@Test

public void test18() throws IOException {

    File file = new File("hello.txt");

    RandomAccessFile raf = new RandomAccessFile(file, "rw");

    //将指针调到指针为3的位置，读取abc后面的内容，保存到builder中

    raf.seek(3);

    StringBuilder builder = new StringBuilder((int) file.length());

    byte[] buffer = new byte[20];

    int len;

    //此时只会读取指针为3及其后面的内容

    while ((len = raf.read(buffer)) != -1){

        builder.append(new String(buffer, 0, len));

    }

    //调回指针，写入“xyz”

    raf.seek(3);

    raf.write("xyz".getBytes());

    //指针此时已经自动移动到xyz后面。再写入复制的abc后面的内容

    raf.write(builder.toString().getBytes());

    raf.close();

}