How do I check to see if a character is NOT contained in a String?

Question

I am trying to create a program which will read in user inputted text. If the text contains an email address, then the program will output it to screen. However, I also need to put some sort of fall back in there, in case an email address is not contained in the string.
This is the part I am having problems with. If I enter an email address, it will output it to screen. Maybe someone can help me with a "no valid mail" message.
I decided to use the '@' character as a reference point, since it is a unique character and should not be used in normal sentances. Since an email address is a continous block of text with no spaces in it,I created 2 substrings - one from '@' going backwards towards the previous space, and one from '@' going forward to the next space.
First, I tried the following:
//if(input.indexOf('@') == -1){
     // System.out.println("-no valid email-");
The program ran and when I entered text containing an email adress, it displayed it on screen. If I didn't include one, it gave me an error: java.lang.StringIndexOutOfBoundsException: -1
I'm guessing this is happening because if there is no '@' symbol in the user input, and so there is an error with the indexOf('@') statement. Any way to get around this?
Then I tried the following, bearing in mind the variable length is defined as "input.length()".
for(int i = 0; i <= length; i++){
     if(input.charAt(i) = '@'){
          break;
     if(i == length){
     System.out.println("-no valid email-");
this gives me an "unexpected type" error with the if(input.charAt(i) = '@') statement.
It says it has found a value, but requires a variable. Is the "i" in the FOR loop not a variable?
Please can anybody help me. Any suggestions appreciated.

Answer 1

Whenever I run the program, it is ok if I input an email address among the text. IT displays the email address to screen. However, if no email address is entered (ie. just a string of text), it displays the message "-no valid email". But it still gives me an error:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String ind
ex out of range: -1
at java.lang.String.charAt(String.java:558)
at CA3a.main(CA3a.java:56)
Here is my code in full:
//I decided to use the '@' character as a reference point, since it is a unique character
//and should not be used in normal sentances. Since an email address is a continous block
//of text with no spaces in it,I created 2 substrings - one from '@' going
//backwards towards the previous space, and one from '@' going forward to the next space.
class CA3a{
   public static void main(String args[]){
//variable declerations
     String input;
     int indexat, length;
     String part1 = "";
     String part2 = "";
//prompt user for input
     System.out.println("This program will detect the 1st email address in a string of text.");
     System.out.println("Please enter a string of text");
//take in input into a stringbuffer
     input = Keyboard.readString();
     StringBuffer buff = new StringBuffer(input);
//assignments
     length = input.length();
     indexat = input.indexOf('@');
//check to see if '@' character is present in string
     if(indexat < 0){
        System.out.println("-no valid email-");
//check for substring part1
     for(int i = indexat; i >= 0; i--){
        if(i == 0){
          part1 = buff.substring(i, indexat);
          break;
        if(input.charAt(i-1) == ' '){
          part1 = buff.substring(i, indexat);
          break;
//check for substring part2
     for(int i = indexat; i <= length; i++){
        if(i == length){
          part2 = buff.substring(indexat, i);
          break;
        if(input.charAt(i) == ' '){
          part2 = buff.substring(indexat, i);
          break;
//output the 2 substring variables to screen.
System.out.println(part1 + part2);
}