EditText condition loop Android Studio

I have an EditText that I need to contain at least one character.
In case it doesn't, I prompt a Toast and ask the user to try again.
The problem is that the loop goes infinity \ the phone is stuck.
I assume that it's a simple question, yet I cant figure out what to do.
I'm trying to read from user the Course name, grade, and points. I need to have at least one char in the course name. Probably something wrong with the logic. Any hints?

public void readSubjectName() {
    grades.add(new Grade()); // Create new grade
    String tempSubject = inputLabel.getText().toString();
    /** If the Subject name does not contain any characters,
     *  assume it's bad, toast and ask again.
     */
    if (!(tempSubject.matches(".*[a-z].*"))) { // Subject Name does not contain any characters
        Toast.makeText(getApplicationContext(), "Please enter a valid course name", Toast.LENGTH_LONG).show();
        while (!(tempSubject.matches(".*[a-z].*"))) {
            Toast.makeText(getApplicationContext(), "Please enter a valid course name", Toast.LENGTH_LONG).show();
        }
    }
    grades.get(grades.size() - 1).setSubject(tempSubject); //Add subject
    inputLabel.setText(null);
    inputLabel.setHint("Enter Grade");
    subjectButton.setVisibility(View.INVISIBLE);
    gradeButton.setVisibility(View.VISIBLE);
}

Try this code

public void readSubjectName() {
    grades.add(new Grade()); // Create new grade
    String tempSubject = inputLabel.getText().toString();
    
    if (tempSubject.isEmpty()) { 
        Toast.makeText(getApplicationContext(), "Please enter a valid course name", Toast.LENGTH_LONG).show();


      //inputLabel.setError("Fill this field"); TODO: Check whether this works else display Toast Message
      inputLabel.requestFocus();

    }
    grades.get(grades.size() - 1).setSubject(tempSubject); //Add subject
    inputLabel.setText(null);
    inputLabel.setHint("Enter Grade");
    subjectButton.setVisibility(View.GONE);
    gradeButton.setVisibility(View.VISIBLE);
}

Hope this helps. Feel free to ask for clarifications...

With below code, you can evaluate the length of the string/text entered by the user and check if it matches your criterion,

       //read value from EditText to a String variable
        val tempSubject: String = inputLabel.text.toString()

        //check if the EditText have values or not
        if(tempSubject.trim().length>1) {
            Toast.makeText(applicationContext, "Message : "+tempSubject, Toast.LENGTH_SHORT).show()
        }else{
            Toast.makeText(applicationContext, "Please enter some message! ", Toast.LENGTH_SHORT).show()
        }

So I figured out what was mistake, I needed to keep the current state and not move to the next state until I get a proper input, using simple the "else" statement:

        //check if the EditText have values or not
    if (!(tempSubject.matches(".*[a-z].*"))) {
        inputLabel.setError("Invalid name!");
        inputLabel.requestFocus();
    } else {  
        grades.get(grades.size() - 1).setSubject(tempSubject); //Add subject
        inputLabel.setText(null);
        inputLabel.setHint("Enter Grade");
        subjectButton.setVisibility(View.INVISIBLE);
        gradeButton.setVisibility(View.VISIBLE);
    }

I was missing the else statement, the rest of the code was out of the condition so the code just continued regardless.