I just recently finished a game using pure Java (no libraries like libgdx or lwjgl). When I exported it from Eclipse (Right-Click Project > Export… > Java > Runnable JAR File > Extract / Package Required Libraries Into Generated JAR > Finish) when opened, the JAR file showed a blank (grey) JFrame without any graphics drawn onto it. Could this be a problem with my code, (even though it runs fine in Eclipse)? Or possibly the way that I am exporting it? I am also using a sprite sheet for a pixel font in the game, but the background and characters are created in the game, so that most likely isn’t the problem.
