于hadoop当处理复杂的业务，需要使用组合键，与单纯的复杂的继承Writable接口，但继承WritableComparable<T>接口。事实上。WritableComparable<T>接口继承Writable和Comparable<T>接口，假设仅仅须要使用某一个类作为传值对象而不是作为key，继承Writable接口就可以。

上源代码：

public interface WritableComparable
     
       extends Writable, Comparable
      
        {}
      
     

public interface Writable {  void write(DataOutput out) throws IOException;  void readFields(DataInput in) throws IOException;}

public interface Comparable
     
       {        public int compareTo(T o);}
     

下面是实现复合key的实例，亲測。可用

public class SortKey implements WritableComparable
     
      {		private Text name;	private IntWritable right;		public SortKey() {		set(new Text(), new IntWritable());	}			public SortKey(Text name, IntWritable right) {		set(name, right);	}	private void set(Text name,IntWritable right){		this.name = name;		this.right = right;	}			/**	 * @return the name	 */	public Text getName() {		return name;	}	/**	 * @param name the name to set	 */	public void setName(Text name) {		this.name = name;	}	/**	 * @return the right	 */	public IntWritable getRight() {		return right;	}	/**	 * @param right the right to set	 */	public void setRight(IntWritable right) {		this.right = right;	}	@Override	public void write(DataOutput out) throws IOException {		name.write(out);		right.write(out);	}	@Override	public void readFields(DataInput in) throws IOException {		name.readFields(in);		right.readFields(in);	}	@Override	public int compareTo(SortKey o) {		int cmp = name.compareTo(o.name);		if(cmp != 0){			return cmp;		}else{			return right.compareTo(o.right);		}	}
     

	//到眼下为止，你仅仅能将其作为key来使用，可是假设你须要依照key的某一个值来排序，下面是重点

static{		WritableComparator.define(SortKey.class, new Comparator());	}		public static class Comparator extends WritableComparator{			    private static final Text.Comparator TEXT_COMPARATOR = new Text.Comparator();				protected Comparator() {			super(SortKey.class);		}		/* (non-Javadoc)		 * @see org.apache.hadoop.io.WritableComparator#compare(byte[], int, int, byte[], int, int)		 */		@Override		public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {			try{				int firstL1 = WritableUtils.decodeVIntSize(b1[s1]) + readVInt(b1, s1);				int firstL2 = WritableUtils.decodeVIntSize(b2[s2]) + readVInt(b2, s2);	            return TEXT_COMPARATOR.compare(b1, s1, firstL1, b2, s2, firstL2);			}catch(Exception e){				throw new IllegalArgumentException(e);			}		}		}}

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