How to import a class from default package

问题

Possible Duplicate: How to access java-classes in the default-package?

I am using Eclipse 3.5 and I have created a project with some package structure along with the default package. I have one class in default package - Calculations.java and I want to make the use of that class in any of the package (for instance in com.company.calc). When I try to make the use of the class which is in the default package, it\'s giving me a compiler error. It\'s not able to recognise the class in default package. Where is the problem?

Calculations.java - source code

public class Calculations {
    native public int Calculate(int contextId);
    native public double GetProgress(int contextId);
    static  {
        System.loadLibrary(\"Calc\");
    }
}

I can\'t put my class in any other package. This class has some native methods which are implemented in Delphi. If I put that class in any of the folders, I will have to make changes to that DLL which I want to avoid (really - I can not). That\'s why I put my class in the default package.

回答1:

From the Java language spec:

It is a compile time error to import a type from the unnamed package.

You'll have to access the class via reflection or some other indirect method.

回答2:

Classes in the default package cannot be imported by classes in packages. This is why you should not use the default package.

回答3:

There is a workaround for your problem. You can use reflection to achieve it.

First, create an interface for your target class Calculatons :

package mypackage;

public interface CalculationsInterface {  
    int Calculate(int contextId);  
    double GetProgress(int contextId);  

}

Next, make your target class implements that interface:

public class Calculations implements mypackage.CalculationsInterface {
    @Override
    native public int Calculate(int contextId);
    @Override
    native public double GetProgress(int contextId);
    static  {
        System.loadLibrary("Calc");
    }
}

Finally, use reflection to create an instance of Calculations class and assign it to a variable of type CalculationsInterface :

Class<?> calcClass = Class.forName("Calculations");
CalculationsInterface api = (CalculationsInterface)calcClass.newInstance();
// Use it 
double res = api.GetProgress(10);

回答4:

I can give you this suggestion, As far as know from my C and C++ Programming experience, Once, when I had the same kinda problem, I solved it by changing the dll written structure in ".C" File by changing the name of the function which implemented the JNI native functionality. for example, If you would like to add your program in the package "com.mypackage", You change the prototype of the JNI implementing ".C" File's function/method to this one:

JNIEXPORT jint JNICALL
Java_com_mypackage_Calculations_Calculate(JNIEnv *env, jobject obj, jint contextId)
{
   //code goes here
}

JNIEXPORT jdouble JNICALL
Java_com_mypackage_Calculations_GetProgress(JNIEnv *env, jobject obj, jint contextId)
{
  //code goes here
}

Since I am new to delphi, I can not guarantee you but will say this finally, (I learned few things after googling about Delphi and JNI): Ask those people (If you are not the one) who provided the Delphi implementation of the native code to change the function names to something like this:

function Java_com_mypackage_Calculations_Calculate(PEnv: PJNIEnv; Obj: JObject; contextId: JInt):JInt; {$IFDEF WIN32} stdcall; {$ENDIF} {$IFDEF LINUX} cdecl; {$ENDIF}
var
//Any variables you might be interested in
begin
  //Some code goes here
end;



function Java_com_mypackage_Calculations_GetProgress(PEnv: PJNIEnv; Obj: JObject; contextId: JInt):JDouble; {$IFDEF WIN32} stdcall; {$ENDIF} {$IFDEF LINUX} cdecl; {$ENDIF}
var
//Any variables you might be interested in
begin
//Some code goes here
end;

But, A final advice: Although you (If you are the delphi programmer) or them will change the prototypes of these functions and recompile the dll file, once the dll file is compiled, you will not be able to change the package name of your "Java" file again & again. Because, this will again require you or them to change the prototypes of the functions in delphi with changed prefixes (e.g. JAVA_yourpackage_with_underscores_for_inner_packages_JavaFileName_MethodName)

I think this solves the problem. Thanks and regards, Harshal Malshe

回答5:

From some where I found below :-

In fact, you can.

Using reflections API you can access any class so far. At least I was able to :)

Class fooClass = Class.forName("FooBar");
Method fooMethod =
    fooClass.getMethod("fooMethod", new Class[] { String.class });

String fooReturned =
    (String) fooMethod.invoke(fooClass.newInstance(), "I did it");

回答6:

Unfortunately, you can't import a class without it being in a package. This is one of the reasons it's highly discouraged. What I would try is a sort of proxy -- put your code into a package which anything can use, but if you really need something in the default package, make that a very simple class which forwards calls to the class with the real code. Or, even simpler, just have it extend.

To give an example:

import my.packaged.DefaultClass;

public class MyDefaultClass extends DefaultClass {}

package my.packaged.DefaultClass;

public class DefaultClass {

   // Code here

}

回答7:

Create a new package And then move the classes of default package in new package and use those classes

回答8:

Create a new package.
Move your files from the default package to the new one.

回答9:

Create "root" package (folder) in your project, for example.

package source; (.../path_to_project/source/)
Move YourClass.class into a source folder. (.../path_to_project/source/YourClass.class)
Import like this

import source.YourClass;

来源：https://stackoverflow.com/questions/2193226/how-to-import-a-class-from-default-package

标签

java

java-native-interface

native-methods

default-package