== Why is this an issue?

Not all classes in the standard Java library were written to be thread-safe. Using them in a multi-threaded manner is highly likely to cause data problems or exceptions at runtime.


This rule raises an issue when an instance of ``++Calendar++``, ``++DateFormat++``, ``++javax.xml.xpath.XPath++``, or ``++javax.xml.validation.SchemaFactory++`` is marked ``++static++``.


=== Noncompliant code example

[source,java]
----
public class MyClass {
  private static SimpleDateFormat format = new SimpleDateFormat("HH-mm-ss");  // Noncompliant
  private static Calendar calendar = Calendar.getInstance();  // Noncompliant
----


=== Compliant solution

[source,java]
----
public class MyClass {
  private SimpleDateFormat format = new SimpleDateFormat("HH-mm-ss");
  private Calendar calendar = Calendar.getInstance();
----

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Make "XXX" an instance variable.


endif::env-github,rspecator-view[]