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The read method of Reader Class in Java is used to read a single character from the stream. This method blocks the stream till: It has taken some input from the stream. Some IOException has occurred; It has reached the end of the stream while reading.

  • It is assumed that (apart from external dependencies) all the files that make up a package live under a common project root directory. This simple example has all the project files in one directory, but most packages will use one or more subdirectories.
  • The Java.io.PushbackReader class is a character-stream reader that allows characters to be pushed back into the stream. Class declaration. Following is the declaration for Java.io.PushbackReader class − public class PushbackReader extends FilterReader Field. Following are the fields for Java.io.PushbackReader class −.
  • Java.io.BufferedReader.skip Method - The java.io.BufferedReader.skip(long n) skips n numbers of characters.
  • Java FileReader Class for beginners and professionals with examples on Java IO or Input Output in Java with input stream, output stream, reader and writer class. The java.io package provides api to reading and writing data.

The JavaReader class, java.io.Reader, is the base class for all Reader subclasses in the Java IO API. A Java Reader is like a Java InputStream except that it is character based rather than byte based. In other words, a Java Reader is intended for reading text (characters), whereas an InputStream is intended for reading raw bytes.

Readers and Sources

A Reader is typically connected to some source of data like a file, char array, network socket etc. This is also explained in more detail in the Java IO Overview text.

Characters in Unicode

Today, many applications use Unicode (UTF-8 or UTF-16) to store text data. It may take one or more bytes to represent a single character in UTF-8. In UTF-16 each character takes 2 bytes to represent. Therefore, when reading text data, a single byte in the data may not correspond to one character in UTF. If you just read one byte at a time of UTF-8 data via an InputStream and try to convert each byte into a char, you may not end up with the text you expected.

To solve this problem we have the Reader class. The Reader class is capable of decoding bytes into characters. You need to tell the Reader what character set to decode. This is done when you instantiate the Reader (actually, when you instantiate one of its subclasses).

Java Reader Subclasses

You will normally use a Reader subclass rather than a Reader directly. Java IO contains a lot of Reader subclasses. Here is a list of the Java Reader subclasses:

Here is an example of creating a Java FileReader which is a subclass of Java Reader:

Read Characters From a Reader

The read() method of a Java Reader returns an int which contains the char value of the next character read. If the read() method returns -1, there is no more data to read in the Reader, and it can be closed. That is, -1 as int value, not -1 as byte or char value. There is a difference here!

Here is an example of reading all characters from a Java Reader:

Notice how the code example first reads a single character from the Java Reader and checks if the char numerical value is equal to -1. If not, it processes that char and continues reading until -1 is returned from the Readerread() method.

Read Array of Characters From Reader

The Java Reader class also has a read() method that takes a char array as parameter, as well as a start offset and length. The char array is where the read() method will read the characters into. The offset parameter is where in the char array the read() method should start reading into. The length parameter is how many characters the read() method should read into the char array from the offset and forward. Here is an example of reading an array of characters into a char array with a Java Reader:

The read(char[], offset, length) method returns the number of characters read into the char array, or -1 if there are no more characters to read in the Reader, for instance if the end of the file the Reader is connected to has been reached.

Read Performance

Reading an array of characters at a time is faster than reading a single character at a time from a Java Reader. The difference can easily be a factor 10 or more in performance increase, by reading an array of characters rather than reading a single character at a time.

The exact speedup gained depends on the size of the char array you read, and the OS, hardware etc. of the computer you are running the code on. You should study the hard disk buffer sizes etc. of the target system before deciding. However buffer sizes of 8KB and up will give a good speedup. However, once your char array exceeds the capacity of the underlying OS and hardware, you won't get a bigger speedup from a bigger char array.

You will probably have to experiment with different byte array size and measure read performance, to find the optimal char array size.

Transparent Buffering via BufferedReader

You can add transparent, automatic reading and buffering of an array of bytes from a Reader using a Java BufferedReader . The BufferedReader reads a chunk of chars into a char array from the underlying Reader. You can then read the bytes one by one from the BufferedReader and still get a lot of the speedup that comes from reading an array of chars rather than one character at a time. Here is an example of wrapping a Java Reader in a BufferedReader :

Notice, that a BufferedReader is a Reader subclass and can be used in any place where an Reader can be used.

Skip Characters

The Java Reader class has a method named skip() which can be used to skip over a number of characters in the input that you do not want to read. You pass the number of characters to skip as parameter to the skip() method. Here is an example of skipping characters from a Java Reader :

This example tells the Java Reader to skip over the next 24 characters in the Reader. The skip() method returns the actual number of characters skipped. In most cases that will be the same number as you requested skipped, but in case there are less characters left in the Reader than the number you request skipped, the returned number of skipped characters can be less than the number of characters you requested skipped.

