在Java中使用常数1中的按位移位的优势

Question

in android source code, there is something like below. Is there any good reason to use bitwise shifting in this situation? Wouldn't be simpler just typing 1 ?

public static final int POP_BACK_STACK_INCLUSIVE = 1<<0;

What is the disadvantage of using like below or advantage of above?

public static final int POP_BACK_STACK_INCLUSIVE = 1;

Answer 1

This was done not for performance but for maintenance of the code.

Upon compilation compiler evaluates constant expressions, and this constant will be stored as 1

But in future, when this constant should be changed via shifting, the maintainer will just change 0 to another number in the source code.

Shift operator just shows to maintainer, that shift operation can be used for this constant in the future releases.

Answer 2

There is no performance advantage for either version. In the first case, the 1<<0 expression is evaluated at compile time, and the Java and Dalvik bytecodes will actually just use a constant 1 ... in both case.

The real reason is to document that this is a "bit flag" value. Writing the expression as 1<<0 is saying that the flag has bit zero set.

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Note that this does NOT give the developers the option of changing the shifting in some future version. POP_BACK_STACK_INCLUSIVE is a public compile-time constant, so changing it could lead to binary compatibility problems. Specifically, code that is compiled against different versions of Android would have different values for the constant. That could make apps non-portable across different Android releases; ie you would need different versions of apps for the different releases.