An example of how to cache a value in a static field.

StaticFieldCacheExample.cls

public class StaticFieldCacheExample
{
    /* Define a static field to hold the value. */
    private static String cachedValue = null;

    public static String getCachedValue()
    {
        /* If the value has been previously calculated, return it immediately
         * instead of performing an expensive calculation. */
        if (StaticFieldCacheExample.cachedValue != null)
            return StaticFieldCacheExample.cachedValue;

        /* Perform the requisite steps to calculate the value to be cached. */
        String calculatedValue = 'whatever';

        /* Before returning the newly-calculated value, update the cache. Now
         * the cache value is non-null, we'll bypass the expensive calculations
         * next time this method is called. */
        StaticFieldCacheExample.cachedValue = calculatedValue;

        return calculatedValue;
    }
}