Method and Loop to check substring of Array List doesn't seem to be returning correct value in Java

working on a few different methods to check that a code entered meets the criteria of 3 Characters and 3 Intergers i.e. ABC123 or JJJ111 etc I then also need to check if this code is used in an array list of objects that include a name and code i.e. Programming in Java, ITC206.

I have written two methods in a subject class to check the rerquirements are met, that the code doesnt already exist, but I am having a couple of issues. the first one isValidCode, seems to run through the loop but only returns false is there are not 6 characters, the .isAlphabetic and .isDigit methods don't seem to be affecting the outcome.

The second method, being codeExists just returns false all the time, even if the code is a new one and not being used.

I have tried changing a couple of things around and can't seem to figure out what is wrong with either..

Below are the methods and the main class I am using to check.

public boolean isValidCode (String subjectCode){
boolean validCode = true;
if (subjectCode.length() !=6){
validCode = false;               
String codeLetters = subjectCode.substring(0, 3);
for (int i = 0; i < codeLetters.length(); i++){
Character letters = subjectCode.charAt(i);
if (!Character.isAlphabetic(letters)) 
validCode = false; break;
}
String codeDigits = subjectCode.substring(3, subjectCode.length());
for (int j = 3; j < codeDigits.length(); j++){
Character digits = subjectCode.charAt(j);
if (!Character.isDigit(digits)) 
validCode = false; break;
} 
}                 
return validCode;
}
public boolean codeExist (String code, List<Subject> subjectList){
boolean exists = false;
for (int i = 0; i < subjectList.size(); i++){
Subject newSubject = subjectList.get(i);
String subject = newSubject.getSubjectCode();
if (subject.equals(code)){  
}
}   
return exists;
}
public static void main(String[] args) {
// create array list        
List<Subject> subjectList = new ArrayList<>();
Scanner input = new Scanner(System.in); // inport scanner
//add a new object ot the array list
subjectList.add(new Subject("Programming in Java", "ITC206"));
subjectList.add(new Subject("Cyber Ethics", "ITC311"));
subjectList.add(new Subject("IT Helpdesk", "ITC232"));
showSubjectList(subjectList);
//user to add name and codes to list
String newName = "Test";
String newCode = "AAA123";
Subject newSubject = new Subject(newName, newCode);
boolean valid = newSubject.isValidCode(newCode);
System.out.println(valid);
boolean exists = newSubject.codeExist(newCode, subjectList);
System.out.println(exists);
showSubjectList(subjectList);

codeExist() does return the variable exists which is always false. You should just return true if your check matches.

public boolean codeExist (String code, List<Subject> subjectList){
for (int i = 0; i < subjectList.size(); i++){
Subject newSubject = subjectList.get(i);
String subject = newSubject.getSubjectCode();
if (subject.equals(code)){  
return true;
}
}   
return false;
}

I would use regex in your isValidCode method to check if your code follows a certain pattern:

in your case "[A-Z]{3}[0-9]{3}" 3 capital letters followed by 3 digits:

public boolean isValidCode (String subjectCode){
String regex = "[A-Z]{3}[0-9]{3}";
return subjectCode.matches(regex);
}

Your codeExist method, provided you use java 8 or higher, can be rewritten as follows:

public boolean codeExist (String code, List<Subject> subjectList){
return subjectList.stream()
.map(Subject::getSubjectCode)
.anyMatch(code::equals);
}