Attempting to change characters to upper case in a mexican wave style pattern

I'm trying to take a string and convert one character at a time to a capital letter, in a mexican wave style pattern. This is what it should look like

Input: hello

Output: Hello
        hEllo
        heLlo
        helLo
        hellO

However, what I am getting is just

Input: hello

Output: hello
        hello
        hello
        hello
        hello

This is what my loop currently looks like

for(int i=0;i<s.length();i++)
   {
     a1=s.charAt(i);
     a2=Character.toUpperCase(a1);
     System.out.println(s);
    }

s being the String with a1 and a2 being two random chars to store the current value in.

I'm not familiar with the "Character.toUpperCase" but I found it online hoping it would help to solve my problems however it doesnt seem to be

You are printing the same String. To get the desired output, you can do something like following,

String s = "hello";
for(int i=0;i<s.length();i++)
{
    char a1 = s.charAt(i);
    char a2 = Character.toUpperCase(a1);
    if (i == 0)
        System.out.println(a2 +s.substring(i+1));
    else
        System.out.println( s.substring(0,i) +a2 +s.substring(i+1));
}

If yo want to skip if-else condition,

String s = "hello";
System.out.println(s.charAt(0) +s.substring(1)); // for first char
for(int i=1;i<s.length();i++) // for rest of the chars
{
    char a1 = s.charAt(i);
    char a2 = Character.toUpperCase(a1);
    System.out.println( s.substring(0,i) +a2 +s.substring(i+1));
}

String is an immutable class. You can't change a String in place. Hence setting a character in the String to uppercase does not change it.

What you need to do is either use something that is mutable, like StringBuilder or create a new String each time you want to make a change.

The below code creates a new String. First I get an array of the individual characters in the source String - which is hello according to your question. Then I iterate through the characters in the array setting each one, in turn, to uppercase while at the same time reverting the previously changed character back to lowercase. And in each iteration I create a new String for the current array of letters.

char[] letters = "hello".toCharArray();
for (int i = 0; i < letters.length; i++) {
    if (i - 1 >= 0) {
        letters[i - 1] = Character.toLowerCase(letters[i - 1]);
    }
    letters[i] = Character.toUpperCase(letters[i]);
    System.out.println(new String(letters));
}

The result of running the above code is:

Hello
hEllo
heLlo
helLo
hellO