This really is a topic for a whole book, but techniques to make a class thread safe include using synchronized methods, synchronized blocks and volatile fields.
Java 5 also added a lot of additional classes that help with this topic.
If you want to know how it's done in the Vector class,
you should probably start by looking at its source code, which comes with the JDK.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus