错误：不能对构造函数使用带有显式类型参数的原始构造函数引用

Question

I have the following pieces of code:

Main.java

public class Main {
  public static void main(String[] args) {
     TestProducer c = Test::<String>new;
     System.out.println(c.produce("Point"));
  }
}

Test.java

public class Test<N extends CharSequence> {
  private CharSequence name;

  public Test(N k) {
    name = k;
  }

  public String toString() {
    return name.toString();
  }
}

TestProducer.java

@FunctionalInterface
public interface TestProducer {
  public <N extends CharSequence> Test<N> produce(N str);
}

When I compile the code, it produces the following error:

OpenJDK Runtime Environment (build 10.0.2+13-Ubuntu-1ubuntu0.18.04.4)
> javac -classpath .:/run_dir/junit-4.12.jar -d . Main.java Test.java TestProducer.java
Main.java:3: error: invalid constructor reference
     TestProducer c = Test::<String>new;
                      ^
  cannot use raw constructor reference with explicit type parameters for constructor
1 error
compiler exit status 1

I have the following questions:

1. What is the "raw constructor reference" here ("explicit type parameters" here is <String> )?
2. Why can't you "use raw constructor reference with explicit type parameters for constructor" (reasoning/why it's not a good idea)?
3. How can I fix this error (create a TestProducer that only accepts String s)?
   

EDIT:

With regards to avoiding the error, here's what I found:
I can modify TestProducer to the following:

@FunctionalInterface
public interface TestProducer<N extends CharSequence>
{
  public Test<N> produce(N str);
}

and then change Main.java like so:

public class Main {
  public static void main(String[] args) {
     TestProducer<String> c = Test::new;
     System.out.println(c.produce("Point"));
  }
}

By doing this, c::produce will only accept String s, which fits my intent for question 3. Questions 1 and 2 are still unclear.

Answer 1

You define your TestProducer interface not to be generic itself but to have a generic method:

Test<N> produce(N str);

Then you try to bind a specific type in the assignment:

TestProducer c = Test::<String>new;

But it's completely valid for me now to say this:

c.produce(new StringBuilder())

which must return a Test<StringBuilder> , not a Test<String> .

You should probably add N as a type parameter on your interface.

Answer 2

1. Remove the type witnesses from around your constructor. That won't work even in Java 8 .

    TestProducer c = Test::new; System.out.println(c.produce("Point"));

2. You don't need to provide type witnesses anyway. TestProducer is providing a strongly-enforced contract based on the produce method definition, and any producer which doesn't satisfy that definition can't be used.

3. If you wanted a TestProducer which only accepted String s, then you'd need to change your bound to <N extends String> .