Hadoop数据类型
hadoop内置数据类型
代码语言：javascript复制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为空时使用用户自定义数据类型的实现
代码语言：javascript复制1.对于只需要作为“值”出现的数据类型，实现Writable接口即可
2.对于有可能作为“键”出现的数据类型，需要实现WritableComparable接口##实现Writable接口：
代码语言：javascript复制/* DataInput and DataOutput 类是java.io的类 */
public interface Writable {
void readFields(DataInput in);
void write(DataOutput out);
}下面是一个小例子：
代码语言：javascript复制public class Point3D implement Writable {
public float x, y, z;
public Point3D(float fx, float fy, float fz) {
this.x = fx;
this.y = fy;
this.z = fz;
}
public Point3D() {
this(0.0f, 0.0f, 0.0f);
}
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 String toString() {
return Float.toString(x) ", "
Float.toString(y) ", "
Float.toString(z);
}
}2、实现WritableComparable接口
代码语言：javascript复制public interface WritableComparable<T> {
public void readFields(DataInput in);
public void write(DataOutput out);
public int compareTo(T other);
}先给出下面的简单例子，再做说明和扩展。
代码语言：javascript复制public class Point3D inplements WritableComparable {
public float x, y, z;
public Point3D(float fx, float fy, float fz) {
this.x = fx;
this.y = fy;
this.z = fz;
}
public Point3D() {
this(0.0f, 0.0f, 0.0f);
}
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 String toString() {
return Float.toString(x) ", "
Float.toString(y) ", "
Float.toString(z);
}
public float distanceFromOrigin() {
return (float) Math.sqrt( x*x y*y z*z);
}
//影响map输出的排序，默认是升序，return值加一个负号变降序
public int compareTo(Point3D other) {
return Float.compareTo(distanceFromOrigin(),other.distanceFromOrigin());
}
public boolean equals(Object o) {
if( !(o instanceof Point3D)) {
return false;
}
Point3D other = (Point3D) o;
return this.x == o.x && this.y == o.y && this.z == o.z;
}
/* 实现 hashCode() 方法很重要
* Hadoop的Partitioners会用到这个方法，后面再说
*/
public int hashCode() {
return Float.floatToIntBits(x)
^ Float.floatToIntBits(y)
^ Float.floatToIntBits(z);
}
}自定义Hadoop数据类型后，需要明确告诉Hadoop来使用它们。这是 JobConf 所能担当的了。
代码语言：javascript复制void setOutputKeyClass(Class<T> theClass)
void setOutputValueClass(Class<T> theClass)通常(默认条件下)，这个函数对Map和Reduce阶段的输出都起到作用，当然也有专门的 setMapOutputKeyClass() / setReduceOutputKeyClass() 接口。
