//继承 WritableComparable接口(继承了writable接口和comparable接口)

public class IntWritable implements WritableComparable<IntWritable> {

//定义普通java类型的成员变量

private int value;

//成员变量的set方法

public void set(int value) { this.value = value; }

//成员变量的get方法

public int get() { return value; }  

//无参构造函数，为MR框架反射机制所调用

public IntWritable() {}

//有参构造函数

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

//反序列化方法

public void readFields(DataInput in) throws IOException {

  value = in.readInt();

}

//序列化方法

public void write(DataOutput out) throws IOException {

  out.writeInt(value);

}

//覆写equals()方法

public boolean equals(Object o) {

  if (!(o instanceof IntWritable))

    return false;

  IntWritable other = (IntWritable)o;

  return this.value == other.value;

}

//覆写hashCode()方法

public int hashCode() {

  return value;

}

//覆写toString()方法

public String toString() {

  return Integer.toString(value);

}

//覆写 comparable接口 中的compareTo()方法【默认升序】

public int compareTo(IntWritable o) {

  int thisValue = this.value;

  int thatValue = o.value;

  return (thisValue<thatValue ? - : (thisValue==thatValue ?  : ));

}

  //1. 定义内部类Comparator【比较器】继承自WritableComparator类

public static class Comparator extends WritableComparator {

  //2. 不可缺少的无参构造函数，反射机制调用

  public Comparator() {

    super(IntWritable.class);

  }

  //3. 覆写 字节流层面的比较排序

  public int compare(byte[] b1, int s1, int l1,

                     byte[] b2, int s2, int l2) {

    //返回 字符数组b1 的编码值

    int thisValue = readInt(b1, s1);

    int thatValue = readInt(b2, s2);

    return (thisValue<thatValue ? - : (thisValue==thatValue ?  : ));

  }

}

//4. 向WritableComparator类注册定制的writable类【Haoop自动调用上述的比较器】

static {

  WritableComparator.define(IntWritable.class, new Comparator());

}

}

/** Parse an integer from a byte array. */

  public static int readInt(byte[] bytes, int start) {

    return (((bytes[start  ] & 0xff) << ) +

            ((bytes[start+] & 0xff) << ) +

            ((bytes[start+] & 0xff) <<  ) +

            ((bytes[start+] & 0xff)));

  }