java basics

Java - Generics

It would be nice if we could write a single sort method that could sort the elements in an Integer array, a String array, or an array of any type that supports ordering. Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods, or with a single class declaration, a set of related types, respectively.

https://www.tutorialspoint.com/java/java_generics.htm

Java - Collections Framework

Prior to Java 2, Java provided ad hoc classes such as Dictionary, Vector, Stack, and Properties to store and manipulate groups of objects. Although these classes were quite useful, they lacked a central, unifying theme. Thus, the way that you used Vector was different from the way that you used Properties.

https://www.tutorialspoint.com/java/java_collections.htm

Java - Regular Expressions

Java provides the java.util.regex package for pattern matching with regular expressions. Java regular expressions are very similar to the Perl programming language and very easy to learn. A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern.

https://www.tutorialspoint.com/java/java_regular_expressions.htm

how to run a command at terminal from java program?

As others said, you may run your external program without xterm. However, if you want to run it in a terminal window, e.g. to let the user interact with it, xterm allows you to specify the program to run as parameter.

https://stackoverflow.com/questions/15356405/how-to-run-a-command-at-terminal-from-java-program