MET03-J. For methods that return an array or collection prefer returning an empty array or collection over a null value

Sometimes null is returned intentionally to account for zero available instances. This practice can lead to vulnerabilities when the client code does not handle the null return case.

Noncompliant Code Example

The erroneous behavior is caused due to the server returning null while the client forgets to add in a check for such a value. This noncompliant example shows how the check item != null condition is missing from the if condition in class Client.

import java.util.Arrays;

class Inventory {
  private static int[] item;
    public Inventory() {
    item = new int[20];
  }

  public static int[] getStock() {
    if(item.length == 0)
      return null;
    else
      return item;
  }
}

  public class Client {
    public static void main(String[] args) {
      Inventory iv = new Inventory();
        int[] item = Inventory.getStock();
	  if (Arrays.asList(item[1]).contains(1)) {
	    System.out.println("Almost out of stock!" + item);
	  }
    }
}

Compliant Solution

This compliant solution eliminates the null return and simply returns the item array as is even if it is zero-length. The client can effectively handle this situation without exhibiting erroneous behavior. Be careful that the client does not try to access individual elements of a zero-length array such as item[1] while following this recommendation.

import java.util.Arrays;

class Inventory {
  private static int[] item;
    public Inventory() {
    item = new int[20];
    item[2] = 1;  //quantity of item 2 remaining is 1, almost out!
  }

  public static int[] getStock() {
    return item;
  }
}

public class Client {
  public static void main(String[] args) {
    Inventory iv = new Inventory();
    int[] item = Inventory.getStock();
    if (Arrays.asList(item[1]).contains(1)) {
      System.out.println("Almost out of stock!" + item);
    }
  }
}

Risk Assessment

Returning null rather than a zero-length array may lead to vulnerabilities when the client code does not handle null properly.

Automated Detection

TODO

Other Languages

This guideline appears in the C Secure Coding Standard as MSC19-C. For functions that return an array, prefer returning an empty array over a null value.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Bloch 08]] Item 43: return empty arrays or collections, not nulls

MET02-J. Avoid ambiguous uses of overloading      09. Methods (MET)      MET30-J. Follow the general contract while overriding the equals method