Lambda return types and overloaded methods

[Greenaway, Chris]

There seems to be a problem with matching the return type of a lambda to the type required by an overloaded method when the lambda parameter isn't explicitly typed.

For example, the code below has compilation failures on the lines that are marked with "// Error". All other lines are acceptable.

There also seems to be an issue with method references when there are multiple methods with the same name but different arity - see the line marked "// Error 8".

            Chris Greenaway.


import java.util.function.Function;
import java.util.function.Predicate;

public class LambdaOverloading {
    public static void main(String[] args) {
        LambdaPredicate predicate = new LambdaPredicate();
        predicate.run((String s) -> s.startsWith("X"));
        predicate.run((String s) -> s.isEmpty());
        predicate.run((String s) -> true);
        predicate.run(s -> s.startsWith("X"));
        predicate.run(s -> s.isEmpty());
        predicate.run(s -> true);
        predicate.run(String::isEmpty);

        LambdaFunction function = new LambdaFunction();
        function.run((String s) -> s.substring(1));
        function.run((String s) -> s.trim());
        function.run((String s) -> s.toUpperCase());
        function.run((String s) -> "");
        function.run(s -> s.substring(1));
        function.run(s -> s.trim());
        function.run(s -> s.toUpperCase());
        function.run(s -> "");
        function.run(String::trim);
        function.run(String::toUpperCase);

        LambdaOverloaded overloaded = new LambdaOverloaded();
        overloaded.run((String s) -> s.startsWith("X"));
        overloaded.run((String s) -> s.substring(1));
        overloaded.run((String s) -> s.isEmpty());
        overloaded.run((String s) -> s.trim());
        overloaded.run((String s) -> s.toUpperCase());
        overloaded.run((String s) -> true);
        overloaded.run((String s) -> "");
        overloaded.run(s -> s.startsWith("X"));                     // Error 1
        overloaded.run(s -> s.substring(1));                        // Error 2
        overloaded.run(s -> s.isEmpty());                           // Error 3
        overloaded.run(s -> s.trim());                              // Error 4
        overloaded.run(s -> s.toUpperCase());                       // Error 5
        overloaded.run(s -> true);                                  // Error 6
        overloaded.run(s -> "");                                    // Error 7
        overloaded.run(String::isEmpty);
        overloaded.run(String::trim);
        overloaded.run(String::toUpperCase);                        // Error 8
    }

    private static class LambdaPredicate {
        public boolean run(Predicate<String> predicate) {
            return predicate.test("test");
        }
    }

    private static class LambdaFunction {
        public String run(Function<String, String> function) {
            return function.apply("test");
        }
    }

    private static class LambdaOverloaded {
        public boolean run(Predicate<String> predicate) {
            return predicate.test("test");
        }

        public String run(Function<String, String> function) {
            return function.apply("test");
        }
    }
}
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