BAEL-1983 Intialize a HashMap in Java (#4819)

core-java-9/src/main/java/com/baeldung/java9/maps/initialize/MapsInitializer.java

@@ -0,0 +1,33 @@
+package com.baeldung.java9.maps.initialize;
+
+import java.util.AbstractMap;
+import java.util.HashMap;
+import java.util.Map;
+
+public class MapsInitializer {
+
+    @SuppressWarnings("unused")
+    public void createMapWithMapOf() {
+        Map<String, String> emptyMap = Map.of();
+        Map<String, String> singletonMap = Map.of("key1", "value");
+        Map<String, String> map = Map.of("key1","value1", "key2", "value2");
+    }
+    
+    public void createMapWithMapEntries() {
+        Map<String, String> map = Map.ofEntries(
+            new AbstractMap.SimpleEntry<String, String>("name", "John"),
+            new AbstractMap.SimpleEntry<String, String>("city", "budapest"),
+            new AbstractMap.SimpleEntry<String, String>("zip", "000000"),
+            new AbstractMap.SimpleEntry<String, String>("home", "1231231231")
+            );
+    }
+    
+    @SuppressWarnings("unused")
+    public void createMutableMaps() {
+        Map<String, String> map = new HashMap<String, String> (Map.of("key1","value1", "key2", "value2"));
+        Map<String, String> map2 = new HashMap<String, String> ( Map.ofEntries(
+            new AbstractMap.SimpleEntry<String, String>("name", "John"),
+            new AbstractMap.SimpleEntry<String, String>("city", "budapest")));
+        
+    }
+}

core-java-collections/src/main/java/com/baeldung/java/map/initialize/MapInitializer.java

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+package com.baeldung.java.map.initialize;
+
+import java.util.AbstractMap;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public class MapInitializer {
+
+    public static Map<String, String> articleMapOne;
+    static {
+        articleMapOne = new HashMap<>();
+        articleMapOne.put("ar01", "Intro to Map");
+        articleMapOne.put("ar02", "Some article");
+    }
+
+    public static Map<String, String> createSingletonMap() {
+        Map<String, String> passwordMap = Collections.singletonMap("username1", "password1");
+        return passwordMap;
+
+    }
+
+    public Map<String, String> createEmptyMap() {
+        Map<String, String> emptyMap = Collections.emptyMap();
+        return emptyMap;
+    }
+
+    public Map<String, String> createUsingDoubleBrace() {
+        Map<String, String> doubleBraceMap = new HashMap<String, String>() {
+
+            /**
+             * 
+             */
+            private static final long serialVersionUID = 1L;
+
+            {
+                put("key1", "value1");
+                put("key2", "value2");
+            }
+        };
+        return doubleBraceMap;
+    }
+
+    public Map<String, String> createMapUsingStreamStringArray() {
+        Map<String, String> map = Stream.of(new String[][] { { "Hello", "World" }, { "John", "Doe" }, })
+            .collect(Collectors.toMap(data -> data[0], data -> data[1]));
+
+        return map;
+    }
+
+    public Map<String, Integer> createMapUsingStreamObjectArray() {
+        Map<String, Integer> map = Stream.of(new Object[][] { { "data1", 1 }, { "data2", 2 }, })
+            .collect(Collectors.toMap(data -> (String) data[0], data -> (Integer) data[1]));
+
+        return map;
+    }
+
+    public Map<String, Integer> createMapUsingStreamSimpleEntry() {
+        Map<String, Integer> map = Stream.of(new AbstractMap.SimpleEntry<>("idea", 1), new AbstractMap.SimpleEntry<>("mobile", 2))
+            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
+
+        return map;
+    }
+
+    public Map<String, Integer> createMapUsingStreamSimpleImmutableEntry() {
+        Map<String, Integer> map = Stream.of(new AbstractMap.SimpleImmutableEntry<>("idea", 1), new AbstractMap.SimpleImmutableEntry<>("mobile", 2))
+            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
+
+        return map;
+    }
+
+    public Map<String, String> createImmutableMapWithStreams() {
+        Map<String, String> map = Stream.of(new String[][] { { "Hello", "World" }, { "John", "Doe" }, })
+            .collect(Collectors.collectingAndThen(Collectors.toMap(data -> data[0], data -> data[1]), Collections::<String, String> unmodifiableMap));
+        return map;
+
+    }
+}

core-java-collections/src/test/java/com/baeldung/java/map/initialize/MapInitializerUnitTest.java

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+package com.baeldung.java.map.initialize;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.Map;
+
+import org.junit.Test;
+
+public class MapInitializerUnitTest {
+    
+    @Test
+    public void givenStaticMap_whenUpdated_thenCorrect() {
+        
+        MapInitializer.articleMapOne.put("NewArticle1", "Convert array to List");
+        
+        assertEquals(MapInitializer.articleMapOne.get("NewArticle1"), "Convert array to List");
+        
+    }
+    
+    @Test(expected=UnsupportedOperationException.class)
+    public void givenSingleTonMap_whenEntriesAdded_throwsException() {
+        
+        Map<String, String> map = MapInitializer.createSingletonMap();
+        map.put("username2", "password2");
+    }
+
+}