Can a javelin be used to fight like a staff? I’m currently working on a character who uses a staff to fight, and I wanted him to have a ranged weapon (he’s a teenager living in Japan so obviously I can’t give him a gun) and I had the idea of a collapsible staff that turns into a javelin.
You couldn’t give him a spear, either.
To answer the first question quickly, it depends. Generally speaking, a javelin would not make a good melee weapon, and some are especially ill suited to hand to hand combat. Javelins are intended to be light weight throwing weapons, and while they do have a bladed tip, they’re not designed for melee combat.
Spear combat does share some similarity to staff combat, and there overlapping techniques that work with both. (The difference is that spears tend to be longer and heavier than javelins.)
As far as I know, collapsible staves are basically a non-start. You can’t have a telescopic staff or spear because the weapon would, “fold up,” during normal combat usage. You might find ones that can be dismantled (such as with multiple segments which screw together), but that wouldn’t be particularly concealable, as you’d need to take the time to reassemble your weapon when needed.
Now, you can find collapsible spears, specifically for LARPing, but those intended to collapse when used to avoid injuring the other participants.
So, a collapsible javelin isn’t and option, but dropping that character in Japan makes that a moot point; Japanese weapon laws are exceptionally strict. Owning a spear would require a permit from the Prefecture Public Safety Commission (this is the same body which issues firearms permits.) Note that I said, “owning,” and not, “carrying,” because it’s very likely that said permit would only allow you to keep it in your house as a display piece.
Private ownership of weapons is governed by the Swords and Firearms Possession Control Law of 1958. This law has been updated several times, and that wasn’t the first Japanese firearms legislation (that goes back to 1629), but the 1958 law separated ownership from carrying. In Japan, permits grant the ability to legally own an item, but not the ability to carry it without further dispensation.
You can dig up a lot of information on the ’58 law because it applies to gun ownership, however it’s far more encompassing than that. Any katana, sword, spear, or halberd is automatically considered a weapon. For, fixed blade knives the magic number is 15cm, if it’s over that, it’s a weapon. (There’s a bit of complexity, with three different groupings. Fixed blade knives have a lower length limit if they’re carried “unpacked,” outside the home, and folding blade knives have an even lower limit. Though, the simple takeaway there is that pocket knives are legal and unregistered, box cutters are weapons.) Incidentally, the spear would be licensed as a, “sword,” under the law.
(You can find an English translation by Mark Allerman published in the Washington International Law Journal here. However, that was published in 2000, and the law was updated in 2008, and again in 2018, so it doesn’t reflect the current version of the text.)
It’s unlikely your character could even obtain a permit to carry a sword outside of the home, even if they were an iaido practitioner. Age alone is a major factor. Normally someone under the age of 18 cannot obtain a permit. (There is a specific exemption for someone who is at least 14 to own an air gun, but, as far as I can tell, this doesn’t apply for sword permits.) So, they wouldn’t be able to even buy this hypothetical collapsing javelin, never mind carrying it.
As I said, Japan’s weapon laws are very strict, to the point that your character couldn’t, realistically, get their hands on much more than a kitchen knife (which would be an illegal weapon once it left the house.) More than that, Japanese authorities are very strict about violations. If your character is carrying any weapon, it will go very badly for them once they start using it.