== Why is this an issue?

Calling ``++toString()++`` or ``++clone()++`` on an object should always return a string or an object. Returning ``++null++`` instead contravenes the method's implicit contract.


=== Noncompliant code example

[source,java]
----
public String toString () {
  if (this.collection.isEmpty()) {
    return null; // Noncompliant
  } else {
    // ...
----

=== Compliant solution

[source,java]
----
public String toString () {
  if (this.collection.isEmpty()) {
    return "";
  } else {
    // ...
----

== Resources

* CWE - https://cwe.mitre.org/data/definitions/476[CWE-476 - NULL Pointer Dereference]
* https://wiki.sei.cmu.edu/confluence/x/aDdGBQ[CERT, EXP01-J.] - Do not use a null in a case where an object is required

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