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Sometimes null is returned intentionally to account for zero available instancesSome APIs intentionally return a null reference to indicate that instances are unavailable. This practice can lead to denial-of-service vulnerabilities when the client code does not fails to explicitly handle the null return value case. A null value is an example of an in-band error indicator, which is discouraged by ERR52-J. Avoid in-band error indicators. For methods that return a set of values using an array or collection, returning an empty array or collection is an excellent alternative to returning a null value, as most callers are better equipped to handle and empty set than a null value.

Noncompliant Code Example

This noncompliant code example returns a null ArrayList when the size of the ArrayList is zero0. The class Inventory contains a getStock() method that constructs a list of items that have zero 0 inventory and returns the list of items to the caller. When the size of this list is zero, a null is returned with the assumption that the client will install the necessary checks. Here, the client omits the check, causing a NullPointerException at runtime. 

class Inventory {
  private final Hashtable<String, Integer> items;
  public Inventory() {
    items = new Hashtable<String, Integer>();	
  }

  public List<String> getStock() {
    List<String> stock = new ArrayList<String>();
    Enumeration itemKeys = items.keys();
    while (itemKeys.hasMoreElements()) {
      Object value = itemKeys.nextElement();
      if((items.get(value)) == 0) {  		
        stock.add((String)value);	 
      }
    }
    
    if(items.size() == 0) {	
      return null;
    } else {
      return stock;
    }	
  }
}

public class Client {
  public static void main(String[] args) {
    Inventory inv = new Inventory();  
    List<String> items = inv.getStock();
    System.out.println(items.size()); // Throws an NPE
  }
}

When the size of this list is 0, a null value is returned with the assumption that the client will install the necessary checks. In this code example, the client lacks any null value check, causing a NullPointerException at runtime.

Compliant Solution

This Instead of returning a null value, this compliant solution eliminates the null return and simply returns the List, even if when it is zero-length. The client can effectively handle this situation without being interrupted by runtime exceptions. When arrays are returned instead of collections, care must be taken to ensure that the client does not access individual elements of a zero-length array. This prevents an ArrayOutOfBoundsException.empty. 

class Inventory {
  private final Hashtable<String, Integer> items;
  public Inventory() {
    items = new Hashtable<String, Integer>();	
  }

  public List<String> getStock() {
    List<String> stock = new ArrayList<String>();
    Integer noOfItems; // Number of items left in the inventory
    Enumeration itemKeys = items.keys();
    while (itemkeysitemKeys.hasMoreElements()) {
      Object value = itemKeys.nextElement();
		
      if((noOfItems = items.get(value)) == 0) {  		
        stock.add((String)value);	 
      }
    }	
    return stock; // Return list (possibly zero-length)
  }
}

public class Client {
  public static void main(String[] args) {
    Inventory inv = new Inventory();  
    List<String> items = inv.getStock();
    System.out.println(items.size()); // Does not throw an NPE
  }
}

The client can handle this situation effectively without being interrupted by runtime exceptions. When returning arrays rather than collections, ensure that the client avoids attempts to access individual elements of a zero-length array. This prevents an ArrayOutOfBoundsException from being thrown.

Compliant Solution

This compliant solution returns an explicit empty list, which is an equivalent, permissible technique.

public List<String> getStock() {
  List<String> stock = new ArrayList<String>();
  Integer noOfItems; // Number of items left in the inventory
  Enumeration itemkeysitemKeys = items.keys();
  while (itemkeysitemKeys.hasMoreElements()) {
    Object value = itemKeys.nextElement();
		
    if((noOfItems = items.get(value)) == 0) {  		
      stock.add((String)value);	 
    }
  }	
  
  if(l.isEmpty()) {
    return Collections.EMPTY_LIST; // Always zero-length
  } else {
    return stock; // Return list 
  }
}

// Class Client ...

Applicability

Returning a null value rather than a zero-length array or collection may lead to denial-of-service vulnerabilities when the client code does not fails to handle null return values properly.

Automatic detection is straightforward but ; fixing the problem will, most probably, require human intervention.typically requires programmer intervention.

Automated Detection

Bibliography

 : Return empty arrays or collections, not nulls

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