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.
*/

import java.math.BigDecimal;
import java.util.List;

import org.bukkit.entity.Player;

public class ExperienceManager {
  
   private Player player;
  
   public ExperienceManager(Player player) {
     this.player = player;
   }
  
   public int getTotalExperience() {
     int experience = 0;
     int level = player.getLevel();
     if(level >= 0 && level <= 15) {
      experience = (int) Math.ceil(Math.pow(level, 2) + (6 * level));
      int requiredExperience = 2 * level + 7;
      double currentExp = Double.parseDouble(Float.toString(player.getExp()));
      experience += Math.ceil(currentExp * requiredExperience);
      return experience;
     } else if(level > 15 && level <= 30) {
       experience = (int) Math.ceil((2.5 * Math.pow(level, 2) - (40.5 * level) + 360));
       int requiredExperience = 5 * level - 38;
       double currentExp = Double.parseDouble(Float.toString(player.getExp()));
       experience += Math.ceil(currentExp * requiredExperience);
       return experience;
     } else {
       experience = (int) Math.ceil(((4.5 * Math.pow(level, 2) - (162.5 * level) + 2220)));
       int requiredExperience = 9 * level - 158;
       double currentExp = Double.parseDouble(Float.toString(player.getExp()));
       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);
       System.out.println("xpForLevel: " + xpForLevel);
       System.out.println(experience);
      
       //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);
       System.out.println("xpForLevel: " + xpForLevel);
       System.out.println(experience);
      
       //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);
       System.out.println("xpForLevel: " + xpForLevel);
       System.out.println(experience);
      
       //Set Everything
       player.setLevel(level);
       player.setExp(experience);      
     }
   }
  
  private float round(float d, int decimalPlace) {
  BigDecimal bd = new BigDecimal(Float.toString(d));
  bd = bd.setScale(decimalPlace, BigDecimal.ROUND_HALF_DOWN);
  return bd.floatValue();
  }
}

On the setTotalExperience method, you have a bug.

You say if(xp >=0 && xp < 351) then you say else if(xp >=352 && xp < 1507)

This excludes the value of xp == 351