Write a program that reads a Java source file and produces an index of all identifiers

in the file. For each identifier, print all lines in which it occurs. For simplicity, we will

consider each string consisting only of letters, numbers, and underscores an identifier.

Use int = 1729

Declare a Scanner in for reading from the source file and call

in.useDelimiter("[^A-Za-z0-9_]+")

Then each call to next returns an identifier.

Expected output - 1, 7, 2, 9,