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
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public class MyClass {
private static SimpleDateFormat format = new SimpleDateFormat("HH-mm-ss"); // Noncompliant