Lambdas.java

import java.lang.invoke.LambdaMetafactory;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Objects;
import java.util.function.IntConsumer;

public class Lambdas {
    private static <F, T> F createLambda(
            Class<? extends F> output, T object, String methodName, Class<?>... inputTypes
    ) throws NoSuchMethodException {
        MethodHandles.Lookup caller = MethodHandles.lookup();
        Class<?> returnType = object.getClass().getMethod(methodName, inputTypes).getReturnType();
        MethodType actualMethodType = MethodType.methodType(returnType, inputTypes);
        Method foundInterfaceMethod = getInterfaceMethod(output.getMethods());
        try {
            @SuppressWarnings("unchecked")
            F result = (F) LambdaMetafactory.metafactory(
                    caller,
                    foundInterfaceMethod.getName(),
                    MethodType.methodType(output, object.getClass()),
                    MethodType.methodType(foundInterfaceMethod.getReturnType(), foundInterfaceMethod.getParameterTypes()),
                    caller.findVirtual(object.getClass(), methodName, actualMethodType),
                    actualMethodType
            ).getTarget().invoke(object);
            return result;
        } catch (Throwable ex) {
            throw new RuntimeException(ex);
        }
    }

    private static Method getInterfaceMethod(Method[] interfaceMethods) {
        Method foundInterfaceMethod = null;
        for (Method interfaceMethod : interfaceMethods) {
            if ((interfaceMethod.getModifiers() & Modifier.ABSTRACT) == Modifier.ABSTRACT) {
                if (foundInterfaceMethod == null) {
                    foundInterfaceMethod = interfaceMethod;
                } else {
                    throw new RuntimeException("Too many interface methods to implement");
                }
            }
        }
        return Objects.requireNonNull(foundInterfaceMethod);
    }

    public static void main(String[] args) throws NoSuchMethodException {
        IntConsumer c = createLambda(IntConsumer.class, System.out, "println", int.class);
        c.accept(42);
    }
}