Learning Java

Inner Classes

Java 1.1 added to the language a large heap of syntactic sugar called inner classes. Simply put, classes in Java can be declared at any level of scope. That is, you can declare a class within any set of curly braces (i.e., almost anywhere that you could put any other Java statement), and its visibility is limited to that scope in the same way that the name of a variable or method would be. Inner classes are a powerful and aesthetically pleasing facility for structuring code. Their even sweeter cousins, anonymous inner classes , are another powerful shorthand that make it seem as if you can create classes dynamically within Java’s statically typed environment.

However, if you delve into the inner workings of Java, inner classes are not quite as aesthetically pleasing or dynamic. We said that they are syntactic sugar; this means that they let you leverage the compiler by writing a few lines of code that trigger a lot of behind-the-scenes work somewhere between the compiler’s front end and the byte-code. Inner classes rely on code generation; they are a feature of the Java language, but not of the Java virtual machine. As a programmer you may never need be aware of this; you can simply rely on inner classes like any other language construct. However, you should know a little about how inner classes work, to better understand the results and a few potential side effects.

To this point, all of our classes have been top-level classes. We have declared them, freestanding, ...