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
• Deque instead of Stack
• HashMap instead of Hashtable
• StringBuilder instead of StringBuffer

Noncompliant Code Example

Vector cats = new Vector(); 

Compliant Solution

ArrayList cats = new ArrayList();

Exceptions

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

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