BAEL-5349 Example for the difference between map and hashmap (#11827)

* example for the difference between map and hashmap

* unit tests for the example

* rename unit tests

Co-authored-by: eugene.kovko <eugene.kovko@tensquaregames.com>

core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/mapandhashmap/Main.java

@@ -0,0 +1,25 @@
+package com.baeldung.mapandhashmap;
+
+import com.baeldung.mapandhashmap.printer.HashMapPrinter;
+import com.baeldung.mapandhashmap.printer.MapPrinter;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.TreeMap;
+
+public class Main {
+    public static void main(String[] args) {
+        Map<String, String> map = new HashMap<>();
+        HashMap<String, String> hashMap = new HashMap<>();
+        TreeMap<String, String> treeMap = new TreeMap<>();
+
+        HashMapPrinter hashMapPrinter = new HashMapPrinter();
+        hashMapPrinter.printMap(hashMap);
+//        hashMapPrinter.printMap(treeMap); Compile time error
+//        hashMapPrinter.printMap(map); Compile time error
+
+        MapPrinter mapPrinter = new MapPrinter();
+        mapPrinter.printMap(hashMap);
+        mapPrinter.printMap(treeMap);
+        mapPrinter.printMap(map);
+    }
+}

core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/mapandhashmap/printer/HashMapPrinter.java

@@ -0,0 +1,13 @@
+package com.baeldung.mapandhashmap.printer;
+
+import java.util.HashMap;
+import java.util.Map.Entry;
+
+public class HashMapPrinter {
+
+    public void printMap(final HashMap<?, ?> map) {
+        for (final Entry<?, ?> entry : map.entrySet()) {
+            System.out.println(entry.getKey() + " " + entry.getValue());
+        }
+    }
+}

core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/mapandhashmap/printer/MapPrinter.java

@@ -0,0 +1,13 @@
+package com.baeldung.mapandhashmap.printer;
+
+import java.util.Map;
+import java.util.Map.Entry;
+
+public class MapPrinter {
+
+    public void printMap(final Map<?, ?> map) {
+        for (final Entry<?, ?> entry : map.entrySet()) {
+            System.out.println(entry.getKey() + " " + entry.getValue());
+        }
+    }
+}

core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/mapandhashmap/printer/MapReporter.java

@@ -0,0 +1,19 @@
+package com.baeldung.mapandhashmap.printer;
+
+import java.util.Map;
+import java.util.Map.Entry;
+
+public class MapReporter {
+
+    private final Map<?, ?> map;
+
+    public MapReporter(final Map<?, ?> map) {
+        this.map = map;
+    }
+
+    public void printMap() {
+        for (final Entry<?, ?> entry : this.map.entrySet()) {
+            System.out.println(entry.getKey() + " " + entry.getValue());
+        }
+    }
+}

core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/mapandhashmap/printer/HashMapPrinterUnitTest.java

@@ -0,0 +1,56 @@
+package com.baeldung.mapandhashmap.printer;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class HashMapPrinterUnitTest {
+
+    private final HashMapPrinter mapPrinter = new HashMapPrinter();
+    private final ByteArrayOutputStream outputStreamCaptor = new ByteArrayOutputStream();
+
+    @BeforeEach
+    public void setUp() {
+        System.setOut(new PrintStream(outputStreamCaptor));
+    }
+
+    @Test
+    @DisplayName("Test hash map printer with HashMap")
+    void testPrintHashMap() {
+        // given
+        String key = "HashMap";
+        String value = "Main default implementation for the Map interface";
+        String expected = key + " " + value;
+        HashMap<String, String> map = new HashMap<>();
+        map.put(key, value);
+        // when
+        mapPrinter.printMap(map);
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+
+    }
+
+    @Test
+    @DisplayName("Test hash map printer with LinkedHash")
+    void testPrintLinkedHashMap() {
+        // given
+        String key = "LinkedHashMap";
+        String value = "Use this implementation if you need keep the order of elements";
+        String expected = key + " " + value;
+        LinkedHashMap<String, String> map = new LinkedHashMap<>();
+        map.put(key, value);
+        // when
+        mapPrinter.printMap(map);
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+
+    }
+}

core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/mapandhashmap/printer/MapPrinterUnitTest.java

