Kotlin makes things easier

Let's take a look at each basic plugin subsystem...

Listeners

Null Safety

Listeners are pretty similar to how we do them in Java. Although in some events, null is very common and very annoying to handle, a great example being in PlayerInteractEvent. With Kotlin, its null-safety makes everything much easier. The following in Java:

String s = "Unnamed";
if(e.hasItem() && e.getItem().hasItemMeta() && e.getItem().getItemMeta().hasDisplayName()) {
    s = e.getItem().getItemMeta().getDisplayName();
}

can be condensed all the way down to a single line of code:

val s = e.item?.itemMeta?.displayName ?: "Unnamed"

Infix Functions

Infix functions makes code look much cleaner. Take a common operation done in PlayerMoveEvent, making it so that the calculation is only done if the player moves a block. Normally what you would see is...

Location from = e.getFrom();
Location to = e.getTo();
if(!(from.getBlockX() == to.getBlockX() && from.getBlockY() == to.getBlockY() && from.getBlockZ() == to.getBlockZ())) {
    e.getPlayer().damage(0.0);
}

Some people would make it look a bit cleaner by doing this...

public boolean equalsBlock(Location from, Location to) {
    return from.getBlockX() == to.getBlockX() && from.getBlockY() == to.getBlockY() && from.getBlockZ() == to.getBlockZ()
}
public void onMove(PlayerMoveEvent e) {
    if(!equalsBlock(e.getFrom(), e.getTo()) {
        e.getPlayer().damage(0.0);
    }
}

But with Kotlin, we can make it look even CLEANER with an infix function!

infix fun Location.equalsBlock(other: Location) =
        this.blockX == other.blockX && this.blockY == other.blockY && this.blockZ == other.blockZ

public fun onMove(e: PlayerMoveEvent) {
    if(!(e.from equalsBlock e.to)) {
        e.player.damage(0.0);
    }
}

(File for reference: KotlinListener.kt)

Commands

Echo

Commands are now also much easier. An echo command can be made in one line without use of other external libraries (such as google guava's Joiner class). In Kotlin, we can just do sender!!.sendMessage(args?.joinToString(separator=" ")) to echo the arguments back to the sender.

(File for reference: KotlinEchoCmd.kt)

Safe Casting

Also, Kotlin has safe casting as well. This makes running player-specific funtions on a CommandSender very easy!

(sender as? Player)?.chat("I just ran the /player command!") ?: sender?.sendMessage("You aren't a player :(")

(File for reference: KotlinPlayerCmd.kt)

Before you say I missed something...

Check out the wiki

Epilogue

I hope you found this useful! Kotlin is very useful for creating Bukkit plugins, and I hope to see some really cool plugins in the future made with this language. If you found this useful, feel free to star the repo and check it out whenever you want. I'll definitely add more to it in the future!