为什么java.lang.reflect.Modifier具有公共构造函数？

Question

I'm learning java reflection and I'm curious to know what use java.lang.reflect.Modifier has for a public constructor. The java documentation doesn't give any description or use for it and the class has only static methods and constants.

EDIT: Added some test code.

import java.lang.reflect.*;

  public class TestModifier {
        Modifier modifier = new Modifier(); // Compiles fine.
       // Math math = new Math(); // Won't compile.  Math() has private access.
}

Answer 1

The constructor exists and it's present in the javadoc although no info is present. I found some information about it in the a source code comment in the GNU classpath , a project that is and I quote:

(GNU Classpath, Essential Libraries for Java,) is a GNU project to create free core class libraries for use with virtual machines and compilers for the java programming language.

In their source code for the Modifier class you can find the following non-javadoc comment for the constructor:

This constructor really shouldn't be here ... there are no instance methods or variables of this class, so instantiation is worthless. However, this function is in the 1.1 spec, so it is added for completeness.

I don't know if this is totally accurate since I could only access Java 1.3.1 spec and in the reflection model part of it what you can read is (bold's mine):

The Modifier class is an uninstantiable class that exports class methods to decode Java language modifiers for classes and members. The language modifiers are encoded in an integer, and use the encoding constants defined by The Java Virtual Machine Specification.

I do wanna believe in the comment from the source code, but I can't really point you at the specification where such thing is said.. Even stranger to me is that the previous quote even being from spec 1.3 already states that it's uninstantiable.

Answer 2

This is definitely not an error in javadoc. You can't find such a constructor means this is a default constructor (no arg constructor) which is by default public because the java.lang.reflect.Modifier is a public class.

Answer 3

如果与default options一起使用，则它可能是javadoc的属性，如果未为该类定义无参数构造函数，并且没有其他带参数的default constructor ，则它将在文档中提供default constructor函数。

Answer 4

Good find, this is an oversight of the API designer.

But, it also proves that it is not a big deal at all to instantiate a utility class; new Modifier() has never caused anybody any problem. Neither would new Math() .

Don't listen to chicken little.