[1] The reason parameters are vals is that vals are easier to reason about. You needn't look further to determine if a val is reassigned, as you must do with a var.
[2] We used a cache here to show a singleton object with a field. A cache such as this is a performance optimization that trades off memory for computation time. In general, you would likely use such acache only if you encountered a performance problem that the cache solves, and might use a weak map, such as WeakHashMap in scala.collection.jcl, so that entries in the cache could be garbage collected if memory becomes scarce.
[3] Because the keyword new is only used to instantiate classes, the new object created here is an instance of the ChecksumAccumulator class, not the singleton object of the same name.
[4] The name of the synthetic class is the object name plus a dollar sign. Thus the synthetic class for the singleton object named ChecksumAccumulator is ChecksumAccumulator$.
[5] If you're a Java programmer, you can think of this import as similar to the static import feature introduced in Java 5. One difference in Scala, however, is that you can import members from any object, not just singleton objects.
[6] The actual mechanism that the scala program uses to "interpret" a Scala source file is that it compiles the Scala source code to Java bytecodes, loads them immediately via a class loader, and executes them.