I vote for Karthik T's answer. you don't need to open a terminal to run commands.

For example,

// file: RunShellCommandFromJava.java
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class RunShellCommandFromJava {

    public static void main(String[] args) {

        String command = "ping -c 3 www.google.com";

        Process proc = Runtime.getRuntime().exec(command);

        // Read the output

        BufferedReader reader =  
              new BufferedReader(new InputStreamReader(proc.getInputStream()));

        String line = "";
        while((line = reader.readLine()) != null) {
            System.out.print(line + "\n");
        }

        proc.waitFor();   

    }
} 

The output:

$ javac RunShellCommandFromJava.java
$ java RunShellCommandFromJava
PING http://google.com (123.125.81.12): 56 data bytes
64 bytes from 123.125.81.12: icmp_seq=0 ttl=59 time=108.771 ms
64 bytes from 123.125.81.12: icmp_seq=1 ttl=59 time=119.601 ms
64 bytes from 123.125.81.12: icmp_seq=2 ttl=59 time=11.004 ms

--- http://google.com ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 11.004/79.792/119.601/48.841 ms

The online docs should be your first recourse: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html which tell us that the command line is

java [options] classname [args]

In more depth, and linked from somewhere on that page, you can read https://docs.oracle.com/javase/8/docs/technotes/tools/unix/classpath.html#CBHHCGFB

which is applicable to pretty much all the Java tools. These docs contain the answer to your question. You can either cd into the directory that is the root of your classpath and use the default classpath, as one answer suggested, or use the classpath options described in the docs to set the directory(-ies) at the top of your classpath. E.g.,

java -cp /Users/mac/Documents/workspace/Calculator/bin Calculator