Effective Java 39 Make defensive copies when needed

Principle

1. It is essential to make a defensive copy of each mutable parameter to the constructor.
2. Defensive copies are made before checking the validity of the parameters (Item 38), and the validity check is performed on the copies rather than on the originals.  

   // Repaired constructor - makes defensive copies of parameters

   public Period(Date start, Date end) {

   this.start = new Date(start.getTime());

   this.end = new Date(end.getTime());

   //Make the defensive copies of the parameters before using them.

   if (this.start.compareTo(this.end) > 0)

   throw new IllegalArgumentException(start +" after "+ end);

   }

   TOCTOU = time of check/ time of use.

3. Do not use the clone method to make a defensive copy of a parameter whose type is subclassable by untrusted parties.
4. Return defensive copies of mutable internal fields.  

   // Repaired accessors - make defensive copies of internal fields

   public Date start() {

   return new Date(start.getTime());

   }

   public Date end() {

   return new Date(end.getTime());

   }

Summary

If a class has mutable components that it gets from or returns to its clients, the class must defensively copy these components. If the cost of the copy would be prohibitive and the class trusts its clients not to modify the components inappropriately, then the defensive copy may be replaced by documentation outlining the client's responsibility not to modify the affected components.