use of static variable

[Paul Chamsay]

Hi everyone,

I would like to ask if it is a 'good' practice to use a static variable (currency) for something like this?

import java.text.*;

    class Test
   {
      static NumberFormat currency = NumberFormat.getCurrencyInstance();
   			
       public static void main(String[] args)
      {
         int money = 12000;
         Test obj = new Test();
         
         obj.money();
         obj.anotherMoney();

         System.out.println(currency.format(money));

       }

       void money()
       {
           int money = 5000;
           System.out.println(currency.format(money));
       }

       void anotherMoney()
      {
          int money = 23500;
          System.out.println(currency.format(money));
      }
   }

[Steve Luke]

In a single threaded environment, that would be safe, but if you plan on allowing your application to run with multiple threads it is no longer safe. From the NumberFormat API:

Synchronization

Number formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

[Rob Spoor]

Also, don't make it public. If you do, other code can modify your NumberFormat object without you knowing. The results may be devastating in some applications.

[Rob Spoor]

Steve Luke wrote:In a single threaded environment, that would be safe, but if you plan on allowing your application to run with multiple threads it is no longer safe. From the NumberFormat API:

Synchronization

Number formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

ThreadLocal can be quite useful in multi-threaded environments:

ThreadLocal<NumberFormat> formatGenerator = new ThreadLocal<NumberFormat>()
{
    protected NumberFormat initialValue()
    {
        return NumberFormat.getCurrencyInstance();
    }
};

[Campbell Ritchie]

That will only work with a static variable if you are sure every sum is to be expressed in the same currency.