public class IntSortReducer extends Reducer<IntWritable, IntWritable, IntWritable, IntWritable> { private static final IntWritable lineNumber = new IntWritable(0); @Override protected void reduce(IntWritable key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException { // ignore values: they'll always be 1 (as long as paired with IntMapper) synchronized (lineNumber) { /* TODO: Advance line number */ context.write(/* TODO: Write new, reduced <K,V> pair to context */); } } }

补全后的代码如下：

public class IntSortReducer extends Reducer<IntWritable, IntWritable, IntWritable, IntWritable> {
    private static final IntWritable lineNumber = new IntWritable(0);
    private int count = 0;
    
    @Override
    protected void reduce(IntWritable key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        // ignore values: they'll always be 1 (as long as paired with IntMapper)
        synchronized (lineNumber) {
            /* Advance line number */
            count++;
            lineNumber.set(count);
            /* Write new, reduced <K,V> pair to context */
            context.write(lineNumber, key);
        }
    }
}

在补全的代码中，我们通过使用 count 计数器和线程安全的 synchronized 关键字来实现了一个自增的 lineNumber，然后将新生成的键值对通过调用 context.write() 写入到 context 中。