public static void main(String[] args) throws IOException {
        try(
            FileInputStream fis = new FileInputStream("xxx.txt");
            FileOutputStream fos = new FileOutputStream("yyy.txt");
        ){
            int b;
            while ((b = fis.read()) != -1){
                fos.write(b);
            }
        }
    }
 

try(...){...}

為什麼可以呢？

因為IO流的類實現了AutoCloseable介面。