Java io 中的带缓冲的 inputstream 读取时，缓冲区为什么不用清除啊？

public class Trans {
  public static void main(String[] args) {
    try {
      readFile();
    } catch (IOException e) {
      e.printStackTrace();
  }
}
  public static void readFile() throws IOException {
    try (InputStreamReader reader=new InputStreamReader(new FileInputStream("se.md"),"UTF-8");) {
      char[] buffer = new char[1000];
      int n;
      while ((n = reader.read(buffer)) != -1) {
        System.out.println("read " + n + " chars.");
     }
   }
  }
}

打印如下

read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 1000 chars.
read 305 chars.

char 类型数组 buffer 难道不是应该读一次清空一次吗？
如果不清空的话，buffer 一次就都读满了，没空间放新读的了就应该停下呀？