Java - Can't declare object in static class

Question

I'm trying to implement a singleton pattern for a System class. The examples I found do not compile (e.g. http://www.tutorialspoint.com/java/java_using_singleton.htm). There is a static method in a non-static class. So I made the class static and all was well until I tried to make a member variable for my Timer class.

Now I get the message "No enclosing instance of type scene_3d is accessible. Must qualify the allocation with an enclosing instance...

I have searched around but nobody's singleton patterns compile for me. By the way I am using Processing (a Java IDE/extension). Any ideas on how to fix this will be of great help. Thanks!

static public class DemoSystem {
  private static DemoSystem instance = null;  
  protected DemoSystem() {}

  public static DemoSystem Inst() {
    if( instance == null ) {
      instance = new DemoSystem();
    }
    return instance;
  }

  void init() {
    Timer timer = new Timer();  
  }

  int getTime() {
    return timer.elapsedTime;
  }
}

possible duplicate of [Implementing the singleton pattern in Java](http://stackoverflow.com/questions/2008912/implementing-the-singleton-pattern-in-java) — Brian Roach, Feb 13 '13 at 06:25

score 2 · Answer 1 · answered Feb 13 '13 at 06:26

The standard singleton pattern is to have a private constructor and a static instance variable, so:

public class DemoSystem {
  private static DemoSystem instance = null;
  private Timer timer;
  protected DemoSystem() {}

  public static DemoSystem Inst() {
    if( instance == null ) {
      instance = new DemoSystem();
    }
    return instance;
  }

  void init() {
    timer = new Timer();  
  }

  int getTime() {
    return timer.elapsedTime;
  }
}

Once you fix your imports, this should work fine.

score 1 · Answer 2 · answered Feb 13 '13 at 06:24

timer is declared inside init. It has to be up in the class so the get method can access it, e.g.:

static public class DemoSystem {
  private static DemoSystem instance = null;
  private Timer timer;
  protected DemoSystem() {}

  public static DemoSystem Inst() {
    if( instance == null ) {
      instance = new DemoSystem();
    }
    return instance;
  }

  public void init() {
    timer = new Timer();  
  }

  public int getTime() {
    return timer.elapsedTime;
  }
}

You were also missing the public qualifiers on the two methods.

score 1 · Answer 3 · answered Feb 13 '13 at 06:26

I don't think you can declare your entire class as static:

static public class DemoSystem {

should be:

public class DemoSystem {

also, and no 100%, may have a syntax error (you are missing a semicolon...):

   protected DemoSystem() {}

should be

   protected DemoSystem() {};

score 1 · Answer 4 · answered Feb 13 '13 at 11:45

There are some points related to the origin code snippet posted.

First of all the top level class can not be declared as static.
The public method you have created ( public static DemoSystem Inst() ) won't take guarantee that it will create singleton object only in case of concurrency. For that initialize the object at the time of declaration only like : private static DemoSystem instance = new DemoSystem();

or synchronize the method properly.

Make Timer timer , a class level variable .
What is timer.elapsedTime . I don't think there is any such property in Timer class. If you are using different api , make necessary imports.

So the final code will look like this :

import java.util.Timer;

public class DemoSystem {
    // Declared the variable at class level.
    Timer timer = null;

    // Initializing the object here only.
    private static DemoSystem instance = new DemoSystem();

    // Made the constructer private.
    private DemoSystem() {
    }

    // Static method to get singleton instance
    public static DemoSystem Inst() {
        return instance;
    }

    void init() {
        timer = new Timer();
    }

    int getTime() {
        //commented the statement so to compile the class  
        //return timer.elapsedTime;
        return 1;
    }
}

This fails to compile. I get the error "The field instance cannot be declared static; static fields can only be declared in static or top level types." Do you think this is a limitation of Processing? I'm using my own Timer class by the way. — ballaw, Feb 14 '13 at 01:55
Hi , I think this might be a limitation with Processing because i have successfully compiled the same code in eclipse juno. — Java World, Feb 14 '13 at 12:42
Well are u able to compile and run your code now. If not pls tell which IDE and Processing plugin are u using for the development. — Java World, Feb 14 '13 at 12:42

Java - Can't declare object in static class

4 Answers4