stuck on this assignment i have been trying to do for the last 6 hours. Quite new to java would love some feedback [duplicate]

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So I have an assignment and the directions is as follows : Implement a program such that, it asks the user for a grade, and then it prints the corresponding letter. If the user enters a grade lower than zero or higher than 10, print an error message (as in example below). ALL GRADES MUST END IN EITHER .0 or .5, including 10.0 and 0.0.

Basically if a user inputs for example a "4" then that would cause the output to be : "Error 4" but if it was say 4.0 it would say "Your letter grade is F"

This is my code please somebody help me get on the right path because I am quite new at this and completely lost!!!

import java.util.Scanner;

class Grades{

public static void main(String [] args){

  String grade;
  String score = "0";

  Scanner gradeScan = new Scanner(System.in);
  System.out.print("Please enter the grade you earned: ");
  String s = score;
  score = gradeScan.nextLine();

     if(s.matches("^\\d+")){
     System.out.println("Error " + score); }

    else if (score == "10.0"){
        grade = "A";   
    System.out.println("letter grade is " + grade);}
    else if (score == "9.5"){
        grade = "A";
    System.out.println("letter grade is " + grade);}
    else if (score == "9.0"){
        grade = "A";
    System.out.println("letter grade is " + grade);}
    else if (score == "8.5"){
        grade = "A";
    System.out.println("letter grade is " + grade);}
    else if (score == "8.0"){
        grade = "B";
    System.out.println("letter grade is " + grade);}
    else if (score == "7.5"){
        grade = "B";
    System.out.println("letter grade is " + grade);}
    else if (score == "7.0"){
        grade = "C";
    System.out.println("letter grade is " + grade);}
    else if (score == "6.5"){
        grade = "C";
    System.out.println("letter grade is " + grade);}
    else if (score == "6.0"){
        grade = "D";
    System.out.println("letter grade is " + grade);}
    else if (score == "5.5"){
        grade = "D";
    System.out.println("letter grade is " + grade);}
    else if (score == "5.0"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "4.5"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "4.0"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "3.5"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "3.0"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "2.5"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "2.0"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "1.5"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "1.0"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "0.5"){
        grade = "F";
    System.out.println("letter grade is " + grade);}
    else if (score == "0.0"){
        grade = "F";
    System.out.println("letter grade is " + grade);}

    else{
        grade = "Error ";
    System.out.println(grade + score);}

  gradeScan.close();

} }