Question

i was looking some code of some GWT classes, and, shamefully, i was unable to understand this part of code:

private void toggleHover() {
    // Toggle hovering.
    int newFaceID = getCurrentFace().getFaceID() ^ HOVERING_ATTRIBUTE;

    // Remove disabled.
    newFaceID &= ~DISABLED_ATTRIBUTE;
    setCurrentFace(newFaceID);
  }

I do not know very well the java operators, so, my question is:

What exactly happens in:

a) getCurrentFace().getFaceID() ^ HOVERING_ATTRIBUTE

b) newFaceID &= ~DISABLED_ATTRIBUTE;

The & is a bitwise AND and ^ is a bitwise exclusive OR operator. This is all I knew about this. But, I dont understand the ~DISABLED_ATTRIBUTE and what happens in the assings of the values.

This piece of code is from GWT com.google.gwt.user.client.ui.CustomButton class.

Thanks in advance.

Answer 1

There must be a bit in the ID that is reserved for the hover status. The expression getCurrentFace().getFaceID() ^ HOVERING_ATTRIBUTE appears to toggle the hovering attribute of the current face. In other words, if the HOVERING_ATTRIBUTE bit is one, it is set to zero; if it’s zero, it is set to one.

Another bit must be the disabled status. The expression newFaceID &= ~DISABLED_ATTRIBUTE clears this bit. That is, the DISABLED_ATTRIBUTE bit is set to zero.