diff --git a/java-collections-conversions-2/src/main/java/com/baeldung/convertlisttomap/ListToMapConverter.java b/java-collections-conversions-2/src/main/java/com/baeldung/convertlisttomap/ListToMapConverter.java new file mode 100644 index 0000000000..8450f54f9d --- /dev/null +++ b/java-collections-conversions-2/src/main/java/com/baeldung/convertlisttomap/ListToMapConverter.java @@ -0,0 +1,70 @@ +package com.baeldung.convertlisttomap; + +import java.util.List; +import java.util.Map; +import java.util.function.BiConsumer; +import java.util.function.BinaryOperator; +import java.util.function.Function; +import java.util.function.Supplier; +import java.util.stream.Collectors; + +/** + * Convert a string list to a map whose key is the string's length and value is the collection with same length. + * Give a list {"Baeldung", "is", "very", "cool"}. + * After conversion we'll get a map like: + * {8 : ["Baeldung"], 2 : ["is"], 4 : ["very", "cool"]}. + * + * @author leasy.zhang + * + */ +public class ListToMapConverter { + + public Map> groupingByStringLength(List source, + Supplier>> mapSupplier, + Supplier> listSupplier) { + + return source.stream() + .collect(Collectors.groupingBy(String::length, mapSupplier, Collectors.toCollection(listSupplier))); + } + + public Map> streamCollectByStringLength(List source, + Supplier>> mapSupplier, + Supplier> listSupplier) { + + BiConsumer>, String> accumulator = (response, element) -> { + Integer key = element.length(); + List values = response.getOrDefault(key, listSupplier.get()); + values.add(element); + response.put(key, values); + }; + + BiConsumer>, Map>> combiner = (res1, res2) -> { + res1.putAll(res2); + }; + + return source.stream() + .collect(mapSupplier, accumulator, combiner); + } + + public Map> collectorToMapByStringLength(List source, + Supplier>> mapSupplier, + Supplier> listSupplier) { + + Function keyMapper = String::length; + + Function> valueMapper = (element) -> { + List collection = listSupplier.get(); + collection.add(element); + return collection; + }; + + BinaryOperator> mergeFunction = (existing, replacement) -> { + existing.addAll(replacement); + return existing; + }; + + return source.stream() + .collect(Collectors.toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)); + } + +} diff --git a/java-collections-conversions-2/src/test/java/com/baeldung/convertlisttomap/ListToMapUnitTest.java b/java-collections-conversions-2/src/test/java/com/baeldung/convertlisttomap/ListToMapUnitTest.java new file mode 100644 index 0000000000..2b43813822 --- /dev/null +++ b/java-collections-conversions-2/src/test/java/com/baeldung/convertlisttomap/ListToMapUnitTest.java @@ -0,0 +1,46 @@ +package com.baeldung.convertlisttomap; + +import static org.junit.Assert.assertTrue; + +import java.util.ArrayList; +import java.util.Arrays; +import java.util.HashMap; +import java.util.List; +import java.util.Map; + +import org.junit.Before; +import org.junit.Test; + +public class ListToMapUnitTest { + + private ListToMapConverter converter; + private List source; + + @Before + public void setUp() { + converter = new ListToMapConverter(); + source = Arrays.asList("List", "Map", "Set", "Tree"); + } + + @Test + public void givenAList_whenConvertWithJava8GroupBy_thenReturnMap() { + Map> convertedMap = converter.groupingByStringLength(source, HashMap::new, ArrayList::new); + assertTrue(convertedMap.get(3) + .contains("Map")); + } + + @Test + public void givenAList_whenConvertWithJava8Collect_thenReturnMap() { + Map> convertedMap = converter.streamCollectByStringLength(source, HashMap::new, ArrayList::new); + assertTrue(convertedMap.get(3) + .contains("Map")); + } + + @Test + public void givenAList_whenConvertWithCollectorToMap_thenReturnMap() { + Map> convertedMap = converter.collectorToMapByStringLength(source, HashMap::new, ArrayList::new); + assertTrue(convertedMap.get(3) + .contains("Map")); + } + +}