如何访问私有包类的公共成员？

Question

I am pretty novice in Java and I'am reading "Thinking in Java" at that moment. So while I was trying to understand "Access Control" chapter I've read that there is a possibility to acces public member of package private class and was a bit confused about it. So I've decided to check this statement, but when I tried to compile such code I got an error. Here's a simple example:

PrivatePackageClass.java:

package simple.PrivatePackage;

class PrivatePackageClass {
    public static void print(Object obj) {
        System.out.println(obj);
    }
}

PublicAccessClass.java

package simple;

import simple.PrivatePackage.*;
import java.util.*;

public class PublicAccessClass {
    public static void main(String[] args) {
        PrivatePackageClass.print(Arrays.toString(args));
    }
}

That is the description of error:

PublicAccessClass.java:8: error :
PrivatePackageClass is not public in simple.PrivatePackage; cannot be accessed from outside package
PrivatePackageClass.print(Arrays.toString(args));

So am I right that there's no simple possibility to access public members of package-private classes?

UPD! Thank you all, I know that if both pieces of code were in the same package it will be Ok. But I am still confusing about statement in the book. Here it is:

As previously mentioned, if you don't put an access specifier for class access, it defaults to package access. This means that an object of that class can be created by any other class in the package, but not outside the package. However, if a static member of that class is public , the client programmer can still access that static member even though they cannot create an object of that class.

I would be grateful if somebody could explain me how could it be.

Answer 1

PublicAccessClass has package private access, so only classes within its package can access it.

simple.PublicAccessClass is not in the same package as simple.PrivatePackage.PublicAccessClass , and therefore PublicAccessClass can't access it. If they were in the same package, your code would work.

Answer 2

this will help you in understanding the concept behind accessing of members in java

take a moment to read it out

http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

In your case you cant access method as it is not declared public in its package

and default members can only be accessed within same package.

Answer 3

No, you can't. Seems to me that you're now confusing the member of a package, which is a class, to the method of a class.

Answer 4

Your class PrivatePackageClass must be preceded by the access modifier public for it to be accessible outside the current file

To answer your second query static member/methods don't need to create an object of the class to access. Static members belong to a class and not specific to an object

Answer 5

Public members in package private class

package private class can have members with public or protected access, however, such a member can not be accessed outside the package.

"As previously mentioned, if you don't put an access specifier for class access, it defaults to package access. This means that an object of that class can be created by any other class in the package, but not outside the package. (Remember, all the files within the same directory that don't have explicit package declarations are implicitly part of the default package for that directory.) However, if a static member of that class is public, the client programmer can still access that static member even though they cannot create an object of that class.package."

I think the last sentence in the paragraph means singleton.

Answer 6

I have read this book Think in Java (Fourth Edition). I am also confusing about this statement.

There are some cases, which may be not exactly match the statement, but may be as a supplementary explanation.

I am not sure those cases is applicable or not, thanks for reading and comments welcomed.

Case 1 : The public static main() method of a package access class can be invoked using "java" command on the command line, like the statement on page 169 :

Even if you have a lot of classes in a program, only the main( ) for the class invoked on the command line will be called. So in this case, when you say java Detergent, Detergent.main( ) will be called. But you can also say java Cleanser to invoke Cleanser.main( ), even though Cleanser is not a public class . Even if a class has package access, a public main() is accessible .

eg There are some files(.java" and ".class") in this directory "testThinkingInJava\\src\\main\\java\\":

testMiscellaneous\\Miscellaneous.java:

package testMiscellaneous;

class Miscellaneous {
    public static void main(String[] args) {
        System.out.println("public static void main(String[] args) called");
    }
}

testMiscellaneous/Miscellaneous.class (generated using "javac" command)

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package testMiscellaneous;

class Miscellaneous {
    Miscellaneous() {
    }

    public static void main(String[] var0) {
        System.out.println("public static void main(String[] args) called");
    }
}

When run this command:

java -classpath C:\projects\testThinkingInJava\src\main\java testMiscellaneous.Miscellaneous

you can get the output like this:

PS C:\Users\pie> java -classpath C:\projects\testThinkingInJava\src\main\java testMiscellaneous.Miscellaneous
public static void main(String[] args) called
PS C:\Users\pie>

As you can see that the visibility of Miscellaneous class is package access (both in .java and .class file), and the main method was invoked.

Case 2 : Invoke the method via a public derived class .

eg C:\\projects\\testThinkingInJava\\src\\main\\java\\testMiscellaneous\\Miscellaneous.java

package testMiscellaneous;

class Miscellaneous {
    public static String staticMember = "staticMember";

    public static String staticMethod() {
        String s = "staticMethod";

        System.out.println("public static String staticMethod() called");

        return s;
    }

    public static void main(String[] args) {
        System.out.println("public static void main(String[] args) called");
    }
}

C:\\projects\\testThinkingInJava\\src\\main\\java\\testMiscellaneous\\Miscellaneous2.java

package testMiscellaneous;

public class Miscellaneous2 extends Miscellaneous {
}

C:\\projects\\testThinkingInJava\\src\\main\\java\\testThinkingInJava\\Application.java

package testThinkingInJava;

import testMiscellaneous.Miscellaneous2;

public class Application {
    String s = Miscellaneous2.staticMember;

    public static void main(String[] args) {
        Miscellaneous2 miscellaneous2 = new Miscellaneous2();

        String s2 = Miscellaneous2.staticMethod();

        System.out.println(s2);

        System.out.println(Miscellaneous2.staticMember);
    }
}

When run the application, you can get the output:

public static String staticMethod() called
staticMethod
staticMember

As you can see:

1 class Miscellaneous is package access;

2 class Miscellaneous2 simply derived from Miscellaneous, but given public access;

3 class Application resides in another package, and can access method of Miscellaneous, via Miscellaneous2.