問題

編寫一個程式，程式會啟動4個執行緒，向4個檔案A，B，C，D裡寫入資料，每個執行緒只能寫一個值。 
    執行緒A：只寫1
    執行緒B：只寫2 
    執行緒C：只寫3 
    執行緒D：只寫4 

4個檔案A，B，C，D。 

程式執行起來，4個檔案的寫入結果如下： 
    A：12341234...
    B：23412341... 
    C：34123412... 
    D：41234123...

不解釋，直接上程式碼

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;

/**
 * 
 * @author songjca
 * 
 */
public class MyThreadSEQ extends Thread {

	private String name;

	private char outchar;

	private MyThreadSEQ next = null;

	private Queue<FileOutputStream> outSet = new ConcurrentLinkedQueue<FileOutputStream>();

	public MyThreadSEQ(char outchar) {
		this.outchar = outchar;
		this.name = "thread" + this.outchar;
	}

	public char getOutchar() {
		return this.outchar;
	}

	public void setOutchar(char outchar) {
		this.outchar = outchar;
	}

	public MyThreadSEQ getNext() {
		return this.next;
	}

	public void setNext(MyThreadSEQ next) {
		this.next = next;
	}

	public Queue<FileOutputStream> getOutSet() {
		return this.outSet;
	}

	public void addOut(FileOutputStream out) {
		this.outSet.add(out);
	}

	@Override
	public void run() {
		try {
			while (true) {
				int i = 0;
				while (this.outSet.size() != 0) {
					FileOutputStream out = this.outSet.poll();
					synchronized (out) {
						System.out.println("----------" + name + " start ---------");
						out.write(this.outchar);
						i = i + 1;
						if (i == 4) {
							out.write('\r');
							out.write('\n');
							i = 0;
						}
						out.flush();

						this.next.addOut(out);
						out.notifyAll();
						System.out.println("----------" + name + " end ---------");
					}

				}
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public static void main(String... args) {
		try {
			MyThreadSEQ t1 = new MyThreadSEQ('1');
			MyThreadSEQ t2 = new MyThreadSEQ('2');
			MyThreadSEQ t3 = new MyThreadSEQ('3');
			MyThreadSEQ t4 = new MyThreadSEQ('4');
			t1.setNext(t2);
			t2.setNext(t3);
			t3.setNext(t4);
			t4.setNext(t1);
			FileOutputStream out1 = new FileOutputStream("d:/a.txt");
			FileOutputStream out2 = new FileOutputStream("d:/b.txt");
			FileOutputStream out3 = new FileOutputStream("d:/c.txt");
			FileOutputStream out4 = new FileOutputStream("d:/d.txt");
			t1.addOut(out1);
			t2.addOut(out2);
			t3.addOut(out3);
			t4.addOut(out4);
			t1.start();
			t2.start();
			t3.start();
			t4.start();
		} catch (FileNotFoundException e) {
			// e.printStackTrace();
		} finally {
		}
	}
}