public static class Map extends Mapper<Text, Text, Text, Text> {

我这几年来第一次用java开发。我正在hadoop中实现一些map-reduce方法。有人能解释一下这个问题吗 <Text, Text, Text, Text> 在这个类定义中？它到底在做什么？

public static class TokenizerMapper
       extends Mapper<Object, Text, Text, IntWritable>{

    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();

    public void map(Object key, Text value, Context context
                    ) throws IOException, InterruptedException {
      StringTokenizer itr = new StringTokenizer(value.toString());
      while (itr.hasMoreTokens()) {
        word.set(itr.nextToken());
        context.write(word, one);
      }
    }
  }