Java - Files and I/O

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You could do it by looking at a few samples. Scanner and Formatterand C-like printf and format methods for formatted output using format specifiers. It also introduces a new try -with-resources syntax to simplify the coding of close method. File can represent either a file or a directory. Pathwhich overcomes many limitations of java.

A path string is used to locate a file or a directory. Unfortunately, path strings are system dependent, e. A path could be absolute beginning from the root or relative which is relative to a reference directory. File class maintains these system-dependent properties, for you to write programs that are portable:. You can construct a File binary io java with a path string or URI, as follows.

A file URL takes the form of file: For applications that you intend to distribute as JAR files, you should use the URL class to reference the resources, as it can reference disk files as well as JAR'ed filesfor example.

The following program recursively lists the contents of a given directory similar to Unix's "ls -r" command. You can apply a filter to list and listFilesto list only files that meet a certain criteria. FilenameFilter declares one abstract method:.

You can program your binary io java criteria in accept. The following program lists only files that meet a certain filtering criteria. Programs read inputs binary io java data sources e. A stream is a sequential and contiguous one-way flow of data just like water or binary io java flows through the pipe. It binary io java important to mention that Java does not differentiate between the various types of data sources or sinks e.

They are all treated as a sequential flow of data. The Java program receives data from a source by opening an input stream, and sends data to a sink by opening an output stream. If your program needs to perform both input and output, you have to open two streams - an input binary io java and an output stream.

Java internally stores characters char type in bit UCS-2 character set. All the byte streams are derived from the abstract superclasses InputStream and OutputStreamas illustrated in the class diagram. The abstract superclass InputStream declares an abstract method read binary io java read one data-byte from the input source:.

The read method returns an binary io java instead of a bytebecause it uses -1 to indicate end-of-stream. The term " binary io java " means that the method and the program will be suspended. The program will resume only when the method returns. Two variations of read methods are implemented in the InputStream for reading a block of bytes into a byte-array. It returns the number of bytes read, or -1 if "end-of-stream" encounters. Similar to the input counterpart, the abstract superclass OutputStream declares an abstract method write to write a data-byte to the output sink.

The least-significant byte of the int argument is written out; the upper 3 bytes are discarded. Similar to the readbinary io java variations of the write method to write a block of bytes from a byte-array are implemented:. Both the InputStream and the OutputStream provides a close method to close the stream, which performs the necessary clean-up operations to free up the system resources.

This could prevent serious resource leaks. Unfortunately, the close binary io java also throws a IOExceptionand needs to be enclosed in a binary io java try-catch statement, as follows. This makes the codes somehow ugly. This produces much neater codes. In addition, the OutputStream provides a binary io java method to flush the remaining bytes from the output buffer.

InputStream and OutputStream are abstract classes that cannot be instantiated. You need to choose an appropriate concrete subclass to establish a connection to a physical device. For example, we can layer a BufferedInputStream to a FileInputStream for buffered input, and stack a DataInputStream in front for formatted data input using primitives such as intdoubleas illustrated in the following diagrams.

This is grossly inefficient, as each call is handled by the underlying operating system which may trigger a disk access, or other expensive operations. To chain the streams together, simply pass an instance of one stream into the constructor of another stream. This example copies a file by reading a byte from the input file and binary io java it to the output file.

The method close binary io java programmed inside the finally binary io java. It is guaranteed to be run after try or catch. However, method close also throws an IOExceptionand therefore must be enclosed inside a nested try-catch block, binary io java makes the codes a little ugly.

The output shows that it took about 4 seconds to copy a KB file. As mentioned, JDK 1. For example, the above example can be re-written in a much neater manner as follow:. This program took only 3 millisecond - a more than times speed-up compared with the previous example. However, there is a trade-off between speed-up the the memory usage. For file copying, a large buffer is certainly recommended.

But for reading just a few bytes from a file, large buffer simply wastes the binary io java. I re-write the program using JDK 1. The JRE decides on the buffer size. The program took 62 milliseconds, about 60 times binary io java compared with example 1, but slower than the programmer-managed buffer. To use DataInputStream for formatted input, you can chain up the input streams as follows:. DataInputStream implements DataInput interface, which provides methods to read formatted primitive data and Stringsuch as:.

DataOutputStream implements DataOutput interface, which provides methods to write formatted primitive data and String. Binary io java following program writes some primitives to a disk file. It then reads the raw bytes to check how the primitives were stored.

Finally, it reads the data as primitives. The data stored in the disk are exactly in the same form as in the Java program internally e. The byte-order is big-endian big byte first, or most significant byte in lowest address. If this character is to be written to a file uses UTF-8, the character stream needs to translate " 60 A8 " to " E6 82 A8 ". The reserve takes place in a reading operation. This is because some charsets use fixed-length of 8-bit e.

When a character stream is used to read an 8-bit ASCII file, binary io java 8-bit data is read from the file and put into the bit char location of the Java program. The abstract superclass Reader operates on char. It declares an abstract method read to read one character from the input source.

There are also two variations of read to read a block of characters into char -array. The abstract superclass Writer declares an abstract method writewhich writes a character to the output sink. The lower 2 bytes of the int argument is written out; while the upper 2 bytes are discarded. The default charset is kept in the JVM's system property " file. You can get the default charset via static method java.

