Following the same principles found in filtering a map by keys and filtering a map by entries, this example will show how to filter a map by values while using java, java 8 and guava. The snippets below will use a map defined in the @setup that contains entries of months where the key is the integer month (2=February) of year and the value represents the month spelled out.
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
Until java 8, the core jdk lack the mechanics for filtering a map by Map.value. The snippet below shows the only effective way is to iterate over the Map.entrySet using a enhanced for loop and then applying criteria in an if statement, in this case checking if entry.value ends with "r".
@Test
public void filter_map_by_values_java () {
Map<Integer, String> torskMeetings = new HashMap<Integer, String>();
for (Entry<Integer, String> entry : MONTHS.entrySet()) {
if (entry.getValue().endsWith("r")) {
torskMeetings.put(entry.getKey(), entry.getValue());
}
}
logger.info(torskMeetings);
assertThat(torskMeetings.values(), contains(
"September", "October", "November", "December"));
}Java 8
This example snippet will show how to filter a map by values using java 8. Using the powerful streams api, we will call entrySet.stream and filter values that end with "r" by creating a java.util.function.predicate 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_values_java8_lambda () {
Map<Integer, String> torskMeetings =
MONTHS.entrySet()
.stream()
.filter(p -> p.getValue().endsWith("r"))
.collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
logger.info(torskMeetings);
assertThat(torskMeetings.values(), contains(
"September", "October", "November", "December"));
}Google Guava
The guava Maps collection utility provides Maps.filterValues which accepts a map and a guava predicate as parameters then returning a map containing the mappings whose values satisfy the predicate.
@Test
public void filter_map_by_values_guava () {
Predicate<String> endsWithR = new Predicate<String>() {
@Override
public boolean apply(String input) {
return input.endsWith("r");
}
};
Map<Integer, String> torskMeetings = Maps.filterValues(MONTHS, endsWithR);
logger.info(torskMeetings);
assertThat(torskMeetings.values(), contains(
"September", "October", "November", "December"));
}