(sender, args) -> {
	//Code here
	return /*some integer here*/ ;
};

Example - Random number result command

Say we want a command that returns a random number as a result. This can then be used by vanilla Minecraft's /execute store result ... command, which can be used for other command block chains.

new CommandAPICommand("randnum")
    .executes((sender, args) -> {
        return new Random().nextInt();
    })
    .register();

This returns a success value of 1 (Because no errors or CommandAPI.fail(String) was thrown) and a result value of a random number.

Example - Lootbox system with /execute command

We can store state using /execute store and we can perform conditional checks using /execute if. By combining these, we can create a system which can be used with commandblocks to say, give players random lootboxes and redeem them. The concept is to create a command that generates a random number from 1 to 100. If the number is 1 (thus, the chance of being chosen is \(\frac{1}{100}\)), then we award a player with some reward, say 64 diamonds.

To do this, we'll declare two commands:

/randomnumber        - returns a random number between 1 and 99 (inclusive)
/givereward <player> - gives a player 64 diamonds and broadcasts it in the chat

Since we're declaring commands that are to be used in /execute, we must ensure that these commands are registered in your plugin's onLoad() method. First, we write our implementation for /randomnumber. It is fairly straight forward using Java's ThreadLocalRandom to generate a random number:

//Register random number generator command from 1 to 99 (inclusive)
new CommandAPICommand("randomnumber")
    .executes((sender, args) -> {
        return ThreadLocalRandom.current().nextInt(1, 100); //Returns random number from 1 <= x < 100
    })
    .register();

Now we write our implementation for /givereward. In this example, we use the EntitySelectorArgument to select a single player. We cast it to Player and then add the items to their inventory.

//Register reward giving system for a target player
new CommandAPICommand("givereward")
    .withArguments(new EntitySelectorArgument("target", EntitySelector.ONE_PLAYER))
    .executes((sender, args) -> {
        Player player = (Player) args[0];
        player.getInventory().addItem(new ItemStack(Material.DIAMOND, 64));
        Bukkit.broadcastMessage(player.getName() + " won a rare 64 diamonds from a loot box!");
    })
    .register();

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Now that we've declared these commands, we can now use them in practice. We can use a command block to store a random number under the scoreboard score randVal for a player called SomePlayer, by executing the command /randomnumber. Since /randomnumber returns an integer, this value is stored in the scoreboard score:

/execute store result score SomePlayer randVal run randomnumber

To check if the random number is equal to 1, we can use the /execute if command. If their score stored in randVal matches 1, then we run the /givereward command.

/execute if score SomePlayer randVal matches 1 run givereward SomePlayer