在hadoop中处理复杂业务时，需要用到复合键，复合不同于单纯的继承Writable接口，而是继承了WritableComparable<T>接口，而实际上，WritableComparable<T>接口继承了Writable和Comparable<T>接口，如果只需要使用某1个类作为传值对象而不是作为key，继承Writable接口便可。
上源码：
public interface WritableComparable<T> extends Writable, Comparable<T> {
}
public interface Writable {
void write(DataOutput out) throws IOException;
void readFields(DataInput in) throws IOException;
}
public interface Comparable<T> {
public int compareTo(T o);
}
以下是实现复合key的实例，亲测，可用
public class SortKey implements WritableComparable<SortKey>{
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);
}
}
<span style="white-space:pre"> </span>//到目前为止，你只能将其作为key来使用，但是如果你需要依照key的某1个值来排序，以下是重点
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);
}
}
}
}