一、Hadoop内置的数据类型

BooleanWritable:标准布尔型数值
ByteWritable:单字节数值
DoubleWritable:双字节数值
FloatWritable:浮点数
IntWritable:整型数
LongWritable:长整型数
Text:使用UTF8格式存储的文本
NullWritable:当<key, value>中的key或value为空时使用

二、用户自定义数据类型的实现

1.继承接口Writable,实现其方法write()和readFields(), 以便该数据能被序列化后完成网络传输或文件输入/输出；

2.如果该数据需要作为主键key使用，或需要比较数值大小时，则需要实现WritalbeComparable接口,实现其方法write(),readFields(),CompareTo() 。

public class Point3D implements Writable<Point3D>

{

    private float x,y,z;

    public float getX(){return x;}

    public float getY(){return y;}

    public float getZ(){return z;}

    public void readFields(DataInput in) throws IOException

    {

        x = in.readFloat();

        y = in.readFloat();

        z = in.readFloat();

    }

    public void write(DataOutput out) throws IOException

    {

         out.writeFloat(x);

         out.writeFloat(y);

         out.writeFloat(z);

    }

}

public class Point3D implements WritableComparable<Point3D>

{

    private float x,y,z;

    public float getX(){return x;}

    public float getY(){return y;}

    public float getZ(){return z;}

    public void readFields(DataInput in) throws IOException

    {
        x = in.readFloat();

        y = in.readFloat();

        z = in.readFloat();

    }

    public void write(DataOutput out) throws IOException

    {
         out.writeFloat(x);

         out.writeFloat(y);

         out.writeFloat(z);

    }

    public int CompareTo(Point3D p)

    {

        //具体实现比较当前的空间坐标点this(x,y,z)与指定的点p(x,y,z)的大小

        // 并输出： -1(小于), 0(等于), 1(大于)

    }

}