The UIDefaults table supports values that are created every time they are fetched. Such values are called active values. 
For values that are created only once, see e871 Setting a UI Default Value That Is Created When Fetched. 

This example declares an active value (a Date) that is created every time it is fetched. 

    // Create an active value
    Object activeValue = new UIDefaults.ActiveValue() {
        // This method is called every time the value is fetched.
        // If this method can be called no more than once, it must be synchronized.
        public Object createValue(UIDefaults table) {
            return new Date();
        }
    };
    
    // Add the active value to the UI defaults table
    UIManager.put("key", activeValue);
    
    // Fetch the value twice; this causes the value to be created twice
    Date d1 = (Date)UIManager.get("key");
    Date d2 = (Date)UIManager.get("key");
    boolean b = d1.equals(d2);   // false