为什么我必须在定义的类中导入嵌套类？

Question

In the following example the import is necessary, otherwise Java's compiler will complain that Nested cannot be resolved to a type in Iterable<Nested> :

package test;

import test.Example.Nested;

public class Example implements Iterable<Nested> {

    public final static class Nested {}

}

(using Iterable<Example.Nested> instead of an import works as well)

This only happens when referencing a nested class in the definition of the outer class, eg when using it as a parametric type, but also when extending/implementing it (which will result in another error once the compiler can resolve the class), or when using it as or in an annotation.

My question is: Why can't the compiler find the nested class without an explicit declaration?

Answer 1

A declaration is visible in the scope it occurs in, and for members, that scope is the stuff between the enclosing curly brackets, or as the spec puts it:

The scope of a declaration of a member m declared in or inherited by a class type C (§8.1.6) is the entire body of C , including any nested type declarations.

Why it has been defined that way is something only the creators of Java can answer with any degree of certainty, but I suspect they wanted as simple a rule as possible, and saying that all members, variables and so on are visible within the surrounding curly brackets is a very simple rule.

And by the way: You are not required to import the type, you can use a qualified name instead. In your example, that would read:

public class Example implements Iterable<Example.Nested> {  }

Answer 2

Nested classes are referenced as <OuterClass>.<InnerClass> , so Iterable<Example.Nested> would be the typical way to do this in Java, though using import is also an option.

See Nested Classes in the Java Tutorial.

Answer 3

The following code is not yet "inside" the class body

public class Example implements Iterable<Nested>
{
     // class body starts here

Nested is refered to outside the class and so you must either import or use Example.Nested to have Nested in scope.

if you put similar code inside the class body it will work:

{
      // class body starts here

      public void foo(Iterable<Nested> x) ...
}

This works just fine because you are now within the class scope where nested is defined and available unqualified