Be Careful When Using Container Classes.

• Vector and Hashtable claim to be threadsafe because they synchronize every method.
• But, this does not mean that they are really safe.
• Is the following thread safe?

import java.util.*;
public synchronized void insertIfAbsent(Vector vector, Object object){
  if (vector.contains(object)) {
    return;
  } 
  vector.add(object);
}

• No. It assumes there is only one instance of the class.
• The right way is to do:

import java.util.*;

public void insertIfAbsent(Vector vector, Object object){
  synchronized (vector){
    if (vector.contains(object)) {
      return;
    } 
    vector.add(object);
  }
}