"Calendars" and "DateFormats" should not be static

级别：bug, multi-threading

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

Compliant Solution：

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