How can I check if a single character appears in a string?

You can use string.indexOf('a').

If the char a is present in string :

it returns the the index of the first occurrence of the character in the character sequence represented by this object, or -1 if the character does not occur.

• String.contains() which checks if the string contains a specified sequence of char values
• String.indexOf() which returns the index within the string of the first occurence of the specified character or substring (there are 4 variations of this method)

I'm not sure what the original poster is asking exactly. Since indexOf(...) and contains(...) both probably use loops internally, perhaps he's looking to see if this is possible at all without a loop? I can think of two ways off hand, one would of course be recurrsion:

public boolean containsChar(String s, char search) {
    if (s.length() == 0)
        return false;
    else
        return s.charAt(0) == search || containsChar(s.substring(1), search);
}

The other is far less elegant, but completeness...:

/**
 * Works for strings of up to 5 characters
 */
public boolean containsChar(String s, char search) {
    if (s.length() > 5) throw IllegalArgumentException();

    try {
        if (s.charAt(0) == search) return true;
        if (s.charAt(1) == search) return true;
        if (s.charAt(2) == search) return true;
        if (s.charAt(3) == search) return true;
        if (s.charAt(4) == search) return true;
    } catch (IndexOutOfBoundsException e) {
        // this should never happen...
        return false;
    }
    return false;
}

The number of lines grow as you need to support longer and longer strings of course. But there are no loops/recurrsions at all. You can even remove the length check if you're concerned that that length() uses a loop.