定制Writable类

//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; }  

//无参构造方法，为MR框架反射机制所调用
public IntWritable() {}
//有参构造方法
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 ? -1 : (thisValue==thatValue ? 0 : 1));
}

  // 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 ? -1 : (thisValue==thatValue ? 0 : 1));
  }
}
//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) << 24) +
            ((bytes[start+1] & 0xff) << 16) +
            ((bytes[start+2] & 0xff) <<  8) +
            ((bytes[start+3] & 0xff)));
  }