InefficientOutputStream.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>

<overview>
  <p>
    The classes <code>java.io.OutputStream</code>
    and <code>java.io.FilterOutputStream</code> only require
    subclasses to implement the method <code>write(byte b)</code>.
    Typically, uses of <code>OutputStream</code>s will not write
    single bytes, but an array via the <code>write(byte[] b, int off,
    int len)</code> method. The default implementation of this method,
    which you are not required to override,
    calls <code>write(byte b)</code> for each byte in the array. If
    this method involves I/O, such as accessing the network or disk,
    this is likely to incur significant overhead.
  </p>
</overview>

<recommendation>
  <p>
    Always provide an implementation of the <code>write(byte[] b, int
      off, int len)</code> method.
  </p>
</recommendation>

<example>

  <p>
    The following example shows a subclass
    of <code>OutputStream</code> that simply wraps
    a <code>DigestOutputStream</code> to confirm that the data it
    writes to a file has the expected MD5 hash. Without an
    implementation of <code>write(byte[] b, int off, int len)</code>
    this will be very slow, because it makes a call
    to <code>DigestOutputStream.write(byte b)</code>
    and <code>FileOutputStream.write(byte b)</code> for each byte
    written.
  </p>

  <sample src="InefficientOutputStreamBad.java" />

  <p>
    The example can be updated to use a more efficient method. In this
    case, calls to <code>write(byte[] b, int off, int len)</code> are
    simply forwarded to <code>DigestOutputStream.write(byte[] b, int
    off, int len)</code>.
  </p>

  <sample src="InefficientOutputStreamGood.java" />

</example>

<references>

<li>
  Java API Specification:
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/OutputStream.html#write(byte[],int,int)">OutputStream.write(byte[] b, int off, int len)</a>,
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/FilterOutputStream.html#write(byte[],int,int)">FilterOutputStream.write(byte[] b, int off, int len)</a>.
</li>

</references>
</qhelp>