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The contracts of the read methods for InputStream and Reader classes and their subclasses are complicated with regard to filling byte or character arrays. According to the Java API [API 2014] for the class InputStream, the read(byte[] b) method and the read(byte[] b, int off, int len) method , provides provide the following behavior:

The default implementation of this number of bytes actually read is returned as an integer. This method blocks until the requested amount of input data len has been readis available, end of file is detected, or an exception is thrown. Subclasses are encouraged to provide a more efficient implementation of this method.

According to the Java API for the However, the read(byte[] b, int off, int len) method

reads some number of bytes from the input stream and stores them into the buffer array b. The number of bytes actually read is returned as an integer. The number of bytes read is, at most, equal to the length of b.

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An attempt is made to read as many as len bytes, but a smaller number may be read, possibly zero. 

Both The read() methods return as soon as they find available input data. As a result, these methods can stop reading data before the array is filled because the available data may be insufficient to fill the array.

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