How can I run multiple commands in just one cmd windows in Java?

With && you can execute more than one commands, one after another:

Runtime.getRuntime().exec("cmd /c \"start somefile.bat && start other.bat && cd C:\\test && test.exe\"");

Using multiple commands and conditional processing symbols

You can run multiple commands from a single command line or script using conditional processing symbols. When you run multiple commands with conditional processing symbols, the commands to the right of the conditional processing symbol act based upon the results of the command to the left of the conditional processing symbol.

For example, you might want to run a command only if the previous command fails. Or, you might want to run a command only if the previous command is successful. You can use the special characters listed in the following table to pass multiple commands.

& [...] command1 & command2
Use to separate multiple commands on one command line. Cmd.exe runs the first command, and then the second command.

&& [...] command1 && command2
Use to run the command following && only if the command preceding the symbol is successful. Cmd.exe runs the first command, and then runs the second command only if the first command completed successfully.

|| [...] command1 || command2
Use to run the command following || only if the command preceding || fails. Cmd.exe runs the first command, and then runs the second command only if the first command did not complete successfully (receives an error code greater than zero).

( ) [...] (command1 & command2)
Use to group or nest multiple commands.

; or , command1 parameter1;parameter2
Use to separate command parameters.

I would use Java's ProcessBuilder or another class which simulates/uses a shell. The following snippet demonstrates the idea (for Linux with bash).

import java.util.Scanner;
import java.io.*;

public class MyExec {
    public static void main(String[] args)
    {
        //init shell
        ProcessBuilder builder = new ProcessBuilder( "/bin/bash" );
        Process p=null;
        try {
            p = builder.start();
        }
        catch (IOException e) {
            System.out.println(e);
        }
        //get stdin of shell
        BufferedWriter p_stdin = 
          new BufferedWriter(new OutputStreamWriter(p.getOutputStream()));

        // execute the desired command (here: ls) n times
        int n=10;
        for (int i=0; i<n; i++) {
            try {
                //single execution
            p_stdin.write("ls");
            p_stdin.newLine();
            p_stdin.flush();
            }
            catch (IOException e) {
            System.out.println(e);
            }
        }

        // finally close the shell by execution exit command
        try {
            p_stdin.write("exit");
            p_stdin.newLine();
            p_stdin.flush();
        }
        catch (IOException e) {
            System.out.println(e);
        }

    // write stdout of shell (=output of all commands)
    Scanner s = new Scanner( p.getInputStream() );
    while (s.hasNext())
    {
        System.out.println( s.next() );
    }
       s.close();
    }
}

Please note that it is only a snippet, which needs to be adapted for Windows, but in general it should work with cmd.exe.