code: public static <T> DataSet<T> sampleWithSize(
  		DataSet <T> input,
  		final boolean withReplacement,
  		final int numSamples,
  		final long seed) {
  
  		SampleInPartition<T> sampleInPartition = new SampleInPartition<>(withReplacement, numSamples, seed);
  		MapPartitionOperator mapPartitionOperator = input.mapPartition(sampleInPartition);
  
  		// There is no previous group, so the parallelism of GroupReduceOperator is always 1.
  		String callLocation = Utils.getCallLocationName();
  		SampleInCoordinator<T> sampleInCoordinator = new SampleInCoordinator<>(withReplacement, numSamples, seed);
  		return new GroupReduceOperator<>(mapPartitionOperator, input.getType(), sampleInCoordinator, callLocation);
  	}
docs: Generate a sample of DataSet which contains fixed size elements.
  
  <p><strong>NOTE:</strong> Sample with fixed size is not as efficient as sample with fraction, use sample with
  fraction unless you need exact precision.
  
  @param withReplacement Whether element can be selected more than once.
  @param numSamples       The expected sample size.
  @param seed            Random number generator seed.
  @return The sampled DataSet