问题:

Looking to the source code of java.nio.DirectByteBuffer class, I have found this:

if ((length << 0) > Bits.JNI_COPY_TO_ARRAY_THRESHOLD) ....

What is the purpose to shift length by zero bits? May this be some perfomance optimization or something else?

回答1:

I think I've solved it.

In the class JavaDocs:

// -- This file was mechanically generated: Do not edit! -- //

So it is not hand coded. It was script-generated and the script writer did not add an optimisation for the case when the amount to bit shift by is zero.

回答2:

Doing i << 0 is a no-op. It evaluates to the same as i.

回答3:

The i << 0 is plainly redundant. There is no good reason for a Java programmer to write this code deliberately.

I'd say that this code is:

written by someone who wasn't thinking,
written by someone who doesn't understand what the << operator does,
the result of some semi-mechanical refactoring, or
originally produced by some sort of code generator or translator.

However, there's a good chance that the bytecode or JIT compiler will optimize this away, or that it won't impact significantly on performance anyway.