I cannot get it to display the grade when you enter a value of 0-100.  The dialogue box keeps looping with no end.  Please help with my java code below:

import javax.swing.JOptionPane;
public class GradeCalcDiao {

public static void main(String[] args)
{
String strYourGrade;
int NumberofGrades = 0;
int grade = 0;
int min = 100;
int max = 0;
double avg = 0;
int sum = 0;
int high = 0;
int low = 0;
int A,B,C,D,F;

A=0;
B=0;
C=0;
D=0;
F=0;

strYourGrade = JOptionPane.showInputDialog("Enter grades in range of 0 - 100");

while(true){

if(grade != -1){

sum = sum + grade;
}

else if(grade >= 0);


else if(grade >= 90 && grade 
JOptionPane.showMessageDialog(null, "Your grade is A");
}
else if(grade >= 80 && grade 
JOptionPane.showMessageDialog(null, "Your grade is B");
}
else if(grade >= 70 && grade 
JOptionPane.showMessageDialog(null, "Your grade is C");
}
else if(grade >= 60 && grade 
JOptionPane.showMessageDialog(null, "Your grade is D");
}
else if(grade >= 0 && grade 
JOptionPane.showMessageDialog(null, "Your grade is F");
}

else if(grade min = grade;
else if(grade > max)
max = grade;



}
}
}