我是个新手。我想知道在hadoop中实现自定义数据类型时readfields和write方法的用途是什么？例如，

public class Point3D implements Writable {
     public float x;
     public float y;
     public float z;

  public Point3D(float x, float y, float z) {
    this.x = x;
    this.y = y;
    this.z = z;
  }

  public Point3D() {
    this(0.0f, 0.0f, 0.0f);
  }

  public void write(DataOutput out) throws IOException {
    out.writeFloat(x);
    out.writeFloat(y);
    out.writeFloat(z);
  }

  public void readFields(DataInput in) throws IOException {
    x = in.readFloat();
    y = in.readFloat();
    z = in.readFloat();
  }

  public String toString() {
    return Float.toString(x) + ", "
    + Float.toString(y) + ", "
    + Float.toString(z);
  }
  public void set(float x, float y, float z)
 {
 this.x=x;
 this.y=y;
 this.z=z;
 }
}

在上面的示例中，自定义recordreader使用set方法来设置x、y和z的值。最后，我们在Map器中得到这些值。但是对于writeable的readfealds和write（）方法有什么需要呢？？请。帮助