Closing a Reader

When you are finished reading characters from a Reader you should remember to close it. Closing an Reader is done by calling its close() method. Here is how closing an Reader looks:

You can also use the Java try with resources construct introduced in Java 7. Here is how to use and close a InputStreamReader looks with the try-with-resources construct:

Notice how there is no longer any explicit close() method call. The try-with-resources construct takes care of that.

Tip

If this is your first time using cabal you should check out the Getting Started guide.

Let’s assume we have created a project directory and already have aHaskell module or two.

Every project needs a name, we’ll call this example “proglet”.

It is assumed that (apart from external dependencies) all the files thatmake up a package live under a common project root directory. Thissimple example has all the project files in one directory, but mostpackages will use one or more subdirectories.

To turn this into a Cabal package we need two extra files in theproject’s root directory:

  • proglet.cabal: containing package metadata and build information.

  • Setup.hs: usually containing a few standardized lines of code,but can be customized if necessary.

We can create both files manually or we can use cabalinit to createthem for us.

4.1.1. Using “cabal init”¶

The cabalinit--interactive command is interactive. If we answer“no” to using the “sensible defaults” it asks a number of questions.

One of the important questions is whether the package contains a libraryand/or an executable. Libraries are collections of Haskell modules thatcan be re-used by other Haskell libraries and programs, while executablesare standalone programs.

For the moment these are the only choices. For more complex packages(e.g. a library and multiple executables or test suites) the .cabalfile can be edited afterwards.

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After you make your selection (executable; library; or: libraryand executable) cabal asks us a number of questions starting withwhich version of the cabal specification to use, our package’s name(for example, “proglet”), and our package’s version.

It also asks questions about various other bits of package metadata. Fora package that you never intend to distribute to others, these fieldscan be left blank.

Finally, cabalinit--interactive creates the initial proglet.cabaland Setup.hs files, and depending on your choice of license, aLICENSE file as well.

At this stage the proglet.cabal is not quite complete and before youare able to build the package you will need to edit the file and addsome build information about the library or executable.

4.1.2. Editing the .cabal file¶

Load up the .cabal file in a text editor. The first part of the.cabal file has the package metadata and towards the end of the fileyou will find the executable or library section.

You will see that the fields that have yet to be filled in are commentedout. Cabal files use “--” Haskell-style comment syntax. (Note thatcomments are only allowed on lines on their own. Trailing comments onother lines are not allowed because they could be confused with programoptions.)

If you selected earlier to create a library package then your .cabalfile will have a section that looks like this:

Alternatively, if you selected an executable then there will be asection like:

The build information fields listed (but commented out) are just the fewmost important and common fields. There are many others that are coveredlater in this chapter.

Most of the build information fields are the same between libraries andexecutables. The difference is that libraries have a number of “exposed”modules that make up the public interface of the library, whileexecutables have a file containing a Main module.

The name of a library always matches the name of the package, so it isnot specified in the library section. Executables often follow the nameof the package too, but this is not required and the name is givenexplicitly.

4.1.3. Modules included in the package¶

For a library, cabalinit looks in the project directory for filesthat look like Haskell modules and adds all the modules to thelibrary:exposed-modules field. For modules that do not form partof your package’s public interface, you can move those modules to theother-modules field. Either way, all modules in the library needto be listed.

For an executable, cabalinit does not try to guess which filecontains your program’s Main module. You will need to fill in theexecutable:main-is field with the file name of your program’sMain module (including .hs or .lhs extension). Other modulesincluded in the executable should be listed in the other-modulesfield.

4.1.4. Modules imported from other packages¶

While your library or executable may include a number of modules, italmost certainly also imports a number of external modules from thestandard libraries or other pre-packaged libraries. (These otherlibraries are of course just Cabal packages that contain a library.)

You have to list all of the library packages that your library orexecutable imports modules from. Or to put it another way: you have tolist all the other packages that your package depends on.

For example, suppose the example Proglet module imports the moduleData.Map. The Data.Map module comes from the containerspackage, so we must list it:

In addition, almost every package also depends on the base librarypackage because it exports the standard Prelude module plus otherbasic modules like Data.List.

You will notice that we have listed base4.*. This gives aconstraint on the version of the base package that our package will workwith. The most common kinds of constraints are:

  • pkgname>=n

  • pkgname^>=n (since Cabal 2.0)

  • pkgname>=n&&<m

  • pkgnamen.* (since Cabal 1.6)

The last is just shorthand, for example base4.* means exactlythe same thing as base>=4&&<5. Please refer to the documentationon the build-depends field for more information.

