hadoop自定义类型异常java.lang.RuntimeException: java.lang.NoSuchMethodException: twicesort.TwoKey.<init>()

java.lang.RuntimeException: java.lang.NoSuchMethodException: twicesort.TwoKey.<init>()
    at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:131)
    at org.apache.hadoop.io.WritableComparator.newKey(WritableComparator.java:144)
    at org.apache.hadoop.io.WritableComparator.<init>(WritableComparator.java:130)
    at org.apache.hadoop.io.WritableComparator.get(WritableComparator.java:65)
    at org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:887)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.init(MapTask.java:1001)
    at org.apache.hadoop.mapred.MapTask.createSortingCollector(MapTask.java:401)
    at org.apache.hadoop.mapred.MapTask.access$100(MapTask.java:81)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:695)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:767)

重点在这里，java.lang.NoSuchMethodException: twicesort.TwoKey，他说没有这个方法，这个方法是TwoKey的无参构造方法
这个细节在hadoop权威指南上也有提到，就在序列化或者自定义类型那章，具体不记得了，我的理解是首先他反序列化的时候先要拿到一个无参的构造对象，然后再进行对象内值的填充。

让我们再来看看hadoop给我提供的类型是不是都有无参的构造方法

public class IntWritable implements WritableComparable<IntWritable> {
  private int value;

  public IntWritable() {}

  public IntWritable(int value) { set(value); }
  }

public class LongWritable implements WritableComparable<LongWritable> {
  private long value;

  public LongWritable() {}

  public LongWritable(long value) { set(value); }
  }

public class Text extends BinaryComparable
    implements WritableComparable<BinaryComparable> {
    public Text() {
    bytes = EMPTY_BYTES;
  }

  /** Construct from a string. 
   */
  public Text(String string) {
    set(string);
  }

  /** Construct from another text. */
  public Text(Text utf8) {
    set(utf8);
  }

  /** Construct from a byte array.
   */
  public Text(byte[] utf8)  {
    set(utf8);
  }
  }

public class BooleanWritable implements WritableComparable<BooleanWritable> {
  private boolean value;

  /** 
   */
  public BooleanWritable() {};
  }

public class ByteWritable implements WritableComparable<ByteWritable> {
  private byte value;

  public ByteWritable() {}

  public ByteWritable(byte value) { set(value); }
  }

public class ShortWritable implements WritableComparable<ShortWritable> {
  private short value;

  public ShortWritable() {
  }

  public ShortWritable(short value) {
    set(value);
  }
}