Can a class have more than one superclass so that it inherits additional methods and behavior?

It is possible with some object-oriented coding languages, but not Java. One of the goals when Java was developed was to provide a simpler language than an object-oriented language such as C++, and limiting inheritance to a single superclass was one way to achieve this. You can use a special type of class called an interface to inherit behavior that isn't received from superclasses.

Most Java programs created up to this point have not used extends to inherit from a superclass. Does this mean they exist outside of the class hierarchy?

All classes you create in Java are part of the hierarchy because the default superclass for the programs you write is Object when the extends keyword is not used. The equals() and toString() methods of all classes are part of the behavior that automatically is inherited from Object.

When is the full name of a class, such as java.applet.Applet, needed in an extends clause instead of a shorter name such as Applet?

You must use the full name whenever you don't use an import javax.swing.JApplet; or import.javax.swing.*; statement at the beginning of your program. The import statement is used solely to make it easier to refer to class names in programs. Each class of objects in Java has a full name that identifies the group of classes to which it belongs. For instance, the Math class is part of the java.lang group of classes. A group of classes is also called a package.

Is there any difference between sea salt and table salt?

Sea salt, table salt, and a third kind—kosher salt—are chemically identical but produced by different means. Sea salt is reclaimed from large quantities of salt water. Table salt and kosher salt are collected from salt deposits, and the main difference between the two is that kosher salt is raked continuously during the evaporation process. The choice of salt is largely a personal preference, because the three different production methods result in largely the same product. Some health advocates believe that sea salt contains more trace minerals than the other kinds.