IntWritable源码学些 - hadoop-1.2.1

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package org.apache.hadoop.io;

import java.io.*;

/** A WritableComparable for ints. */
public class IntWritable implements WritableComparable {
  private int value;

  public IntWritable() {}

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

  /** Set the value of this IntWritable. */
  public void set(int value) { this.value = value; }

  /** Return the value of this IntWritable. */
  public int get() { return value; }

  public void readFields(DataInput in) throws IOException {
    value = in.readInt();
  }

  public void write(DataOutput out) throws IOException {
    out.writeInt(value);
  }

  /** Returns true iff <code>o</code> is a IntWritable with the same value. */
  public boolean equals(Object o) {
    if (!(o instanceof IntWritable))
      return false;
    IntWritable other = (IntWritable)o;
    return this.value == other.value;
  }

  public int hashCode() {
    return value;
  }

  /** Compares two IntWritables. */
  public int compareTo(Object o) {
    int thisValue = this.value;
    int thatValue = ((IntWritable)o).value;
    return (thisValue<thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
  }

  public String toString() {
    return Integer.toString(value);
  }

  /** A Comparator optimized for IntWritable. */ 
  public static class Comparator extends WritableComparator {
    public Comparator() {
      super(IntWritable.class);
    }

    public int compare(byte[] b1, int s1, int l1,
                       byte[] b2, int s2, int l2) {
      int thisValue = readInt(b1, s1);
      int thatValue = readInt(b2, s2);
      return (thisValue<thatValue ? -1 : (thisValue==thatValue ? 0 : 1));
    }
  }

  static {                                        // register this comparator
    WritableComparator.define(IntWritable.class, new Comparator());
  }
}

在学习Hadoop的时候，读源代码是必不可少的，在看到IntWritable时比较困惑它添加Comparator干什么，后来在网上看了一下原来是方便对象在字节状态下的比较提高效率的，将静态类的对象注册到WritableComparator的comparators中去，在利用时直接从这里提出比较，相较于IntWritable里的compareTo要反序列化才能比较效率自然很高。