Code of a Bukkit v1.13 plugin called HealthSteal by Slash9211

HealthStealEvent.java

package me.slash9211;

import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.EntityDamageByEntityEvent;
import org.bukkit.inventory.ItemStack;

public class HealthStealEvent implements Listener {
	
	/*
	 * When a player attacks a player it checks to see if the weapon was a sword (Diamond, Iron, Gold, Stone, or Wood)
	 * Then checks to make sure the sword name is what the configuration says (Default: 'Health Steal')
	 * Also checks to see if a player was damaged and if their health is less than 20 (Full row of hearts) it'll 
	 * change the MAX health from 40 (Two rows of hearts) down to 20 so there's no extra row of empty hearts, same with going up.
	 */
	
	@SuppressWarnings("deprecation")
	@EventHandler
	public void onPlayerHit(EntityDamageByEntityEvent event) {
		if (event.getDamager() instanceof Player && event.getEntity() instanceof Player) { //Makes sure both are players
			Player damager = (Player) event.getDamager();
			
			ItemStack weapon = damager.getInventory().getItemInMainHand();
			if (Main.swordID.contains(weapon.getType().getId()) && weapon.getItemMeta().getDisplayName().contains(Main.healthsteal_name)) { //Making sure its a sword and has the right name
				if (damager.getHealth() >= 19)
					damager.setMaxHealth(40); //Allows the player to have up to two rows of hearts if they have max default health
				
				damager.setHealth(damager.getHealth() + 0.5); //Adds half a heart to the damagers health
			}
			

		} else if (event.getEntity() instanceof Player) {
			Player damaged = (Player) event.getEntity();
			
			if (damaged.getHealth() <= 20) 
				damaged.setMaxHealth(20); //Removing the second row of hearts when it's not needed if they ever dip below 20 health
		}
	}
}

Main.java

package me.slash9211;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.logging.Logger;

import org.bukkit.configuration.file.FileConfiguration;
import org.bukkit.configuration.file.YamlConfiguration;
import org.bukkit.plugin.java.JavaPlugin;

public class Main extends JavaPlugin {
	
	public static String healthsteal_name;
	
	private File config_file;
	private FileConfiguration config;
	
	public static ArrayList<Integer> swordID = new ArrayList<>(Arrays.asList(267, 268, 272, 276, 283));

	public void onEnable() {
		
		/*
		 * Generates a configuration file to save the name of the 'Health Steal' sword for later use
		 */
		
		if (config_file == null) 
			config_file = new File(getDataFolder(), "healthsteal.yml");
		
		config = YamlConfiguration.loadConfiguration(config_file);
		
		if (!config.contains("settings.healthsteal_name")) {
			config.set("settings.healthsteal_name", "Health Steal"); //Defaults to 'Health Steal'
			saveConfig();
			}
		
		healthsteal_name = config.getString("settings.healthsteal_name");
		
		getServer().getPluginManager().registerEvents(new HealthStealEvent(), this);
		
		/*
		 * Prints out to the console the plugin has been enabled
		 */
		
		System.out.println("[HealthSteal] Enabled!");
	}
	
	/*
	 * Prints out to the console the plugin has been disabled
	 */
	
	public void onDisable() {
		System.out.println("Disabled!");
	}
	
	/*
	 * Saves the plugins configuration file
	 */
	
	public void saveConfig() {
		try {
			config.save(config_file);
		} catch (IOException ex) {
			Logger.getLogger("Minecraft").severe("[Health Steal] Unable to Save config");
		}
	}
}