Builders work with object classes, presenting users with properties they can modify and events on one object that they can tie to methods on another object. Ideally, a builder should be able to import new classes, glean all of the property, event and method information and then allow them to be manipulated in the same way as built-in classes. It turns out that much of the information needed by the builder is available in the class file. It's a set method. It's a get method.
Developers can identify set and get methods by inspection of the class file.
But one thing they can't always do is match up set methods with their corresponding get methods. For example, in JDK 1.0 an AWT component implements a get method called location which returns the component's position as a Point object. But the corresponding set method is called move. JavaBeans contains just such a method naming convention as part of its specification. A builder would be able to learn all this not by reading the class file, but instead by using the Beans API to ask the class for the information.