ReplaceAll и toUpperCase

import java.io.PrintWriter;
import java.util.Scanner;
 
public class Main {
    static String regexp = "(_)([a-z]+)";
    public static void main(String[] args) throws Exception {
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        String str;
        str = in.nextLine();
        str = str.replaceAll(regexp, "$2".toUpperCase());
        out.println(str);
        out.flush();
        out.close();
    }
}

import java.io.PrintWriter;
import java.util.Scanner;
 
public class Main {
 
    public static void main(String[] args) throws Exception {
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        final char[] arr = in.nextLine().toCharArray();
        for (int i = 0; i < arr.length - 1; i++) {
            if (arr[i] == '_') {
                arr[i + 1] = Character.toUpperCase(arr[i + 1]);
            }
        }
        out.println(String.valueOf(arr));
        out.flush();
        out.close();
    }
}

Matcher mtch = Pattern.compile("(?<=_)[a-z]").matcher(str);
while (mtch.reset(str).find()) 
    str = mtch.replaceFirst(mtch.group().toUpperCase());