Early classes of the Java API, such as Vector, Hashtable and StringBuffer, were synchronized to make them thread-safe. Unfortunately, synchronization has a big negative impact on performance, even when using these collections from a single thread.

It is better to use their new unsynchronized replacements:

• ArrayList or LinkedList instead of Vector
• HashMap instead of Hashtable
• StringBuilder instead of StringBuffer

The following code:

Vector cats = new Vector();       // Non-Compliant

should be refactored into:

ArrayList cats = new ArrayList(); // Compliant

Use of those synchronized classes is allowed in method signatures when overriding an existing method.

@Override
public Vector getCats() {...}   //Compliant