FilterMapByKeys.java

package com.levelup.java.collections;

import static org.hamcrest.collection.IsIterableContainingInOrder.contains;
import static org.junit.Assert.assertThat;

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;

import org.apache.log4j.Logger;
import org.junit.Before;
import org.junit.Test;

import com.google.common.base.Predicate;
import com.google.common.collect.Maps;

/**
 * This java example will demonstrate how to filter
 * a map by keys.
 *  
 * @author Justin Musgrove
 * @see <a href='http://www.leveluplunch.com/java/examples/filter-map-by-key/'>Filter map by key</a>
 * 
 */
public class FilterMapByKeys {
	
	private static final Logger logger = Logger.getLogger(FilterMapByKeys.class);
	
	Map<Integer, String> MONTHS = new HashMap<Integer, String>();
	
	@Before
	public void setUp () {

		MONTHS.put(new Integer(1), "January");
		MONTHS.put(new Integer(2), "February");
		MONTHS.put(new Integer(3), "March");
		MONTHS.put(new Integer(4), "April");
		MONTHS.put(new Integer(5), "May");
		MONTHS.put(new Integer(6), "June");
		MONTHS.put(new Integer(7), "July");
		MONTHS.put(new Integer(8), "August");
		MONTHS.put(new Integer(9), "September");
		MONTHS.put(new Integer(10), "October");
		MONTHS.put(new Integer(11), "November");
		MONTHS.put(new Integer(12), "December");
	}
	
	@Test
	public void filter_map_by_keys_java () {
		
		Map<Integer, String> firstQuarter = new HashMap<Integer, String>();

		for (Entry<Integer, String> entry : MONTHS.entrySet()) {
			if (entry.getKey().intValue() <= 4) {
				firstQuarter.put(entry.getKey(), entry.getValue());
			}
		}
		
		logger.info(firstQuarter);
		
	    assertThat(firstQuarter.keySet(), contains(
	    		1, 2, 3, 4));
	}

	@Test
	public void filter_map_by_keys_java8_lambda () {
	
		Map<Integer, String> firstQuarter = 
				MONTHS.entrySet()
				.stream()
				.filter(p -> p.getKey().intValue() <= 4)
				.collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
		
		logger.info(firstQuarter);
		
	    assertThat(firstQuarter.keySet(), contains(
	    		1, 2, 3, 4));
	}
	
	@Test
	public void filter_map_by_keys_guava () {
		
		Predicate<Integer> byFirstQuarter = new Predicate<Integer>() {
			@Override
			public boolean apply(Integer input) {
				return input.intValue() <= 4;
			}
		};
		
		Map<Integer, String> firstQuarter = Maps.filterKeys(MONTHS, byFirstQuarter);
		
		logger.info(firstQuarter);
		
	    assertThat(firstQuarter.keySet(), contains(
	    		1, 2, 3, 4));
	}

	
}