A simple and easy to use class that can get and set a player's total experience points in Minecraft. This is based off the Bukkit API. The determines this with a mathematical formula based off the player's current level and their progress towards the next one.

ExperienceManager.java

/*
AUTHOR: Dev_Richard (https://www.spigotmc.org/members/dev_richard.38792/)
DESC: A simple and easy to use class that can get and set a player's total experience points.
Feel free to use this class in both public and private plugins, however if you release your
plugin please link to this gist publicly so that others can contribute and benefit from it.

Source available at https://gist.github.com/warriordog/6160a1ed45134265fb30b8d224da0426
*/

import org.bukkit.entity.Player;

import java.math.BigDecimal;

public class PlayerExp {

    private Player player;

    public PlayerExp(Player player) {
        this.player = player;
    }

    public int getTotalExperience() {
        int level = player.getLevel();
        double currentExp = (double) player.getExp(); //works the same, see http://stackoverflow.com/a/916110
        int experience;
        int requiredExperience;
        if (level >= 0 && level <= 15) {
            experience = (int) Math.ceil(Math.pow(level, 2) + (6 * level));
            requiredExperience = 2 * level + 7;
        } else if (level > 15 && level <= 30) {
            experience = (int) Math.ceil(2.5 * Math.pow(level, 2) - 40.5 * level + 360);
            requiredExperience = 5 * level - 38;
        } else {
            experience = (int) Math.ceil(4.5 * Math.pow(level, 2) - 162.5 * level + 2220);
            requiredExperience = 9 * level - 158;
        }
        experience += Math.ceil(currentExp * requiredExperience);
        return experience;
    }

    public void setTotalExperience(int xp) {
        //Levels 0 through 15
        if (xp >= 0 && xp < 351) {
            //Calculate Everything
            int a = 1;
            int b = 6;
            int c = -xp;
            int level = (int) (-b + Math.sqrt(Math.pow(b, 2) - (4 * a * c))) / (2 * a);
            int xpForLevel = (int) (Math.pow(level, 2) + (6 * level));
            int remainder = xp - xpForLevel;
            int experienceNeeded = (2 * level) + 7;
            float experience = (float) remainder / (float) experienceNeeded;
            experience = round(experience, 2);

            //Set Everything
            player.setLevel(level);
            player.setExp(experience);
            //Levels 16 through 30
        } else if (xp >= 352 && xp < 1507) {
            //Calculate Everything
            double a = 2.5;
            double b = -40.5;
            int c = -xp + 360;
            double dLevel = (-b + Math.sqrt(Math.pow(b, 2) - (4 * a * c))) / (2 * a);
            int level = (int) Math.floor(dLevel);
            int xpForLevel = (int) (2.5 * Math.pow(level, 2) - (40.5 * level) + 360);
            int remainder = xp - xpForLevel;
            int experienceNeeded = (5 * level) - 38;
            float experience = (float) remainder / (float) experienceNeeded;
            experience = round(experience, 2);

            //Set Everything
            player.setLevel(level);
            player.setExp(experience);
            //Level 31 and greater
        } else {
            //Calculate Everything
            double a = 4.5;
            double b = -162.5;
            int c = -xp + 2220;
            double dLevel = (-b + Math.sqrt(Math.pow(b, 2) - (4 * a * c))) / (2 * a);
            int level = (int) Math.floor(dLevel);
            int xpForLevel = (int) (4.5 * Math.pow(level, 2) - (162.5 * level) + 2220);
            int remainder = xp - xpForLevel;
            int experienceNeeded = (9 * level) - 158;
            float experience = (float) remainder / (float) experienceNeeded;
            experience = round(experience, 2);

            //Set Everything
            player.setLevel(level);
            player.setExp(experience);
        }
    }

    private float round(float d, int decimalPlace) {
        BigDecimal bd = BigDecimal.valueOf(d); //works the same, see http://stackoverflow.com/a/916110
        bd = bd.setScale(decimalPlace, BigDecimal.ROUND_HALF_DOWN);
        return bd.floatValue();
    }
}