用户定制数据类型 如果 Point3D还需要作为主键值使用,或者需要比 较大小时,还应该实现 Writab| e comparable接口 pubic class Point 3D implements Writable Comparable< Point3D> private float x, y, z public void read Fields(Datainput in) throws IoException public void write(Data Output out) throws IOException 如果Point3D还需要作为主键值使用，或者需要比 较大小时，还应该实现WritableComparable接口 pubic class Point3D implements WritableComparable<Point3D> { private float x,y,z; public void readFields(DataInput in) throws IOException {……} public void write(DataOutput out) throws IOException {……} }