BufferedReader provides a new method readLinebinary io java reads a line and returns a String without the line delimiter. The main binary io java java. Charset provides static methods for testing whether a particular charset is supported, locating charset instances by name, and listing all the available charsets and the default charset.

The default charset for binary io java encoding is kept in the system property " file. For example, the following command run the program with default charset of UTF The following example encodes some Unicode texts in various encoding scheme, and display the Hex codes of the encoded byte sequences. As mentioned, Java internally stores characters char type in bit UCS-2 character set.

You can list the available charsets via static method java. The commonly-used Charset names supported by Java are:.

The following program writes Unicode texts to a disk file using various charsets for file encoding. It then reads the file byte-by-byte via a byte-based input stream to check the encoded characters in the various charsets. Finally, it reads the file using the character-based reader. Primitives are converted to their string representation for printing. The printf and format were introduced in JDK 1. Instead, it sets an internal flag which can be checked via the checkError method.

A PrintStream can also be created to flush the output automatically. The standard output and error streams System. All characters printed by a PrintStream are converted into bytes using the default character encoding. The PrintWriter class should be used in situations that require writing characters rather than bytes.

The character-stream PrintWriter is similar to PrintStreamexcept that it write in characters instead of bytes.

The PrintWriter also supports all the convenient printing methods printprintlnprintf and format.

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Java views each file as a sequential stream of bytes. When a file is opened, an object is created and a stream is associated with the object. The streams provide communication channels between a program and a particular file or device. The java package java. Classes in the package are divided into input and output streams, and then subdivided into those, which operate with characters data text files and with byte data binary files. The difference between a stream of characters from a stream of bytes is that the former uses Unicode characters, which are represented by 16 bits - that is it, in java, a character is made up of two bytes.

The natural question to ask is how Java deals with these encoding problems: The Java has different tables of encoding and you can choose any of them. To see the default table you print System. This is very important issue if you are concerned with writing applications that operate in an international context.

Standard Output To print output values in our programs, we have been using System. The printf method gives us more control over the output. This format says, that we print a float number of 7 characters that includes 5 digits after the decimal point. Command-line Input Any Java program takes input values from the command line and prints output back to the terminal window. When you use the java command to invoke a Java program Demo from the command line, you type something like this java Demo parameter1 parameter2 Input values parameter1 and parameter2 are passed to a main method as an array of Strings.

Standard Input The string of characters that you type in the terminal window after the command line is the standard input stream. The following code example let us call this class Average. Similarly, we can redirect standard input so that a program reads data from a file instead of the terminal application. Once an exception is thrown you have three options: That is your responsibility as a programmer to validate the input. One way to do this is to handle exceptions, meaning that you have to catch exceptions.

Catching Exceptions To catch an exception, you have to use try and catch blocks. You wrap try around the statement or a group of statements which may throw an exception. Note, you may have several catch-blocks for one try-block: If an exception is thrown, control is immediately transferred to a correspondent catch-block, skipping all statements below the statement which generated an exception.

Regardless to what happens in the try-block, the statements placed inside the finally-block will always execute. Those statements are usually like closing the input file, or database, or network connection. Review the code example ExceptionDemo.

Throwing Exceptions It's common to test parameters of methods or constructors for valid input data. If the value is illegal, you throw an exception by using the throw keyword along with the object you want to throw. It also transmits information. The exception object may contain a detail message that can be retrieved by invoking the getMessage method Any method that may throw an exception, must declare the exception in it's method declaration.

This is implemented by the use of the throws clause. Throws clause follows the method name and lists the exception types that may be thrown: A checked exception means that if you ignore it, your program will not compile. What do you do in this case? Either you use try-catch blocks or propagete the exception usingh throws clause.

Reading Text Files There are two approaches of reading text files: The class throws FileNotFoundException checked exception , so you have to wrap try and catch block around each time you open a file. For the efficient reading, we use the BufferedReader class. The method returns a String, or null if the end of the stream has been reached. The readLine method throws IOException , so you have to handle it.

A text file can also be read using the Scanner class. Using the Scanner offers the advantage for text processing of various data formats.. Scanner has a lot of other features, with support for regular expressions, delimiter definitions, skipping input, searching in a line, reading from different inputs and others.. Review the code examples Read. The class has minimal method support, like a method write , which writes a character or an array of characters or String.

Unless prompt output is required, it is advisable to wrap a BufferedWriter around for efficiency reason. The class has the newLine method, which uses the platform's own notion of line separator. Neither FileWriter nor BufferedWriter provides enough methods to deal with a formatted output.

For such purposes we shall use another class PrintWriter on the top of the previous two classes. The class uses familiar methods print and println. Review the code example Write.

In Java the line separator is defined by the system property line. They are written to the stream with the methods, such as writeInt , writeFloat , and a few others. The class of each serializable object is encoded including the class name and signature of the class , the values of the object's fields and arrays, and the closure of any other objects referenced from the initial objects. Only objects with the Serializable interface can be serialized. The interface does not have any methods, and serves only as a flag to the compiler.

For most classes the default serialization is sufficient. Reading an object from a stream, like writing, is straightforward. Primitive data types are read from the stream with the methods, such as readInt, readFloat. The default serialization process serializes each field of the object except static and transient. Using transient , we can exclude particular information when we serialize the oblect. An example of such excluded information could be a password.