Filter map by key

Following the same principles found in filtering a map by entries and filtering a map by values, this example will show how to filter a map by Map.key while using java, java 8 and guava. Each of the snippets below will use a predefined map created in the @setup where the entries resemble a month. The Map.key is the integer month of year (1=January) and the value represents the month spelled out. A comparable example demonstrates how to filter a map by keys in groovy.

Setup

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");
}

Straight up Java

The core jdk lack the mechanics for filtering a map by Map.key until java 8. As shown in the snippet below, the only effective way is to iterate over the Map.entrySet using a for loop and then applying criteria in an if statement, in this case checking if entry.key is less than four or finding months contained in the first quarter.

@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));
}

Output

{1=January, 2=February, 3=March, 4=April}

Java 8

The snippet below will show how to filter a map by keys using java 8. Using the powerful streams api, we will call entrySet.stream and filter months within the first quarter. We created this java.util.function.predicate by using a lambda expression. Then using Collectors.toMap we will map each element in the stream back into a map.

@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));
}

Output

{1=January, 2=February, 3=March, 4=April}

Google Guava

Using guava Maps collection utility we will call Maps.filterKeys which accepts a map and a guava predicate as parameters then returning a map containing the mappings whose keys satisfy the predicate.

@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));
}

Output

{1=January, 2=February, 3=March, 4=April}