Работа с многопоточностью "Thread"

public class SampleThreads {
    public static void main(String[] args) {
        int count = 10;
        Counter counter = new Counter();
 
        Thread one = new Thread(new MyThread(counter, count, "+", 500), "One");
        Thread two = new Thread(new MyThread(counter, count, "-", 400), "Two");
        Thread three = new Thread(new MyThread(counter, count, "*", 700), "Three");
        Thread four = new Thread(new MyThread(counter, count, "/", 300), "Four");
 
        one.start();
        two.start();
        three.start();
        four.start();
    }
}
 
class MyThread implements Runnable {
    private final Counter counter;
    private int amount;
    private final String operation;
    private final int delayMS;
 
    MyThread(Counter counter, int amount, String operation, int delayMS) {
        this.counter = counter;
        this.amount = amount;
        this.operation = operation;
        this.delayMS = delayMS;
    }
 
    @Override
    public void run() {
        int number;
        int result;
        try {
            while (!Thread.currentThread().isInterrupted() && this.amount != 0) {
                number = this.counter.getCount();
                result = solution(number, this.operation);
                this.counter.setCount(result);
                System.out.printf("Thread [%s]: %d %s 2 = %d;\n",
                        Thread.currentThread().getName(), number, this.operation, result);
                this.amount--;
                Thread.sleep(this.delayMS);
            }
        } catch (InterruptedException e) {
            System.out.println("Thread [" + Thread.currentThread().getName() + "] is stopped.");
        }
    }
 
    private int solution(int number, String operation) {
        if (operation != null) {
            if (operation.equals("+")) {
                number += 2;
            } else if (operation.equals("-")) {
                number -= 2;
            } else if (operation.equals("*")) {
                number *= 2;
            } else {
                number /= 2;
            }
        }
        return number;
    }
}
 
class Counter {
    private int count;
 
    public int getCount() {
        return count;
    }
 
    public void setCount(int count) {
        this.count = count;
    }
 
    @Override
    public String toString() {
        return String.format("%d", this.count);
    }
}