[JAVA-6143] java copy from one hashmap to another (#13497)

Co-authored-by: Bhaskar <bhaskar.dastidar@freshworks.com>

core-java-modules/core-java-collections-maps-6/pom.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <artifactId>core-java-collections-maps-6</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-collections-maps-6</name>
+    <packaging>jar</packaging>
+    <parent>
+        <artifactId>core-java-modules</artifactId>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <properties>
+        <spring.version>5.2.5.RELEASE</spring.version>
+    </properties>
+
+</project>

core-java-modules/core-java-collections-maps-6/src/main/java/com/baeldung/map/hashmapcopy/CopyingAHashMapToAnother.java

@@ -0,0 +1,47 @@
+package com.baeldung.map.hashmapcopy;
+
+import java.util.Map;
+
+import com.google.common.collect.MapDifference;
+import com.google.common.collect.Maps;
+
+public class CopyingAHashMapToAnother {
+    public Map<String, String> copyByIteration(Map<String, String> sourceMap, Map<String, String> targetMap) {
+        for (Map.Entry<String, String> entry : sourceMap.entrySet()) {
+            if (!targetMap.containsKey(entry.getKey())) {
+                targetMap.put(entry.getKey(), entry.getValue());
+            }
+        }
+        return targetMap;
+    }
+
+    public Map<String, String> copyUsingPutAll(Map<String, String> sourceMap, Map<String, String> targetMap) {
+        sourceMap.keySet()
+          .removeAll(targetMap.keySet());
+        targetMap.putAll(sourceMap);
+        return targetMap;
+    }
+
+    public Map<String, String> copyUsingPutIfAbsent(Map<String, String> sourceMap, Map<String, String> targetMap) {
+        for (Map.Entry<String, String> entry : sourceMap.entrySet()) {
+            targetMap.putIfAbsent(entry.getKey(), entry.getValue());
+        }
+        return targetMap;
+    }
+
+    public Map<String, String> copyUsingPutIfAbsentForEach(Map<String, String> sourceMap, Map<String, String> targetMap) {
+        sourceMap.forEach(targetMap::putIfAbsent);
+        return targetMap;
+    }
+
+    public Map<String, String> copyUsingMapMerge(Map<String, String> sourceMap, Map<String, String> targetMap) {
+        sourceMap.forEach((key, value) -> targetMap.merge(key, value, (oldVal, newVal) -> oldVal));
+        return targetMap;
+    }
+
+    public Map<String, String> copyUsingGuavaMapDifference(Map<String, String> sourceMap, Map<String, String> targetMap) {
+        MapDifference<String, String> differenceMap = Maps.difference(sourceMap, targetMap);
+        targetMap.putAll(differenceMap.entriesOnlyOnLeft());
+        return targetMap;
+    }
+}

core-java-modules/core-java-collections-maps-6/src/test/java/com/baeldung/map/hashmapcopy/CopyHashMapIntoAnotherUnitTest.java

@@ -0,0 +1,68 @@
+package com.baeldung.map.hashmapcopy;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import com.baeldung.map.hashmapcopy.CopyingAHashMapToAnother;
+
+public class CopyHashMapIntoAnotherUnitTest {
+    @Test
+    public void givenSourceAndTargetMapsWhenIteratedOverThenCopyingSuccess(){
+        CopyingAHashMapToAnother obj = new CopyingAHashMapToAnother();
+        Assert.assertEquals(generateExpectedResultMap(), obj.copyByIteration(generateSourceMap(), generateTargetMap()));
+    }
+
+    @Test
+    public void givenSourceAndTargetMapsWhenUsedPutAllThenCopyingSuccess(){
+        CopyingAHashMapToAnother obj = new CopyingAHashMapToAnother();
+        Assert.assertEquals(generateExpectedResultMap(), obj.copyUsingPutAll(generateSourceMap(), generateTargetMap()));
+    }
+
+    @Test
+    public void givenSourceAndTargetMapsWhenUsedPutIfAbsentThenCopyingSuccess(){
+        CopyingAHashMapToAnother obj = new CopyingAHashMapToAnother();
+        Assert.assertEquals(generateExpectedResultMap(), obj.copyUsingPutIfAbsent(generateSourceMap(), generateTargetMap()));
+        Assert.assertEquals(generateExpectedResultMap(), obj.copyUsingPutIfAbsentForEach(generateSourceMap(), generateTargetMap()));
+    }
+
+    @Test
+    public void givenSourceAndTargetMapsWhenUsedMapMergeThenCopyingSuccess(){
+        CopyingAHashMapToAnother obj = new CopyingAHashMapToAnother();
+        Assert.assertEquals(generateExpectedResultMap(), obj.copyUsingMapMerge(generateSourceMap(), generateTargetMap()));
+    }
+
+    @Test
+    public void givenSourceAndTargetMapsWhenMapDifferenceUsedThenCopyingSuccess(){
+        CopyingAHashMapToAnother obj = new CopyingAHashMapToAnother();
+        Assert.assertEquals(generateExpectedResultMap(), obj.copyUsingGuavaMapDifference(generateSourceMap(), generateTargetMap()));
+    }
+
+    private Map<String, String> generateSourceMap(){
+        Map<String, String> sourceMap = new HashMap<>();
+        sourceMap.put("India", "Delhi");
+        sourceMap.put("United States", "Washington D.C.");
+        sourceMap.put("United Kingdom", "London DC");
+        return sourceMap;
+    }
+
+    private Map<String, String> generateTargetMap(){
+        Map<String, String> targetMap = new HashMap<>();
+        targetMap.put("Zimbabwe", "Harare");
+        targetMap.put("Norway", "Oslo");
+        targetMap.put("United Kingdom", "London");
+        return targetMap;
+    }
+
+    private Map<String, String> generateExpectedResultMap(){
+        Map<String, String> resultMap = new HashMap<>();
+        resultMap.put("India", "Delhi");
+        resultMap.put("United States", "Washington D.C.");
+        resultMap.put("United Kingdom", "London");
+        resultMap.put("Zimbabwe", "Harare");
+        resultMap.put("Norway", "Oslo");
+        return resultMap;
+    }
+}

core-java-modules/pom.xml

@@ -132,6 +132,7 @@
         <module>core-java-regex-2</module>
         <module>core-java-uuid</module>
         <module>pre-jpms</module>
+        <module>core-java-collections-maps-6</module>
     </modules>
 
     <dependencyManagement>