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Week 92 — What is a "defensive copy" and what is its purpose?

Question of the Week #92

What is a "defensive copy" and what is its purpose?

1 Reply

When storing an object, change from external sources is often unwanted and cause issues. For example, take the following code:

public class SomeClass {
  private List<String> someList;
  public SomeClass(List<String> someList){
    this.someList = someList;
  }
  public String takeFirstElement() {
    if(!someList.isEmpty()) {
      return someList.removeFirst();
    }
    return null;
  }
}

public class SomeClass {
  private List<String> someList;
  public SomeClass(List<String> someList){
    this.someList = someList;
  }
  public String takeFirstElement() {
    if(!someList.isEmpty()) {
      return someList.removeFirst();
    }
    return null;
  }
}

If some other code modifies the List passed to the constructor of SomeClass between the isEmpty and removeFirst method invocations, it would be possible that removeFirst throws a NoSuchElementException even though isEmpty returned false before. In order to prevent issues like that, it is possible to create a copy of the List in the constructor and use that. The idea of making a copy of an object in order to protect that object from unwanted modification is called defensive copying.

public class SomeClass {
  private List<String> someList;
  public SomeClass(List<String> someList){
    this.someList = new ArrayList<>(someList);//make a defensive copy of someList
  }
  public String takeFirstElement() {
    if(!someList.isEmpty()) {
      return someList.removeFirst();
    }
    return null;
  }
}

public class SomeClass {
  private List<String> someList;
  public SomeClass(List<String> someList){
    this.someList = new ArrayList<>(someList);//make a defensive copy of someList
  }
  public String takeFirstElement() {
    if(!someList.isEmpty()) {
      return someList.removeFirst();
    }
    return null;
  }
}

Because external code doesn't have access to the internal List used by SomeClass, that code cannot accidentially cause issues in these methods.

📖 Sample answer from dan1st

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Week 92 — What is a "defensive copy" and what is its purpose?