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class Modifiable {	
  private List&lt;Integer&gt;List<Integer> list = new ArrayList&lt;Integer&gt;ArrayList<Integer>();

  public void listIt() {
    addSomething(list);
    // ...
  }

  private void addSomething(Collection&lt;Integer&gt;Collection<Integer> collection) {
    collection.add(1); 
  }
}

// ...
public void listIt() {
  addSomething(Collections.unmodifiableCollection(list));
  // ...
}

private void addSomething(Collection&lt;Integer&gt;Collection<Integer> collection) {
  collection.add(1); // throws java.lang.UnsupportedOperationException
}