Upper.java

package com.thealgorithms.strings;

public final class Upper {
 private Upper() {
 }

 /**
 * Driver Code
 */
 public static void main(String[] args) {
 String[] strings = {"ABC", "ABC123", "abcABC", "abc123ABC"};
 for (String s : strings) {
 assert toUpperCase(s).equals(s.toUpperCase());
 }
 }

 /**
 * Converts all the characters in this {@code String} to upper case.
 *
 * @param s the string to convert
 * @return the {@code String}, converted to uppercase.
 */
 public static String toUpperCase(String s) {
 if (s == null) {
 throw new IllegalArgumentException("Input string cannot be null");
 }
 if (s.isEmpty()) {
 return s;
 }

 StringBuilder result = new StringBuilder(s.length());

 for (int i = 0; i < s.length(); ++i) {
 char currentChar = s.charAt(i);
 if (Character.isLowerCase(currentChar)) {
 result.append(Character.toUpperCase(currentChar));
 } else {
 result.append(currentChar);
 }
 }
 return result.toString();
 }
}