage is less clumsy, or am I stuck with always specifying type arguments? Thanks!

Answer 1

Holger had the best answer in the comment section in my opinion:

This is a known limitation of Java 8’s type inference: it doesn’t work with chained method invocations like genericFactoryMethod().build().

Thanks! About my API, I will specify the functions before using them as arguments, like this:

Function<Entry<String, Set<String>>, String> keyMapper = Entry::getKey;
Function<Entry<String, Set<String>>, Stream<String>> valueMapper = entry -> entry.getValue().stream();

EDIT: I redesigned the API thanks to Holger's comments (thanks again!). It keeps the original element instead of a key, along with the flattened value.

public static <T, R> Function<? super T, Stream<FlatEntry<T, R>>> flatEntryMapper(Function<? super T, ? extends Stream<? extends R>> mapper)
{
    return element -> mapper.apply(element).map(value -> new FlatEntry<>(element, value));
}

public static class FlatEntry<E, V>
{
    /** The original stream element */
    public final E element;

    /** The flattened value */
    public final V value;

    private FlatEntry (E element, V value)
    {
        this.element = element;
        this.value = value;
    }
}

It is chainable, starting with level 2 the mapper has to process a FlatEntry. The usage is similar to a simple flatMap:

Map<String, Map<String, Map<String, Set<String>>>> level3Map;

// gives a stream of all the flattened values
level3Map.entrySet()
         .stream()
         .flatMap(entry -> entry.getValue().entrySet().stream())
         .flatMap(entry -> entry.getValue().entrySet().stream())
         .flatMap(entry -> entry.getValue().stream());

// gives a stream of FlatEntries with flattened values and all their original elements in nested FlatEntries
level3Map.entrySet()
         .stream()
         .flatMap(GdkStreams.flatEntryMapper(entry -> entry.getValue().entrySet().stream()))
         .flatMap(GdkStreams.flatEntryMapper(flatEntry -> flatEntry.value.getValue().entrySet().stream()))
         .flatMap(GdkStreams.flatEntryMapper(flatEntry -> flatEntry.value.getValue().stream()));