Smalltalk and Object Orientation: an Introduction - Free

13. Streams and Files
13.1 Introduction
This chapter discusses the second most used class hierarchy in Smalltalk; the Stream classes. The
Stream classes are used (amongst other things) for accessing files . The remainder of the chapter is
structured in the following manner: Section two describes the Streams classes. Section three describes
how the stream classes can be used for file access. Section four describes an example of using files and
streams.
13.2 Streams
Figure 13.1: The structure of the Stream class hierarchy
Streams are the second most used set of classes. Streams are objects which serve as sources or sinks of
data. At first this concept can seem a bit strange. The easiest way to think of streams are as streams of
data (as opposed to water) either flowing from a pool of data or into a pool of data.
There are a number of different types of stream in the Smalltalk system, and each can have a
surprisingly wide range of uses. Figure 13.1 illustrates the structure of the stream class hierarchy. The
Stream class is the root class of this hierarchy. Below this are stream classes for reading, writing, for
accessing external files etc.
A stream may be input only (ReadStream ), output only (WriteStream ) or input and output
(ReadWriteStream). They can be internal streams (i.e. that act as a source or sink for data internal to the
image) or external streams (i.e. the source or sink for the data is external to the current image. A file is a
typical example of an external source or sink). Peekable streams also have an internal record of their
current position and can look ahead in the stream for information.
Streams have various access methods defined which allow the next item to be obtained or provided.
Some of the more useful methods include:
• next Answer the next object in the receiver stream and advance the position indicator by one.
• peek Answer the next object in the receiver stream, but do not advance the position indicator.
• nextPut: anObject Write anObject at the current position.
• nextPutAll: aCollection Write all elements of the collection to the receiver stream.
• atEnd Answer true if the stream is at the end else answer false.
Others include next: anInteger, peekFor: anObject and skipTo: anObject. What
operations can be performed on Streams depends on the class of stream being used. Figure 13.1
illustrates the structure of the stream hierarchy.
Typically streams are connected to files (using one of the ExternalStream’s subclasses) or to a
collection of data (using one of the InternalStream’s subclasses). If a stream is connected to an
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