If I combine two objects by pressing Control + J, one side becomes strange

Question

If I select two objects at the same time and press Control + J,

one side turns strange. Why is this happening?

Here is my .blender file: https://drive.google.com/open?id=1krNaVE55Fo-GmDQxwTKkPm0460x-lxL4

Answer 1

From what I see, you've mirrored one object by scaling it negatively from the other (look at the scale values, one is negative on X while the other is positive). Which under the hood kinds of turns the mesh inside out twice like a sock.
But when you join the two meshes together, you lose the scale value from the joined object, which is replaced by the target object. So you end up with reversed faces.

To fix that, simply enter edit mode, select everything with A and hit Ctrl N to recalculate the normals.

Besides that, if you know you're gonna model mirrored parts, you should use the Mirror Modifier. It's a more efficient tool, on top of being reversible, editable at any time, and gives you more modeling possibilities.

One more thing : By looking at your object, you can see that they have uneven scales, not equal to 1, as well as some rotations. It might be intentional, but usually, you want to keep your objects scales equal to 1, or at least even (if uneven, you will have troubles while modeling, texturing and so on). Rotations should be usable ones (90s, 45s, .... are ok, but "-0.000001" are not) unless you do need those. So you might want to apply those transformations or undo them, depending on your needs. To apply transformations, use Ctrl A menu. To undo transformations, use the shortcuts Alt G/R/S (G for location, R for rotation, S for scale).

Answer 2

Don't ever have a negative scale! Don't adjust the shape in object mode, (only in edit mode) or apply the transforms then! (CTRL+A>>rotation&scale) If you make a mirror with negative scaling it will flip the normals. You can recalculate or flip them back in the normals menu in edit mode.