public class ExampleGenerator extends ChunkGenerator {
    @Override
    public ChunkData generateChunkData(World world, Random random, int chunkX, int chunkZ, BiomeGrid grid) {
        // mandatory to create initial data
        ChunkData chunk = super.createChunkData(world);

        // create generator for Y values
        // use world seed so results are the same for each unique seed
        // number of octaves (iterations) to use
        SimplexOctaveGenerator generator = new SimplexOctaveGenerator(new Random(world.getSeed()), 8);

        // lower values create smoother terrain (plains) while high values make steep terrain (mountains)
        generator.setScale(0.005D);

        // current Y value
        int y;

        // loop through the chunk's blocks
        // remember the Y value comes from the octave generator
        for (int x = 0; x < 16; x++) {
            for (int z = 0; z < 16; z++) {

                // set the current Y value
                y = (int)
                        // get the current Y value for the given block at x/z at chunkX/chunkZ
                        (((generator.noise((chunkX * 16) + x, (chunkZ * 16) + z, 0.5D, 0.5D, true) + 1)
                        // multiply the Y value by a number - this is the difference between the highest and lowest points in the world (at 10 world stops at 64)
                        // add a number to the Y value - this number the minimum height of the world
                        // both of these numbers can be changed
                        * 15D) + 7D);

                // this is where you would apply your custom generation using (x, y, z)

                // creating superflat terrain with 3 blocks of stone as an example
                chunk.setBlock(x, y, z, Material.GRASS);
                chunk.setBlock(x, y - 1, z, Material.DIRT);
                chunk.setBlock(x, y - 2, z, Material.DIRT);
                for (int index = y - 3; index > 0; index--) {
                    chunk.setBlock(x, index, z, Material.STONE);
                }
                chunk.setBlock(x, 0, z, Material.BEDROCK);
            }
        }

        // return your chunk
        return chunk;
    }
}