Also, you can factor out shared build-depends (and other fields suchas ghc-options) into a common stanza which you can import inyour libraries and executable sections. For example:

Note that the importmust be the first thing in the stanza. For moreinformation see the Common stanzas section.

4.1.5. Building the package¶

For simple packages that’s it! We can now try configuring and buildingthe package:

Assuming those two steps worked then you can also install the package:

For libraries this makes them available for use in GHCi or to be used byother packages. For executables it installs the program so that you canrun it (though you may first need to adjust your system’s $PATH).

4.1.6. Next steps¶

What we have covered so far should be enough for very simple packagesthat you use on your own system.

The next few sections cover more details needed for more complexpackages and details needed for distributing packages to other people.

The previous chapter covers building and installing packages – your ownpackages or ones developed by other people.

Before diving into the details of writing packages it helps tounderstand a bit about packages in the Haskell world and the particularapproach that Cabal takes.

4.2.1. The point of packages¶

Packages are a mechanism for organising and distributing code. Packagesare particularly suited for “programming in the large”, that is buildingbig systems by using and re-using code written by different people atdifferent times.

People organise code into packages based on functionality anddependencies. Social factors are also important: most packages have asingle author, or a relatively small team of authors.

Packages are also used for distribution: the idea is that a package canbe created in one place and be moved to a different computer and beusable in that different environment. There are a surprising number ofdetails that have to be got right for this to work, and a good packagesystem helps to simplify this process and make it reliable.

Packages come in two main flavours: libraries of reusable code, andcomplete programs. Libraries present a code interface, an API, whileprograms can be run directly. In the Haskell world, library packagesexpose a set of Haskell modules as their public interface. Cabalpackages can contain a library or executables or both.

Some programming languages have packages as a builtin language concept.For example in Java, a package provides a local namespace for types andother definitions. In the Haskell world, packages are not a part of thelanguage itself. Haskell programs consist of a number of modules, andpackages just provide a way to partition the modules into sets ofrelated functionality. Thus the choice of module names in Haskell isstill important, even when using packages.

4.2.2. Package names and versions¶

All packages have a name, e.g. “HUnit”. Package names are assumed to beunique. Cabal package names may contain letters, numbers and hyphens,but not spaces and may also not contain a hyphened section consisting ofonly numbers. The namespace for Cabal packages is flat, nothierarchical.

Packages also have a version, e.g “1.1”. This matches the typical way inwhich packages are developed. Strictly speaking, each version of apackage is independent, but usually they are very similar. Cabal packageversions follow the conventional numeric style, consisting of a sequenceof digits such as “1.0.1” or “2.0”. There are a range of commonconventions for “versioning” packages, that is giving some meaning tothe version number in terms of changes in the package, such ase.g. SemVer; however, for packages intended to bedistributed via Hackage Haskell’s Package Versioning Policy applies(see also the PVP/SemVer FAQ section).

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The combination of package name and version is called the package IDand is written with a hyphen to separate the name and version, e.g.“HUnit-1.1”.

For Cabal packages, the combination of the package name and versionuniquely identifies each package. Or to put it another way: twopackages with the same name and version are considered to be the same.

Strictly speaking, the package ID only identifies each Cabal sourcepackage; the same Cabal source package can be configured and built indifferent ways. There is a separate installed package ID that uniquelyidentifies each installed package instance. Most of the time however,users need not be aware of this detail.

4.2.3. Kinds of package: Cabal vs GHC vs system¶

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It can be slightly confusing at first because there are variousdifferent notions of package floating around. Fortunately the detailsare not very complicated.

Cabal packages

Cabal packages are really source packages. That is they containHaskell (and sometimes C) source code.

Cabal packages can be compiled to produce GHC packages. They canalso be translated into operating system packages.

GHC packages

This is GHC’s view on packages. GHC only cares about librarypackages, not executables. Library packages have to be registeredwith GHC for them to be available in GHCi or to be used whencompiling other programs or packages.

The low-level tool ghc-pkg is used to register GHC packages andto get information on what packages are currently registered.

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You never need to make GHC packages manually. When you build andinstall a Cabal package containing a library then it gets registeredwith GHC automatically.

Haskell implementations other than GHC have essentially the sameconcept of registered packages. For the most part, Cabal hides theslight differences.

Operating system packages

On operating systems like Linux and Mac OS X, the system has aspecific notion of a package and there are tools for installing andmanaging packages.

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The Cabal package format is designed to allow Cabal packages to betranslated, mostly-automatically, into operating system packages.They are usually translated 1:1, that is a single Cabal packagebecomes a single system package.

It is also possible to make Windows installers from Cabal packages,though this is typically done for a program together with all of itslibrary dependencies, rather than packaging each library separately.

