java中的文件读取和排序

如何实现从文件中读取字符后，排序，然后将其写入到另一文件中？请高手指教！

如在文件a中有以下数据：

2

8

5

9

4

1

写入到另一文件b后变为：

1

2

4

5

8

9

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;

public class Test{
public static void main(String[] args) throws Exception
{
String fromPath = "D:\\a.txt";//源文件路径
String toPath = "D:\\b.txt";//排序后文件存放路径
ArrayList list = new ArrayList();
BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(fromPath)));
String str = "";
while((str=br.readLine())!=null){
list.add(Integer.parseInt(str));
}
Collections.sort(list);
PrintWriter pw = new PrintWriter(toPath);
Iterator it = list.iterator();
while(it.hasNext()){
pw.println(it.next());
}
br.close();
pw.flush();
pw.close();
}

}