You can just split the string..
public String[] split(String regex)
Note that java.lang.String.split uses delimiter's regular expression value. Basically like this...
String filename = "abc.def.ghi"; // full file name
String[] parts = filename.split("\\."); // String array, each element is text between dots
String beforeFirstDot = parts[0]; // Text before the first dot
Of course, this is split into multiple lines for clairity. It could be written as
String beforeFirstDot = filename.split("\\.")[0];