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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.