javaperformancememoryvm-implementation

Overwriting vs. Lookup

I was reading through the SparseArray class in android, and came across the following method:

public void removeAt(int index) {
    if (mValues[index] != DELETED) {
        mValues[index] = DELETED;
        mGarbage = true;
    }
}

Clearly this could as well has been written:

public void removeAt(int index) {      Or   public void removeAt(int index) {
    if (mValues[index] != DELETED) {            mValues[index] = DELETED;
        mValues[index] = DELETED;               mGarbage = true;
        if (!mGarbage)                      }
            mGarbage = true;         
    }                                
}

It would seem the android developers believed the array lookup mValues[index] was faster than an array write, but the variable lookup wasn't faster than a variable write.

Is this really true? Does it depend on the VM, or is it general knowledge in compiled languages too?

Solution

  • Certainly the right-hand side version is not equivalent - because then mGarbage is set to true whether or not the value has changed.

    The left-hand side is equivalent to the original, but it's pointless.

    Basically I think you've missed the side-effect of checking whether or not the existing value was allows DELETED: it allows mGarbage to be set to true only if the method has actually had an effect. That has nothing to do with the performance of reading from the array.