Implementing GroupBy using Google guava Multimap and Function

Posted on Posted in java

This is one way of creating a group by like functionality for collections using google guava Multimap and Function.

Code is straight forward we simply use the index method of the Multimap to group our data by, in here we use our 2 column to group it by the department.

Sample output

key = Dev
  1 : Greg
  3 : Roman
key = Support
  2 : Leo
  4 : Jobby

Code

package com.gregbugaj.guava;
 
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
 
import com.google.common.base.Function;
import com.google.common.collect.Lists;
import com.google.common.collect.Multimap;
import com.google.common.collect.Multimaps;
 
public class GroupByMultimap {
	public static void main(String[] args) {
		Object[] o1 = new Object[] { 1, "Greg", "Dev" };
		Object[] o2 = new Object[] { 2, "Leo", "Support" };
		Object[] o3 = new Object[] { 3, "Roman", "Dev" };
		Object[] o4 = new Object[] { 4, "Jobby", "Support" };
 
		List<Object[]> rows = Lists.newArrayList(o1, o2, o3, o4);
		Multimap<String, Object[]> grouped = Multimaps.index(rows,
				new Function<Object[], String>() {
					@Override
					public String apply(Object[] item) {
						return (String) item[2];
					}
				});
 
		Iterator<String> keyIterator = grouped.asMap().keySet().iterator();
		while (keyIterator.hasNext()) {
			String key = keyIterator.next();
			System.out.println("key = " + key);
			Collection<Object[]> dataRows = grouped.get(key);
			for (Object[] o : dataRows) {
				System.out.println(String.format("  %d : %s", o[0], o[1]));
			}
		}
	}
}

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