4.2.4. Unit of distribution¶

The Cabal package is the unit of distribution. What this means is thateach Cabal package can be distributed on its own in source or binaryform. Of course there may be dependencies between packages, but there isusually a degree of flexibility in which versions of packages can worktogether so distributing them independently makes sense.

It is perhaps easiest to see what being “the unit of distribution”means by contrast to an alternative approach. Many projects are made upof several interdependent packages and during development these mightall be kept under one common directory tree and be built and testedtogether. When it comes to distribution however, rather thandistributing them all together in a single tarball, it is required thatthey each be distributed independently in their own tarballs.

Cabal’s approach is to say that if you can specify a dependency on apackage then that package should be able to be distributedindependently. Or to put it the other way round, if you want todistribute it as a single unit, then it should be a single package.

4.2.5. Explicit dependencies and automatic package management¶

Cabal takes the approach that all packages dependencies are specifiedexplicitly and specified in a declarative way. The point is to enableautomatic package management. This means tools like cabal canresolve dependencies and install a package plus all of its dependenciesautomatically. Alternatively, it is possible to mechanically (or mostlymechanically) translate Cabal packages into system packages and let thesystem package manager install dependencies automatically.

It is important to track dependencies accurately so that packages canreliably be moved from one system to another system and still be able tobuild it there. Cabal is therefore relatively strict about specifyingdependencies. For example Cabal’s default build system will not even letcode build if it tries to import a module from a package that isn’tlisted in the .cabal file, even if that package is actuallyinstalled. This helps to ensure that there are no “untrackeddependencies” that could cause the code to fail to build on some othersystem.

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The explicit dependency approach is in contrast to the traditional“./configure” approach where instead of specifying dependenciesdeclaratively, the ./configure script checks if the dependencies arepresent on the system. Some manual work is required to transform a./configure based package into a Linux distribution package (orsimilar). This conversion work is usually done by people other than thepackage author(s). The practical effect of this is that only the mostpopular packages will benefit from automatic package management.Instead, Cabal forces the original author to specify the dependenciesbut the advantage is that every package can benefit from automaticpackage management.

The “./configure” approach tends to encourage packages that adaptthemselves to the environment in which they are built, for example bydisabling optional features so that they can continue to work when aparticular dependency is not available. This approach makes sense in aworld where installing additional dependencies is a tiresome manualprocess and so minimising dependencies is important. The automaticpackage management view is that packages should just declare what theyneed and the package manager will take responsibility for ensuring thatall the dependencies are installed.

Sometimes of course optional features and optional dependencies do makesense. Cabal packages can have optional features and varyingdependencies. These conditional dependencies are still specified in adeclarative way however and remain compatible with automatic packagemanagement. The need to remain compatible with automatic packagemanagement means that Cabal’s conditional dependencies system is a bitless flexible than with the “./configure” approach.

Note

GNU autoconf places restrictions on paths, including thepath that the user builds a package from.Package authors using build-type:configure should be aware ofthese restrictions; because users may be unexpectedly constrained andface mysterious errors, it is recommended that build-type:configureis only used where strictly necessary.

4.2.6. Portability¶

One of the purposes of Cabal is to make it easier to build packages ondifferent platforms (operating systems and CPU architectures), withdifferent compiler versions and indeed even with different Haskellimplementations. (Yes, there are Haskell implementations other thanGHC!)

Cabal provides abstractions of features present in different Haskellimplementations and wherever possible it is best to take advantage ofthese to increase portability. Where necessary however it is possible touse specific features of specific implementations.

For example a package author can list in the package’s .cabal whatlanguage extensions the code uses. This allows Cabal to figure out ifthe language extension is supported by the Haskell implementation thatthe user picks. Additionally, certain language extensions such asTemplate Haskell require special handling from the build system and bylisting the extension it provides the build system with enoughinformation to do the right thing.

Another similar example is linking with foreign libraries. Rather thanspecifying GHC flags directly, the package author can list the librariesthat are needed and the build system will take care of using the rightflags for the compiler. Additionally this makes it easier for tools todiscover what system C libraries a package needs, which is useful fortracking dependencies on system libraries (e.g. when translating intoLinux distribution packages).

In fact both of these examples fall into the category of explicitlyspecifying dependencies. Not all dependencies are other Cabal packages.Foreign libraries are clearly another kind of dependency. It’s alsopossible to think of language extensions as dependencies: the packagedepends on a Haskell implementation that supports all those extensions.

Where compiler-specific options are needed however, there is an “escapehatch” available. The developer can specify implementation-specificoptions and more generally there is a configuration mechanism tocustomise many aspects of how a package is built depending on theHaskell implementation, the operating system, computer architecture anduser-specified configuration flags.