Does android always use the newest API to run an app?

Question

I was messing around with MeasureSpec when I came across this bit of text:

Note: On API level 17 and lower, makeMeasureSpec's implementation was such that the order of arguments did not matter and overflow in either value could impact the resulting MeasureSpec. RelativeLayout was affected by this bug. Apps targeting API levels greater than 17 will get the fixed, more strict behavior.

So that got me wondering: If I build an app for API 14 but I run it on an API 22 device will it fix the bug or will < API 17's bug still exist on the 22 device?

Answer 1

makeMeasureSpec(API 17<) method's implementation is the following:

 public static int makeMeasureSpec(int size, int mode) {
        if (sUseBrokenMakeMeasureSpec) {
            return size + mode;
        } else {
            return (size & ~MODE_MASK) | (mode & MODE_MASK);
        }
    }

As you can see it's return value is depending on the value of sUseBrokenMakeMeasureSpec which value is assigned in the View class's constructor:

  sUseBrokenMakeMeasureSpec = targetSdkVersion <= JELLY_BEAN_MR1;

So only the app's target will determine the behaviour. By doing it this way a newer system can maintain compatibility with an older app which expets the old behaviour.