Add a method to userdata

I am trying to add a method to an existing userdata like this, this however game me an error.

local userData = luajava.newInstance("Objects.Block") --creates a userdata from a Java class
userData.newMethod = function()
        -- Do stuff
end

I found this example on a site but it doesn't work either

local userData = luajava.newInstance("Objects.Block")
local mt = getmetatable(userData)
mt.__index.newMethod = function()
        -- Do stuff
end

is there a working way to add a method/function to an existing userdata from Lua

The second method you posted will work if the userdata already has a metatable with a table assigned to the __index field. A metatable cannot be assigned to a userdata from Lua for safety reasons. So, you must assign the userdata a metatable from your Java binding code.

lua_newuserdata(L, SOME_SIZE);
luaL_newmetatable(L, "userData.mt");
lua_setmetatable(L, -2);

If the metatable does not already have an __index table, then create one.

local mt = getmetatable(userData)
mt.__index = {
    newMethod = function()
        print('It works!')
    end
}