关于使用线程调用run方法的问题

/**
 * 
 */

public class TwoThread extends Thread {

    private Thread creatorThread;
    
    public TwoThread(){
        creatorThread = Thread.currentThread();
    }
    
    public void run(){
        for(int i=0; i<5; i++){
            printMsg();
        }
    }
    
    public void printMsg(){
        Thread t = Thread.currentThread();
        if(t==creatorThread){
            System.out.println("Creator thread");
        } else if(t==this){
            System.out.println("New thread");
        }
    }
    
    /**
     * @param args
     */
    public static void main(String[] args) {
        
        TwoThread tt = new TwoThread();
        tt.start();
        for(int i=0; i<10; i++){
            tt.printMsg();
        }
    }

}

大家好，该程序在调用tt.start()方法时为什么没有直接去调用run()方法，而是执行完for循环后才调用的run()方法?