Question

Suppose I have the following interface deceleration :

public interface classListener<classEvent> {

    void classMethod1(classEvent x);
    void classMethod2(classEvent y);
    void classMethod3(classEvent z);
}

and suppose I want to add this interface to whatever.something package.

What should i do ?

Answer 1

Put a package statement at the top of the file:

package your.package.name;

public interface classListener<classEvent> {

    void classMethod1(classEvent x);
    void classMethod2(classEvent y);
    void classMethod3(classEvent z);
}

suppose I want to add this interface to java.something package

You cannot add to the java. packages. That’s just for the JDK (currently controlled by Oracle since they acquired Sun). It’ll compile, but when you try to use it you’ll get a SecurityException:

Exception in thread "main" java.lang.SecurityException: Prohibited package name: java.util
    at java.lang.ClassLoader.preDefineClass(ClassLoader.java:479)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:625)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Could not find the main class: UsePackage.  Program will exit.

My code that gave me that error:

In ~/temp/java/util/AddPackage.java:

package java.util;

public interface AddPackage {
    void doSomething();
}

In ~/temp/UsePackage.java:

import java.util.AddPackage;

class UsePackage implements AddPackage {
    public static final void main(String[] args) {
        new UsePackage().doSomething();
    }

    public void doSomething() {
        System.out.println("Hi");
    }
}

Commands:

~/temp$ javac java/util/AddPackage.java
~/temp$ javac UsePackage.java
~/temp$ java UsePackage

…whereupon the error occured. Using Oracle’s (Sun’s) JVM.