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
public class MyClass {
private static SimpleDateFormat format = new SimpleDateFormat("HH-mm-ss"); // Noncompliant
private static Calendar calendar = Calendar.getInstance(); // Noncompliant
Compliant Solution
public class MyClass {
private SimpleDateFormat format = new SimpleDateFormat("HH-mm-ss");
private Calendar calendar = Calendar.getInstance();
