WritableComparable接口

Writable接口大家可能都知道，它是一个实现了序列化协议的序列化对象。在Hadoop中定义一个结构化对象都要实现Writable接口，使得该结构化对象可以序列化为字节流，字节流也可以反序列化为结构化对象。那WritableComparable接口是可序列化并且可比较的接口。MapReduce中所有的key值类型都必须实现这个接口，既然是可序列化的那就必须得实现readFiels（）和write（）这两个序列化和反序列化函数，既然也是可比较的那就必须得实现compareTo（）函数，该函数即是比较和排序规则的实现。这样MR中的key值就既能可序列化又是可比较的。下面几符图是API中对WritableComparable接口的解释及其方法，还有一个实现了该接口的对象的列子：

public interface WritableComparable
     
           extends     Writable,     Comparable
      
             A Writable which is alsoComparable.WritableComparables can be compared to each other, typically via Comparators. Any type which is to be used as a key in the Hadoop Map-Reduce framework should implement this interface.Example:        public class MyWritableComparable implements WritableComparable {       // Some data       private int counter;       private long timestamp;                public void write(DataOutput out) throws IOException {         out.writeInt(counter);         out.writeLong(timestamp);       }                public void readFields(DataInput in) throws IOException {         counter = in.readInt();         timestamp = in.readLong();       }                public int compareTo(MyWritableComparable w) {         int thisValue = this.value;         int thatValue = ((IntWritable)o).value;         return (thisValue < thatValue ? -1 : (thisValue==thatValue ? 0 : 1));       }     }