個人覺得這個解法最簡單而且好理解：

public class ToUperCase {
    public static void main(String[] args) {
        String str = "goOd gooD stUdy dAy dAy up";
        //切割儲存在陣列，再進行轉換
        String[] strings = str.split(" ");
        ArrayList<String> list = new ArrayList<>();
        for (String s : strings){
            String lowerCase = s.toLowerCase();
            list.add(lowerCase);
        }
        for (String s : list){
            String substring = s.replace(s.substring(0,1),s.substring(0,1).toUpperCase());
            System.out.print(substring+" ");
        }
    }