File Handling in Java

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In the earlier sections we worked on the basis that all of a program s data was read into memory in one go, processed, and then all written out in one go Modern computers have so much RAM that this is a perfectly viable approach, even for large data sets However, in some situations holding the data on disk and just reading the bits we need and writing back changes might be a better solution The disk-based random access approach is most easily done using a key value database (a DBM ), or a full SQL database both are covered in 12 but in this section we will show how to handle random access les by hand We will rst present the BinaryRecordFileBinaryRecordFile class Instances of this class represent a generic readable/writable binary le, structured as a sequence of xed length records We will then look at the BikeStockBikeStock class which holds a collection of BikeStockBike objects as records in a BinaryRecordFileBinaryRecordFile to see how to make use of binary random access les
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The BinaryRecordFileBinaryRecordFile class s API is similar to a list in that we can get/set/delete a record at a given index position When a record is deleted, it is simply marked deleted ; this saves having to move all the records that follow it up to ll the gap, and also means that after a deletion all the original index positions remain valid Another bene t is that a record can be undeleted simply by unmarking it The price we pay for this is that deleting records doesn t save any disk space We will solve this by providing methods to compact the le, eliminating deleted records (and invalidating index positions) Before reviewing the implementation, let s look at some basic usage:
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Contact = structStruct("<15si") contacts = BinaryRecordFileBinaryRecordFile(filename, Contactsize)
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Here we create a struct (little-endian byte order, a 15-byte byte string, and a 4-byte signed integer) that we will use to represent each record Then we create a BinaryRecordFileBinaryRecordFile instance with a lename and with a record size to match the struct we are using If the le exists it will be opened with its contents left intact; otherwise, it will be created In either case it will be opened in binary read/write mode, and once open, we can write data to it:
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contacts[4] = Contactpack("Abe Baker"encode("utf8"), 762) contacts[5] = Contactpack("Cindy Dove"encode("utf8"), 987)
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Table 74 File Object Attributes and Methods #1
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Syntax fclose() fclosed fencoding ffileno() fflush() fisatty() fmode fname fnewlines f__next__() fpeek(n) fread(count)
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Description
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Closes le object f and sets attribute fclosed to True Returns True if the le is closed The encoding used for bytes str conversions Returns the underlying le s le descriptor (Available only for le objects that have le descriptors) Flushes the le object f Returns True if the le object is associated with a console (Available only for le objects that refer to actual les) The mode le object f was opened with File object f s lename (if it has one) The kinds of newline strings encountered in text le f Returns the next line from le object f In most cases, this method is used implicitly, for example, for line in f Returns n bytes without moving the le pointer position Reads at most count bytes from le object f If count is not speci ed then every byte is read from the current le position to the end Returns a bytes object when reading in binary mode and a str when reading in text mode If there is no more to read (end of le), an empty bytes or str is returned Returns True if f was opened for reading Reads at most len(ba) bytes into bytearray ba and returns the number of bytes read this is 0 at end of le (Available only in binary mode) Reads the next line (or up to count bytes if count is speci ed and reached before the \n character), including the \n Reads all the lines to the end of the le and returns them as a list If sizehint is given, then reads approximately up to sizehint bytes if the underlying le object supports this Moves the le pointer position (where the next read or write will take place) to the given offset if whence is not given or is osSEEK_SET Moves the le pointer to the given offset (which may be negative) relative to the current position if whence is osSEEK_CUR or relative to the end if whence is osSEEK_END Writes are always done at the end in append "a" mode no matter where the le pointer is In text mode only the return value of tell() method calls should be used as offsets
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freadable() freadinto( ba) freadline( count) freadlines( sizehint) fseek( offset, whence)
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