Merge pull request #9519 from LeasyZhang/convert-list-to-map

Leasy Zhang/shiwangzhihe@gmail.com

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<Integer, List<String>> groupingByStringLength(List<String> source, 
+        Supplier<Map<Integer, List<String>>> mapSupplier, 
+        Supplier<List<String>> listSupplier) {
+        
+        return source.stream()
+            .collect(Collectors.groupingBy(String::length, mapSupplier, Collectors.toCollection(listSupplier)));
+    }
+    
+    public Map<Integer, List<String>> streamCollectByStringLength(List<String> source, 
+        Supplier<Map<Integer, List<String>>> mapSupplier, 
+        Supplier<List<String>> listSupplier) {
+        
+        BiConsumer<Map<Integer, List<String>>, String> accumulator = (response, element) -> {
+            Integer key = element.length();
+            List<String> values = response.getOrDefault(key, listSupplier.get());
+            values.add(element);
+            response.put(key, values);
+        };
+
+        BiConsumer<Map<Integer, List<String>>, Map<Integer, List<String>>> combiner = (res1, res2) -> {
+            res1.putAll(res2);
+        };
+
+        return source.stream()
+            .collect(mapSupplier, accumulator, combiner);
+    }
+    
+    public Map<Integer, List<String>> collectorToMapByStringLength(List<String> source, 
+        Supplier<Map<Integer, List<String>>> mapSupplier, 
+        Supplier<List<String>> listSupplier) {
+        
+        Function<String, Integer> keyMapper = String::length;
+
+        Function<String, List<String>> valueMapper = (element) -> {
+            List<String> collection = listSupplier.get();
+            collection.add(element);
+            return collection;
+        };
+
+        BinaryOperator<List<String>> mergeFunction = (existing, replacement) -> {
+            existing.addAll(replacement);
+            return existing;
+        };
+
+        return source.stream()
+            .collect(Collectors.toMap(keyMapper, valueMapper, mergeFunction, mapSupplier));
+    }
+
+}

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<String> source;
+
+    @Before
+    public void setUp() {
+        converter = new ListToMapConverter();
+        source = Arrays.asList("List", "Map", "Set", "Tree");
+    }
+
+    @Test
+    public void givenAList_whenConvertWithJava8GroupBy_thenReturnMap() {
+        Map<Integer, List<String>> convertedMap = converter.groupingByStringLength(source, HashMap::new, ArrayList::new);
+        assertTrue(convertedMap.get(3)
+            .contains("Map"));
+    }
+
+    @Test
+    public void givenAList_whenConvertWithJava8Collect_thenReturnMap() {
+        Map<Integer, List<String>> convertedMap = converter.streamCollectByStringLength(source, HashMap::new, ArrayList::new);
+        assertTrue(convertedMap.get(3)
+            .contains("Map"));
+    }
+
+    @Test
+    public void givenAList_whenConvertWithCollectorToMap_thenReturnMap() {
+        Map<Integer, List<String>> convertedMap = converter.collectorToMapByStringLength(source, HashMap::new, ArrayList::new);
+        assertTrue(convertedMap.get(3)
+            .contains("Map"));
+    }
+
+}