I'm a newbie to Java, so I'm sorry if this is a silly question. I'll post the code first, then explain my problem.

import java.util.Scanner;

class CharacterOperations{

    public void run(){
        System.out.println("1. toUpper");
        System.out.println("2. toLower");
        System.out.println("3. Alphabet");
        System.out.println("4. Quit");
        
        int input;
        Scanner sc = new Scanner(System.in);
        input = sc.nextInt();
        
        while (input == 1 || input == 2 || input == 3){
            input = sc.nextInt();
            if (input == 1){
                System.out.println(toUpper());
            }
        }
    }
    public void toUpper(char lowerCase){
        int decimal;
        decimal = (int) lowerCase - 32;
        if(decimal >= 65 && decimal <= 90){
            System.out.println( (char) decimal);
        }
        
    }
    public void toLower(char upperCase){
        int decimal;
        decimal = (int) upperCase + 32;
        if(decimal >= 97 && decimal <= 122){
            System.out.println( (char) decimal);
        }
    }
public void alphabet(char start, char end){
        int decimalStart = (int) start;
        int decimalEnd = (int) end;
        
        while(decimalStart != decimalEnd){
            
            if(( (int) start >= 65 && (int) start <= 90) && ( (int) end >= 97 && (int) end <= 122)){
                decimalStart = (int) start + 32;
                System.out.println( (char) decimalStart);
                start++;
            }
            else if(( (int) start >= 97 && (int) start <= 122) && ( (int) end >= 65 && (int) end <= 90)){
                decimalEnd = (int) end + 32;
                end = (char) decimalEnd;
                System.out.println( (char) decimalStart);
                start++;
            }
             else {
               decimalStart = (int) start;
                System.out.println( (char) start);
                start++;
            }
                
        }
        
    }
}

When the user inputs 1, 2, or 3 into the terminal window, I want the run method to call the toUpper, toLower, or alphabet method, respectively. The methods work properly when selected from an object (I'm using BlueJ) but I can't seem to properly call the methods within the run method. As the code stand now, I get the following error when I try to compile: "toUpper(char) in CharacterOperations cannot be applied to ()". I can't use the class Character from the Java library, nor can the class contain properties.
Can anyone tell me what I'm doing wrong?

The method toUpper has a parameter, when you call the method you are not sending it a parameter. Line 18 is where you call the toUpper method.
This is how is should look.

toUpper('a');

The reason why I took the System.out.println out of the code is because the method is void, which means it doesn't return anything that can be printed out. And because there is a print statement in the method.

Yes, actually. I didn't realize that calling toUpper doesn't seem to do anything until I've called the alphabet method, and toLower doesn't seem to do anything when called through the run method.

You can use the scanner to read in a char,

import java.util.*;

public class readInChar
{
    public static void main(String[] args)
	{
		Scanner scan = new Scanner(System.in);
		String s;
		char u; 
		System.out.println("Enter you char: ");
		s = scan.next();
		u = s.charAt(0);
		System.out.println(u);
	}
}

scan.next() returns a String, that's why I used the charAt(0) method. The 0 in this method refers to the first element in the String.
Include reading in the userChar in the while loop and send that a parameter in the method calls and you should be good to go.

That works for the first two methods, but I'm having trouble applying to the alphabet method.

else if(input == 3){
                String str;
                str = sc.next();
                char letter1 = str.charAt(0);
                String str2 = sc.next();
                char letter2 = str.charAt(0);
                alphabet(letter1, letter2);
                input = sc.nextInt();
                
            }

That yields a vertical blinking line that just keeps accepting user inputs.

This question has already been answered. Start a new discussion instead.