Question

I’m implementing an interface in Java and was wondering why this code:

package threaddemo;

// Create a new thread...
class NewThread implements Runnable {
      Thread t;
      NewThread(){
        // Create a second, new thread...
        t = new Thread(this, "Demo Thread");
        System.out.println("Child thread: " + t);
        t.start();
      }

    // This is the entry point for the second thread...
    public void run(){
           try {
               for (int i=0; i<5; i++){
                   System.out.println("Child thread: " + i);
                    // Let the thread sleep for a while...
                    Thread.sleep(500);
               }
           } catch (InterruptedException e) {
                   System.out.println("Child interrupted...");
               }
                   System.out.println("Exiting child thread...");
           } }

public class ThreadDemo {

    public static void main(String[] args) {
            // Create a new thread...
            new NewThread(); 
            try {
                for (int i=0; i<5; i++){
                    System.out.println("Main thread: " + i);
                       Thread.sleep(1000);
                }
            } catch (InterruptedException e){
                System.out.println("Main thread itnerrupted...");
            }
            System.out.println("Main thread exiting...");
    } }

generates the following warning to the left of it:

Package Field

When you implement interfaces, do you gain access to the classes in the package that contain the interface? I have no other files in my package yet and I haven’t done any kind of import either, so I’m actually kind of confused as to why this is accessible to begin with…

Answer 1

Never seen that warning, so going out on a limb here… Do you have a package defined for the class? Otherwise it could mean that the Thread t-member has what most books call default visibility or package-private visibility (which means package- and class-level visibility), because there’s no visibility modifier for the field. Java has 4 different visibilities: public, default, protected, private. See here for more information: Controlling Access to Members of a Class