Hadoop WritableComparable接口

hadoop WritableComparable接口的作用，

两个作用 A 实现了一个序列化协议

B 可以比较的实现，实现方法compareTo()方法，

MapReduce中所有的key值类型都必须实现这个接口，避免重复的key啊

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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<T>
extends
Writable,
Comparable<T>

A

Writable

which is also

Comparable

.

WritableComparable

s can be compared to each other, typically via

Comparator

s. 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));
}
}

转自：http://www.cnblogs.com/dyllove98/archive/2013/07/31/3228579.html 感谢作者