@@ -0,0 +1,71 @@
+package com.baeldung.mapandhashmap.printer;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class MapPrinterUnitTest {
+
+    private final MapPrinter mapPrinter = new MapPrinter();
+    private final ByteArrayOutputStream outputStreamCaptor = new ByteArrayOutputStream();
+
+    @BeforeEach
+    public void setUp() {
+        System.setOut(new PrintStream(outputStreamCaptor));
+    }
+
+    @Test
+    @DisplayName("Test printer with TreeMap")
+    void testPrintTreeMap() {
+        // given
+        String key = "TreeMap";
+        String value = "Used when sorting is needed";
+        String expected = key + " " + value;
+        TreeMap<String, String> map = new TreeMap<>();
+        map.put(key, value);
+        // when
+        mapPrinter.printMap(map);
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+    }
+
+    @Test
+    @DisplayName("Test printer with HashMap")
+    void testPrintHashMap() {
+        // given
+        String key = "HashMap";
+        String value = "Main default implementation for the Map interface";
+        String expected = key + " " + value;
+        HashMap<String, String> map = new HashMap<>();
+        map.put(key, value);
+        // when
+        mapPrinter.printMap(map);
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+    }
+
+    @Test
+    @DisplayName("Test printer with LinkedHash")
+    void testPrintLinkedHashMap() {
+        // given
+        String key = "LinkedHashMap";
+        String value = "Use this implementation if you need keep the order of elements";
+        String expected = key + " " + value;
+        LinkedHashMap<String, String> map = new LinkedHashMap<>();
+        map.put(key, value);
+        // when
+        mapPrinter.printMap(map);
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+    }
+}

core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/mapandhashmap/printer/MapReporterUnitTest.java

@@ -0,0 +1,74 @@
+package com.baeldung.mapandhashmap.printer;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class MapReporterUnitTest {
+
+    private final ByteArrayOutputStream outputStreamCaptor = new ByteArrayOutputStream();
+
+    @BeforeEach
+    public void setUp() {
+        System.setOut(new PrintStream(outputStreamCaptor));
+    }
+
+    @Test
+    @DisplayName("Test reporter with TreeMap")
+    void testPrintTreeMap() {
+        // given
+        String key = "TreeMap";
+        String value = "Used when sorting is needed";
+        String expected = key + " " + value;
+        TreeMap<String, String> map = new TreeMap<>();
+        map.put(key, value);
+        // when
+        MapReporter mapReporter = new MapReporter(map);
+        mapReporter.printMap();
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+    }
+
+    @Test
+    @DisplayName("Test reporter with HashMap")
+    void testPrintHashMap() {
+        // given
+        String key = "HashMap";
+        String value = "Main default implementation for the Map interface";
+        String expected = key + " " + value;
+        HashMap<String, String> map = new HashMap<>();
+        map.put(key, value);
+        // when
+        MapReporter mapReporter = new MapReporter(map);
+        mapReporter.printMap();
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+    }
+
+    @Test
+    @DisplayName("Test reporter with LinkedHash")
+    void testPrintLinkedHashMap() {
+        // given
+        String key = "LinkedHashMap";
+        String value = "Use this implementation if you need keep the order of elements";
+        String expected = key + " " + value;
+        LinkedHashMap<String, String> map = new LinkedHashMap<>();
+        map.put(key, value);
+        // when
+        MapReporter mapReporter = new MapReporter(map);
+        mapReporter.printMap();
+        // then
+        String actual = outputStreamCaptor.toString().trim();
+        assertThat(actual).isEqualTo(expected);
+    }
+
+}