Since you are inside of an activity it is enough to write
int resId = YourActivity.this.getResources().getIdentifier(
"ball_red",
"drawable",
YourActivity.this.getPackageName()
);
or if you’re not calling it from an inner class
int resourceID = getResources().getIdentifier(
"ball_red",
"drawable",
getPackageName()
);
Note
getIdentifier() Returns 0 if no such resource was found. (0 is not a valid resource ID.)
Check
Check also in your R.java whether there is a drawable
with the name ball_red
e.g.:
public static final class drawable {
public static final int ball_red = 0x7f020000;
}
EDIT
If you’re not in any activity then you must pass a context instead of resources as parameter
then do this
int resId = context.getResources().getIdentifier(
"ball_red",
"drawable",
context.getPackageName()
);