private static boolean containsOnly(String input, char ch) {
    if (input.isEmpty())
        return false;
    for (int i = 0; i < input.length(); i++)
        if (input.charAt(i) != ch)
            return false;
    return true;
}

System.out.println(containsOnly("++++", '+')); // prints: true
System.out.println(containsOnly("----", '+')); // prints: false
System.out.println(containsOnly("+-+-", '+')); // prints: false

// escape special character '+'
input.matches("\++")

// '+' not special in a character class
input.matches("[+]+")

// if "+" is dynamic value at runtime, use quote() to escape for you,
// then use a repeating non-capturing group around that
input.matches("(?:" + Pattern.quote("+") + ")+")

^         #From start of string
[^+]*    #Match 0 or more non plus characters
+        #Match 1 plus character
[^+]*    #Match 0 or more non plus characters
$         #End of string

input.matches("[^\+]*?+[^\+]*")