Inconsistent Execution of Code

Question

The following class works fine when called from the testclass but shows no graphics when called from another class in my project.
import java.awt.*;
import java.applet.*;
import java.io.*;
import javax.swing.*;
import java.awt.event.*;
import javax.imageio.*;
import java.util.*;
import java.awt.image.*;
public class ExpScreen{
     int[] expN={0,0,200,500,800,1200,1800,2500,3000,4000,5000,5500,6000,6750,7500,8750,10000,12000,14000,16700,18500,20000,22222,25000,30000,35000,40000,50000,60000,75000,100000,150000,200000,250000,300000,350000,500000,1000000};
     public ExpScreen(Stats play,int HPn,int MPn, int exp,int curLvl,int curExp){
          System.out.println(play);
          System.out.println(HPn);
          System.out.println(MPn);
          System.out.println(exp);
          System.out.println(curLvl);
          System.out.println(curExp);
          JFrame fr=new JFrame("You earned " + exp + " experience!");
          Dimension d=Toolkit.getDefaultToolkit().getScreenSize();
          fr.setSize(d.width,d.height);
          fr.setResizable(false);
          JLabel hp=new JLabel("HP  "+ play.getCurHP()+ "/" + play.getHP());
          JLabel BLANK=new JLabel(" ");
          JLabel mp=new JLabel("MP  "+ play.getCurMP()+ "/" + play.getMP());
          JLabel lvl=new JLabel("You are currently level " + curLvl);
          JLabel Exp=new JLabel("Experience  "+ curExp+ "/" + expN[curLvl]);
          JProgressBar HP=new JProgressBar(0,100);
          JProgressBar MP=new JProgressBar(0,100);
          JProgressBar EXP=new JProgressBar(0,100);
          HP.setValue(100);
          MP.setValue(100);
          EXP.setValue(100);
          fr.getContentPane().setLayout(new GridLayout(4,2));
          fr.getContentPane().add(lvl);
          fr.getContentPane().add(BLANK);
          fr.getContentPane().add(hp);
          fr.getContentPane().add(HP);
          fr.getContentPane().add(mp);
          fr.getContentPane().add(MP);
          fr.getContentPane().add(Exp);
          fr.getContentPane().add(EXP);
          HP.setValue((int)((play.getCurHP())/play.getHP()*100));
          MP.setValue((int)((play.getCurMP())/play.getMP()*100));
          HP.setVisible(true);
          HP.setStringPainted(true);
          MP.setStringPainted(true);
          fr.setVisible(true);
          if(expN[curLvl]>0)
               EXP.setValue((int)(((curExp-expN[curLvl-1])/(expN[curLvl]-expN[curLvl-1]))*100));
          else
               EXP.setValue((int)(curExp/2));
          for(int x=0;x<HPn;x++){
               System.gc();
               if(play.getCurHP()!=play.getHP()){
                    //System.out.println("1");
                    play.setCurHP(play.getCurHP()+1);
                    HP.setValue((int)(((double)play.getCurHP())/(double)play.getHP()*100));
                    hp.setText("HP  "+ play.getCurHP() + "/" + play.getHP());
               else
                    x=HPn;
          //fr.setVisible(true);
          for(int x=0;x<MPn;x++){
               System.gc();
               if(play.getCurMP()!=play.getMP()){
                    //System.out.println("2");
                    play.setCurMP(play.getCurMP()+1);
                    MP.setValue((int)(((double)play.getCurMP())/(double)play.getMP()*100));
                    mp.setText("MP  "+ play.getCurMP() + "/" + play.getMP());
               else
                    x=MPn;
          //fr.setVisible(true);
          for(int x=0;x<exp;x++){
               System.gc();
               curExp++;
               Exp.setText("Experience  "+ curExp+ "/" + expN[curLvl]);
               if(expN[curLvl]>0)
                    EXP.setValue((int)(((double)(curExp-expN[curLvl-1])/(double)(expN[curLvl]-expN[curLvl-1]))*100));
               else
                    EXP.setValue((int)(curExp/2));
               if(curExp==expN[curLvl]){
                    curLvl++;
                    play.setLevel(curLvl);
                    lvl.setText("You are currently level " + curLvl);
                    JOptionPane.showMessageDialog(null,"LEVEL UP");
                    Exp.setText("Experience  "+ curExp+ "/" + expN[curLvl]);
                    EXP.setValue((int)(((curExp-expN[curLvl-1])/(expN[curLvl]-expN[curLvl-1]))*100));
          try{
               Thread.sleep(2000);
          catch(Exception e){
          //fr.setVisible(true);
          play.setExp(play.getExp()+exp);
          fr.setVisible(false);     
class testclass{
     public static void main(String args[]){
          ExpScreen ex = new ExpScreen(new Stats(1,500,500,1,1,1,1,1,1,100,false,100),99,100,50,2,199);
}Here is stats:
public class Stats{
     private int level;
     private int HP;
     private int MP;
     private int curHP;
     private int curMP;
     private int str;
     private int mag;
     private int def;
     private int mDef;
     private int eva;
     private int dex;
     private int lim;
     private boolean death;
     private int exp;
     public Stats(int LEVEL,int hp, int mp, int STR, int MAG,int DEF, int MDEF, int EVA, int DEX, int LIM,boolean DEATH){
          level=LEVEL;
          HP=hp;
          MP=mp;
          curHP=hp;
          curMP=mp;
          str=STR;
          mag=MAG;
          def=DEF;
          mDef=MDEF;
          eva=EVA;
          dex=DEX;
          lim=LIM;
          death=DEATH;
          exp=0;
     public String toString(){
          return level+" "+HP+" "+MP+" "+curHP+" "+curMP+" "+mag+" "+death;
     public Stats(int LEVEL,int hp, int mp, int STR, int MAG,int DEF, int MDEF, int EVA, int DEX, int LIM,boolean DEATH,int EXP){
          level=LEVEL;
          HP=hp;
          MP=mp;
          curHP=hp;
          curMP=mp;
          str=STR;
          mag=MAG;
          def=DEF;
          mDef=MDEF;
          eva=EVA;
          dex=DEX;
          lim=LIM;
          death=DEATH;
          exp=EXP;
     public int getExp(){
          return exp;
     public void setExp(int EXP){
          exp=EXP;
     public boolean death(){
          return death;
     public int getCurHP(){
          return curHP;
     public int getLimit(){
          return lim;
     public int getCurMP(){
          return curMP;
     public void setLimit(int num){
          lim=num;
     public void setCurHP(int num){
          curHP=num;
     public void setCurMP(int num){
          curMP=num;
     public int getHP(){
          return HP;
     public int getMP(){
          return MP;
     public int getLevel(){
          return level;
     public void setLevel(int a){
          level=a;
     public int getStr(){
          return str;
     public int getMag(){
          return mag;
     public int getDef(){
          return def;
     public int getMDef(){
          return mDef;
     public int getEva(){
          return eva;
     public int getDex(){
          return dex;
}

Answer 1

It's amazing that such cleatr code would act inconsistently.
Note that your posted code will create a 'memory leak' because you never call dispose on the Frame.
You are also calling System.gc() which is generally not a good idea and you have two loops that look very similar if not identical.
Other than that, I have no idea without seeing how you call it from the other method. Of course, with all those constructor parameters, I doubt I'd seee the problem then either.