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Java Program: Convert Lowercase ↔ Uppercase Without Built-in Methods

Java

import java.util.Scanner;
 
public class CaseConversionManual {
    public static void main(String[] args) {
 
        Scanner sc = new Scanner(System.in);
 
        System.out.print("Enter a character: ");
        char ch = sc.next().charAt(0);
 
        char converted;
 
        // Check if lowercase (ASCII 97–122)
        if (ch >= 'a' && ch <= 'z') {
            converted = (char) (ch - 32);  // Convert to uppercase
            System.out.println("Converted character: " + converted);
        }
        // Check if uppercase (ASCII 65–90)
        else if (ch >= 'A' && ch <= 'Z') {
            converted = (char) (ch + 32);  // Convert to lowercase
            System.out.println("Converted character: " + converted);
        }
        else {
            System.out.println("Not an alphabet character.");
        }
 
        sc.close();
    }
}

Output

 
OUTPUT 1:
Enter a character: g
Converted character: G 

OUTPUT 2:
Enter a character: T
Converted character: t

OUTPUT 3:
Enter a character: 5
Not an alphabet character.

Explanation

✅ ASCII Range Logic

'a' to 'z' → 97 to 122
'A' to 'Z' → 65 to 90

The difference between uppercase and lowercase letters is 32.

✅ Conversion Rules

Lowercase → Uppercase

converted = (char)(ch - 32);

Uppercase → Lowercase

converted = (char)(ch + 32);

✅ Why No Built-in Methods?

Instead of using:

Character.toUpperCase()
Character.toLowerCase()

The program manually converts using ASCII arithmetic.

Java Programs

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Introduction to Java

Java Program: Convert Lowercase ↔ Uppercase Without Built-in Methods

Output

Explanation