public class Test4 {
    public static void main(String[] args) {
        computer4 c1 = new computer4('a');
        computer4 c2 = new computer4('b');
        c1.start();
        c2.start();

    }

}

class computer4 extends Thread {
    char ch;
    Object obj = new Object();

    computer4(char ch) {
        this.ch = ch;
    }

    public void print(char ch) {

        for (int i = 0; i < 10; i++) {
            System.out.print(ch);
        }
    }

    public void run() {
        synchronized (obj) {
            print(this.ch);
            System.out.println();
        }

    }
}

synchronized (computer4.class)

------解决方案--------------------
Java code

Object obj = new Object();