trim @domain.xxx from email leaving just username

To find: int index = string.indexOf('@');

To replace: email = email.substring(0, index);

To summarize:

email = "[email protected]";
int index = email.indexOf('@');
email = email.substring(0,index);

Another approach is to split an email on a nickname and on a domain. Look at javadoc

There is a code example:

String email = "[email protected]";
String[] parts = email.split('@');

// now parts[0] contains "example"
// and parts[1] contains "domain.com"