自定义MapReduce中数据类型

数据类型(都实现了Writable接口)

BooleanWritable		布尔类型
ByteWritable		单字节数值
DoubleWritable		双字节数值
FloatWritable		浮点数
IntWritable			整型数
LongWritable		长整型
Text				UTF-8格式存储的文本
NullWritable		空类型

因为shuffle中排序依据是key,若定义的数据类型为Key,必须实现writable和comparable接口，即WritableComparable接口

Writable

write()把每个对象序列化到输出流				
readFilds()把输入流字节反序列化到输入流		

自定义数据类型实例

1.定义私有变量
2.setter，getter方法
3.无参有参构造器
4.set()方法，帮助构造器初始化数据（Hadoop偏爱）
5.hashCode()方法和equals()方法
6.toString()方法
7.implement Writable并实现write()方法readFilds()方法
8.implement WritableComparable并实现compareTo()方法

package com.cenzhongman.io;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;

public class UserWritable implements WritableComparable<UserWritable> {
	private int ip;
	private String name;

	public UserWritable() {
	}

	public UserWritable(int ip, String name) {
		this.set(ip, name);
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ip;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public String toString() {
		return ip + "\t" + name;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		UserWritable other = (UserWritable) obj;
		if (ip != other.ip)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}

	public void set(int ip, String name) {
		this.setIp(ip);
		this.setName(name);
	}

	public int getIp() {
		return ip;
	}

	public void setIp(int ip) {
		this.ip = ip;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	// read和write方法元素的顺序必须一致
	@Override
	public void readFields(DataInput arg0) throws IOException {
		this.ip = arg0.readInt();
		this.name = arg0.readUTF();
	}

	@Override
	public void write(DataOutput arg0) throws IOException {
		arg0.writeInt(ip);
		arg0.writeUTF(name);
	}

	@Override
	public int compareTo(UserWritable o) {
		int comp = Integer.valueOf(this.getIp()).compareTo(o.getIp());

		if (comp != 0) {
			return comp;
		}
		return this.getName().compareTo(o.getName());
	}
}