Question

I am comparing example Java code from the O’Reilly book Hadoop: The Definitive Guide (by Tom White, 3rd edition) and my own attempt at recreating/understanding it. The issue I am having is as follows:

The class from the book compiles just fine:

import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public class MaxTemperatureReducer
  extends Reducer<Text, IntWritable, Text, IntWritable> {

  @Override
  public void reduce(Text key, Iterable<IntWritable> values,
      Context context)
      throws IOException, InterruptedException {

    int maxValue = Integer.MIN_VALUE;
    for (IntWritable value : values) {
      maxValue = Math.max(maxValue, value.get());
    }
    context.write(key, new IntWritable(maxValue));
  }
}

But when I try to test a portion of it on my own, I get the compile error of “int cannot be dereferenced:”

public class TestMinValue {
    public static void main(String[] args){
        int[] values = {1,2,3,4,5};
        int maxValue = Integer.MIN_VALUE;
        for(int value : values){
            maxValue = Math.max(maxValue, value.get());
}
}
}

I am new to Java and would like to understand the difference; why is the example class working, but my snippet of it isn’t?

Answer 1

The IntWritable type is a class, which has a get() method. You are using the primitive type int instead. Primitives do not have methods in Java.