while((remain = fileInputStream.read()) != -1)

import *;
public class FileStreamDemo {
    public static void main(String[] args) {
        try {
            byte[] buffer = new byte[1024];
            FileInputStream fileInputStream =
            new FileInputStream(new File(args[0]));
            FileOutputStream fileOutputStream =
            new FileOutputStream(new File(args[1]));
            System.out.println("复制档案：" +
            fileInputStream.available() + "字节");
            while(true) { // 从来源档案读取数据至缓冲区
                if(fileInputStream.available() < 1024) {
                    int remain;
                    while((remain = fileInputStream.read())
                    != -1) {
                        fileOutputStream.write(remain);
                    }
                    break;
                }
                else {
                    fileInputStream.read(buffer);
                    // 将数组数据写入目的档案
                    fileOutputStream.write(buffer);
                }
            }
            // 关闭串流
            fileInputStream.close();
            fileOutputStream.close();
            System.out.println("复制完成");
        }
        catch(ArrayIndexOutOfBoundsException e) {
            System.out.println(
            "using: java FileStreamDemo src des");
            e.printStackTrace();
        }
        catch(IOException e) {
            e.printStackTrace();
        }
    }
}
 int remain;
while((remain = fileInputStream.read())!= -1)
不明白是什么意思，求指导！！！