Defensive Copies

The simplest way to avoid problems with concurrency is to share only immutable data between threads. Immutable data is data which cannot changed

  • An immutable class may have some mutable data which is uses to manage its state but from the outside this class nor any attribute of this class can get changed.
  • For all mutable fields, e.g. Arrays, that are passed from the outside to the class during the construction phase, the class needs to make a defensive-copy of the elements to make sure that no other object from the outside still can change the data.
  • You must protected your classes from calling code. Assume that calling code will do its best to change your data in a way you didn’t expect it. While this is especially true in case of immutable data it is also true for non-immutable data which you still not expect that this data is changed outside your class.
  • To protect your class against that you should copy data you receive and only return copies of data to calling code.-

The following example creates a copy of a list (ArrayList) and returns only the copy of the list. This way the client of this class cannot remove elements from the list.

public class MyDataStructure {
  List<String> list = new ArrayList<String>();
  public void add(String s) {
   * Makes a defensive copy of the List and return it
   * This way cannot modify the list itself
   * @return List<String>
  public List<String> getList() {
    return Collections.unmodifiableList(list)  }

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