IntWritable详解

1.Hadoop数据类型如下图：

      由上图的Writable层次结构图可以看到绝大多数的数据类型都实现了Writable、WritableComparable接口，在此先分析一下这两个接口情况。自顶下下逐步分析。

Writable接口的定义如下：

<span style="font-family:SimSun;font-size:14px;">package org.apache.hadoop.io;

import java.io.DataOutput;
import java.io.DataInput;
import java.io.IOException;
public interface Writable {
/*
    object将自身字段序列化后的的字节流写入输出流out中。
参数:
    out - 接收object序列化后的字节流的输出流.
*/
　　void write(DataOutput out) throws IOException;
　　
　　/*
　　    将输入流in中的字节流反序列化然后写入object的字段
　　参数:
　　    字节流的出处
　　*/
    void readFields(DataInput in) throws IOException;
}</span>

WritableComparable接口定义如下：

<span style="font-family:SimSun;font-size:14px;">package org.apache.hadoop.io;
public interface WritableComparable<T> extends Writable, comparable<T> {
}</span>

<span style="font-family:SimSun;font-size:14px;">void write(DataOutput out) throws IOException；

void readFields(DataInput in) throws IOException；</span>

<span style="font-family:SimSun;font-size:14px;">int compareTo( T other);
/*
    比较此对象与指定对象other的顺序。如果该对象小于、等于或大于指定对象，则分别返回负整数、零或正整数。

    参数：o - 要比较的对象。

    返回：负整数、零或正整数，根据此对象是小于、等于还是大于指定对象。 
*/</span>

2.IntWritable类定义如下：

<span style="font-family:SimSun;font-size:14px;">package org.apache.hadoop.io;

import java.io.*;

/** A WritableComparable for ints. */
public class IntWritable implements WritableComparable {
  private int value;

  public IntWritable() {}

  public IntWritable(int value) { set(value); }

  /** Set the value of this IntWritable. */
  public void set(int value) { this.value = value; }

  /** Return the value of this IntWritable. */
  public int get() { return value; }

  public void readFields(DataInput in) throws IOException {
    value = in.readInt();
  }

  public void write(DataOutput out) throws IOException {
    out.writeInt(value);
  }

  /** Returns true iff o is a IntWritable with the same value. */
  public boolean equals(Object o) {
    if (!(o instanceof IntWritable))
      return false;
    IntWritable other = (IntWritable)o;
    return this.value == other.value;
  }

  public int hashCode() {
    return value;
  }

  /** Compares two IntWritables. */
  public int compareTo(Object o) {
    int thisValue = this.value;
    int thatValue = ((IntWritable)o).value;
    return (thisValue<thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
  }

  public String toString() {
    return Integer.toString(value);
  }

  /** A Comparator optimized for IntWritable. */ 
  public static class Comparator extends WritableComparator {
    public Comparator() {
      super(IntWritable.class);
    }

    public int compare(byte[] b1, int s1, int l1,
                       byte[] b2, int s2, int l2) {
      int thisValue = readInt(b1, s1);
      int thatValue = readInt(b2, s2);
      return (thisValue<thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
    }
  }

  static {                                        // register this comparator
    WritableComparator.define(IntWritable.class, new Comparator());
  }
}</span>

如：

<span style="font-family:SimSun;font-size:14px;">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)

    }

}</span>

http://www.cnblogs.com/zhengyuhong/p/3952954.html