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public interface CallBack {
  void callMethod();
}
  
class UserLookupCallBack implements CallBack {
  private int uid;
  private String name;
 
  public UserLookupCallBack(int uid) {
    this.uid = uid;
  }
 
  public String getName() {
    return name;
  }
 
  public void callMethod() {
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {
          try (InputStream fis = new FileInputStream("/etc/passwd")) {
            // Look up userid & assign to UserLookupCallBack.this.name
          } catch (IOException x) {
            UserLookupCallBack.this.name = null;
          }
          return null;
        }
      });
  }
}
 
class Client {
  CallBack callback;
 
  public void registerCallBack(CallBack callback) {
    this.callback = callback;
  }
  
  public void doSomething() {
    callback.callMethod();
  }
 
  public static void main(String[] args) {
    int uid = Integer.parseInt(args[0]);
 
    Client client = new Client();
    CallBack callBack = new UserLookupCallBack(uid);
 
    client.registerCallBack(callBack);
    // ...
    client.doSomething(); // Looks up user name
    System.out.println("User " + uid + " is named " + callBack.getName());
  }
}