Streams & Files

CS 240 – Advanced Programming Concepts

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I/O Overview

Ways to Read/Write Files

• Streams
• Read or write a file (or other source or destination of bytes) sequentially
• Scanner Class
• Tokenize stream input (read one token at a time)
• Files Class
• Read, copy, etc. whole files
• RandomAccessFile Class
• Use a file pointer to read from / write to any location in a file

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The File Class

• Used to represent, create, or delete a file, but not to read one
• Check file existence:

       File file = new File("/user/MyFile.txt");

       if(file.exists()) {

• Create a file:

       File file = new File("/user/MyFile.txt");

       file.createNewFile();

• Delete a file:

       File file = new File("/user/MyFile.txt");

       file.delete();

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Streams

Reading and Writing Bytes

Java I/O Streams (overview)

• Writing data to / Reading data from files (or other data sources)
• Two Choices: Binary-Formatted or Text-Formatted Data
• Binary Formatted: 00 00 04 D2 (4 bytes)
• Text Formatted: 1234 (4 characters)
• InputStream and OutputStream
• Reading/writing bytes
• Reading/writing binary-formatted data
• Reader and Writer
• Reading/writing characters
• Reading/writing text-formatted data

Reading/Writing Bytes

• The InputStream interface is used to read bytes sequentially from a data source
• FileInputStream
• PipedInputStream
• URLConnection.getInputStream()
• HttpExchange.getRequestBody()
• ResultSet.getBinaryStream(int columnIndex)
• Many more examples in the Java API

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Filter Input Streams

• There are many features you may want to enable when consuming data from an InputStream
• Decompress data as it comes out of the stream
• Decrypt data as it comes out of the stream
• Compute a “digest” of the stream (a fixed-length value that summarizes the data in the stream)
• Byte counting
• Line Counting
• Buffering

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Filter Input Streams (cont.)

• Open an InputStream on a data source (file, socket, etc.), and then wrap it in one or more “filter input streams” that provide the features you want (decompression, decryption, etc.)

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• Filter input streams all implement the InputStream interface, and can be arranged as a pipeline with the real data source at the end

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• Each filter input stream reads data from the next InputStream in line, and then performs a transformation or calculation on the data

OutputStream

• Writing bytes works the same way, just in reverse
• The OutputStream interface is used to write bytes sequentially to a data destination
• FileOutputStream
• PipedOutputStream
• URLConnection.getOutputStream()
• HttpExchange.getResponseBody()
• Many more examples in the Java API

Filter Output Streams

• There are many features you may want to enable when writing data to an OutputStream
• Compress data as it goes into the stream
• Encrypt data as it goes into the stream
• Compute a “digest” of the stream (a fixed-length value that summarizes the data in the stream)
• Buffering

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Filter Output Streams (cont.)

• Open an OutputStream on a data destination (file, socket, etc.), and then wrap it in one or more “filter output streams” that provide the features you want (compression, encryption, etc.)

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• Filter output streams all implement the OutputStream interface, and can be arranged as a pipeline with the real data destination at the end

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• Each filter output stream performs a transformation or calculation on the data, and then writes it to the next OutputStream in line

Filter Stream Example

• Compress a file to GZIP format
• Compress Example
• LegacyCompress Example
• Decompress a file from GZIP format
• Decompress Example
• LegacyDecompress Example

Reading/Writing Binary-Formatted Data

• Reading/writing bytes is useful, but usually we want to read/write larger data values, such as: float, int, boolean, etc.

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• The DataOutputStream class lets you write binary-formatted data values

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• The DataOutputStream(OutputStream out) constructor wraps a DataOutputStream around any OutputStream

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• The DataInputStream class lets you read binary-formatted data values

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• The DataInputStream(InputStream in) constructor wraps a DataInputStream around any InputStream

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Readers and Writers

Reading and Writing Character Data

Reading/Writing Characters

• The Reader interface is used to read characters sequentially from a data source

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• The Writer interface is used to write characters sequentially to a data destination

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• See CopyFileExample

Reading/Writing Text-Formatted Data

• The PrintWriter class lets you write text-formatted data values (String, int, float, boolean, etc.)

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• The Scanner class lets you read text-formatted data values

Convert from Streams to Readers or Writers 

• Convert from an input stream to a reader with InputStreamReader:

new InputStreamReader(new FileInputStream(“myfile.txt”));

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• Convert from an output stream to a writer with OutputStreamWriter:

new OutputStreamWriter(new FileOutputStream(“myfile.txt”));

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The Scanner Class

Tokenizing Input

Scanner: Tokenize Data As You Read It

• Can read "tokens" one at a time from a source of characters
• Read from a File, InputStream, or Reader
• Can specify the delimit character(s) as a regular expression
• Defaults to delimiting by whitespace: \s+

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• Code Examples:
• ScannerExample1.java
• ScannerExample2.java
• ScannerExample3.java

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Other Ways to Read and Write Files

Files: Read Entire File into a List

public List<String> readFile(File file) throws IOException {� Path path = Paths.get(file.getPath());� List<String> fileContents = Files.readAllLines(path); � return fileContents; �}�

Random Access Files

• The RandomAccessFile class allows “random access” to a file’s contents for both reading and writing

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• Random Access
• The “file pointer” represents the current location in the file (similar to an array index)
• Use the seek(long) or skipBytes(int) methods to move the “file pointer” to any location in the file
• Read or write bytes at the current file pointer location using various overloaded read and write methods

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