//NOTE: this isn't the full Stream class, but contains stuff you should add
//on to your existing class.
public class Stream extends Thread {

    private boolean triggerStartCrawling = false;
    private Thread crawlingThread = ...;

    public void run() {
        while(true) {
            if(triggerStartCrawling) {
                triggerStartCrawling = false;
                System.out.println("Started crawling!");
                //Start crawling
                crawlingThread.start();
            }
            //Hopefully, you understand the idea here- constantly check for a boolean being true, in which case execute
            //some code, but if not, simply try again. You can do this with other operations as well.
            //The crawling code can be encased within another thread, so it is simple to stop it at any time with      //java.lang.Thread.join()
        }
    }

    public void triggerStartCrawling() {
        triggerStartCrawling = true;
    }

    public void stopCrawling() throws InterruptedException {
        crawlingThread.join();
    }
}