Coerce type without creating temporary variable

I have the following function:

  <K, P, Q> Map<K, List<Q>> convert(Map<K, ? extends List<? extends P>> input) {
    return HashMap.empty();
  }

I am calling it in the following way:

Map<Integer, List<? extends String>> stringMap = HashMap.empty();
Map<Integer, List<? extends Double>> doubleMap = HashMap.empty();
Map<Integer, List<Boolean>> stringMapConverted = convert(stringMap);
Map<Integer, List<Boolean>> merged = stringMapConverted.merge(convert(doubleMap));

However, I want to do this without having to create a temporary variable first.
Something like this:

Map<Integer, List<? extends String>> stringMap = HashMap.empty();
Map<Integer, List<? extends Double>> doubleMap = HashMap.empty();
Map<Integer, List<Boolean>> merged = convert(stringMap).merge(convert(doubleMap));

However, I am getting a compilation error here:

'merge(io.vavr.collection.Map<? extends java.lang.Integer,? extends io.vavr.collection.List<java.lang.Object>>)' in 'io.vavr.collection.Map' cannot be applied to '(io.vavr.collection.Map<java.lang.Integer,io.vavr.collection.List<? extends java.lang.Double>>)'

Required type: Map<? extends Integer,? extends List<Object>>
Provided: Map<Integer,List<? extends Double>>

I also tried casting explicitly:

Map<Integer, List<Boolean>> merged2 = ((Map<Integer,List<Boolean>>)convert(stringMap)).merge(doubleMap);

but its throwing a compilation error:

Inconvertible types; cannot cast 'io.vavr.collection.Map<java.lang.Integer,io.vavr.collection.List<java.lang.Object>>' to 'io.vavr.collection.Map<java.lang.Integer,io.vavr.collection.List<java.lang.Boolean>>'

The signature of Map.merge() is as follows:

Map<K, V> merge(Map<? extends K, ? extends V> that);

The Map and List classes are from vavr.

Solution

Use type witness, or so called TypeArguments mentioned in 15.12 in SE11 JLS to specify the type argument when invoking convert.

MethodInvocation:
  MethodName ( [ArgumentList] )
  TypeName . [TypeArguments] Identifier ( [ArgumentList] )
  ExpressionName . [TypeArguments] Identifier ( [ArgumentList] )
  Primary . [TypeArguments] Identifier ( [ArgumentList] )
  super . [TypeArguments] Identifier ( [ArgumentList] )
  TypeName . super . [TypeArguments] Identifier ( [ArgumentList] )
ArgumentList: Expression {, Expression}

Note that we cannot omit the instance name->(this) when using TypeArguments.

Map<Integer, List<Boolean>> merged = this.<Integer, String, Boolean>convert(stringMap).merge(convert(doubleMap));