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Appending to a File
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Think of a file as a long strip of tape, where the first byte written to the file is at the beginning of the tape, and subsequent bytes fall into place behind the first byte When you wrote data to a file previously in this chapter, the data was always written at the beginning of the file This is fine if you don t want to retain the data already stored in the file, because the new data will overwrite the existing data in the file However, programmers usually want to add data to a file rather than replace existing data To do this, new data must be written after the last byte in the file rather than being written at the beginning of the file Programmers call this appending data to a file
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You append data to a file by setting the second parameter of the FileOutputStream constructor to true As you ll recall from the Writing to a File section of this chapter, the FileOutputStream constructor is used to open a file for writing The version of the constructor used in the Writing to a File section consisted of one argument, which is the filename Bytes are written at the beginning of the file by default Another version of the FileOutputStream constructor uses two arguments The first argument is again the filename, and the second argument is the Boolean value true This causes bytes to be written at the end of the file The file is created if it doesn t exist The following example illustrates how to append data to a file This example is nearly the same program as the example shown in the Writing to a File section, except we use the second version of the FileOutputStream constructor and pass it a Boolean true as the second argument, causing data to be written at the end of the file
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import javaio*; import javautil*; public class Demo { public static void main( String args[] ) { String studentFirstName = "Mary"; String studentLastName = " Jones "; String grade = "A"; try { PrintWriter out = new PrintWriter( new FileOutputStream("Studentdat , true)); outprint(studentFirstName); outprint(studentLastName); outprintln(grade); } catch (IOException e) { Systemoutprintln(e); } finally { outclose(); } } }
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CHAPTER 11 Files and Streams
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Previously in this chapter, you learned how to write data to a file and read data from a file that is not encapsulated in the class definition That is, the student name and grade used in previous examples were not associated with each other in a class In the real world, many data elements you want stored in a file will be data members of a class As you ll recall, a class is a cookie cutter that describes attributes and behaviors of an object, such as a student Attributes are basically data, and behaviors are methods When you press the cookie cutter into the dough, you make a real cookie When you declare an instance of a class, you create a real object of the class (in this case, a real student) Many attributes of a class are held in instance variables An instance variable represents a memory location used to store data for an instance of the object Suppose two instance variables are defined in a class definition, and you created five objects of that class Java sets aside five sets of instance variables, each independent of the other However, each of these five objects share the same set of methods associated with the class When a programmer needs to retain an instance of a class, the programmer saves the instance to the file In doing so, only the instance variables are stored in the file Methods are not saved in the file This is a different technique than we used previously in this chapter, where we saved individual data elements We retrieve this data from a file by reading an object from the file rather than reading individual data elements That is, we read the entire set of instance variables from the file, which are stored in memory and accessed by using a reference to the class Here are the steps you must follow in order to be able to write and read an object to and from a file The initial step is to implement the Serializable interface in the class whose objects you are going to write to a file The Serializable interface enables instances of the class to be converted to a byte stream that can be written to disk or transmitted over a network Programmers call this serialization The byte stream is then deserialized when the object is read and reconstituted back to the instance of the class Next, you create an instance of the class and assign values to instance variables With the data in place, you re ready to open the file You open a file by using the new operator to call the constructor of the FileOutputStream class and pass it the name of the file, which you learned how to do previously in this chapter The new operator returns a reference to FileOutputStream This reference is passed to the constructor of the ObjectOutputStream class, which defines the methods used to write an object to a file
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