(a)
lock( ... ) 
   if( Z==null )
      Z = new Foo();
 ... = Z;

(b)
// Z must be declared volatile
if( Z==null ) 
   lock( ... ) 
      if( Z==null )
         Z = new Foo();
 ... = Z

Example 3: Double check is an optimization of lazy initialization that avoids acquiring a lock in the common case that Z is already initialized. (a) Lazy initialization; (b) double check.