[JAVA-5730] convert list of objects to list of strings (#13166)

* [JAVA-5730] convert list of objects to list of strings

* [JAVA-5730] added new unit tests with null and non-null lists. modified pom.xml with java version to 16

* [JAVA-5730] simplified User class and formatted the codebase

* [JAVA-5730] added module in jdk-9 and above profile

* [JAVA-5730] removed arraylist module from other profiles of core-java pom

* [JAVA-5730] java util opened for reflection surefire plugin

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

core-java-modules/core-java-collections-array-list/pom.xml

@@ -5,6 +5,33 @@
     <modelVersion>4.0.0</modelVersion>
     <artifactId>core-java-collections-array-list</artifactId>
     <version>0.1.0-SNAPSHOT</version>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>${maven-compiler-plugin.source}</source>
+                    <target>${maven-compiler-plugin.target}</target>
+                </configuration>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>${surefire.plugin.version}</version>
+                <configuration>
+                    <argLine>
+                        --add-opens java.base/java.util=ALL-UNNAMED
+                    </argLine>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+    <properties>
+        <maven-compiler-plugin.source>16</maven-compiler-plugin.source>
+        <maven-compiler-plugin.target>16</maven-compiler-plugin.target>
+        <surefire.plugin.version>3.0.0-M3</surefire.plugin.version>
+    </properties>
     <name>core-java-collections-array-list</name>
     <packaging>jar</packaging>
 
@@ -20,6 +47,12 @@
             <artifactId>commons-collections4</artifactId>
             <version>${commons-collections4.version}</version>
         </dependency>
+        <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+            <version>31.1-jre</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
 </project>

core-java-modules/core-java-collections-array-list/src/main/java/com/baeldung/listofobjectstolistofstring/Node.java

@@ -0,0 +1,17 @@
+package com.baeldung.listofobjectstolistofstring;
+
+public class Node {
+
+    private final int x;
+    private final int y;
+
+    public Node(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    @Override
+    public String toString() {
+        return "Node (" + "x=" + x + ", y=" + y + ')';
+    }
+}

core-java-modules/core-java-collections-array-list/src/main/java/com/baeldung/listofobjectstolistofstring/User.java

@@ -0,0 +1,14 @@
+package com.baeldung.listofobjectstolistofstring;
+
+public class User {
+    private final String fullName;
+
+    public User(String fullName) {
+        this.fullName = fullName;
+    }
+
+    @Override
+    public String toString() {
+        return "User (" + "full name='" + fullName + ')';
+    }
+}

core-java-modules/core-java-collections-array-list/src/test/java/com/baeldung/listofobjectstolistofstring/ConvertObjectListToStringListUnitTest.java

@@ -0,0 +1,92 @@
+package com.baeldung.listofobjectstolistofstring;
+
+import com.google.common.collect.Lists;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+public class ConvertObjectListToStringListUnitTest {
+
+    @Test
+    public void givenObjectList_whenForEachUsedToConvert_thenReturnSuccess() {
+        List<String> outputList = new ArrayList<>(objectListWithNull().size());
+        for (Object obj : objectListWithNull()) {
+            outputList.add(Objects.toString(obj, null));
+        }
+        Assert.assertEquals(expectedStringListWithNull(), outputList);
+    }
+
+    @Test
+    public void givenObjectList_whenUsingStreamsToConvert_thenReturnSuccess() {
+        List<String> outputList;
+        outputList = objectListWithNull().stream()
+          .map((obj) -> Objects.toString(obj, null))
+          .collect(Collectors.toList());
+        Assert.assertEquals(expectedStringListWithNull(), outputList);
+
+    }
+
+    @Test
+    public void givenObjectList_whenUsingStreamsUnmodifiableListToConvert_thenReturnSuccess() {
+        List<String> outputList;
+        outputList = objectListWithNull().stream()
+          .filter(Objects::nonNull)
+          .map((obj) -> Objects.toString(obj, null))
+          .collect(Collectors.toUnmodifiableList());
+        Assert.assertEquals(expectedStringListWithoutNull(), outputList);
+
+    }
+
+    @Test
+    public void givenObjectList_whenUsingGuavaTransform_thenReturnSuccess() {
+        List<String> outputList;
+        outputList = Lists.transform(objectListWithNull(), obj -> Objects.toString(obj, null));
+        Assert.assertEquals(expectedStringListWithNull(), outputList);
+    }
+
+    @Test
+    public void givenObjectListWithNoNull_whenUsingToList_thenReturnSuccess() {
+        List<String> outputList;
+        outputList = objectListWithoutNull().stream()
+          .map((obj) -> Objects.toString(obj, null))
+          .toList();
+        Assert.assertEquals(expectedStringListWithoutNull(), outputList);
+    }
+
+    private List<String> expectedStringListWithNull() {
+        List<String> listOfStrings = new ArrayList<>();
+        listOfStrings.add("1");
+        listOfStrings.add("true");
+        listOfStrings.add("hello");
+        listOfStrings.add(Double.toString(273773.98));
+        listOfStrings.add(null);
+        listOfStrings.add(new Node(2, 4).toString());
+        listOfStrings.add(new User("John Doe").toString());
+        return listOfStrings;
+    }
+
+    private List<Object> objectListWithNull() {
+        List<Object> listOfStrings = new ArrayList<>();
+        listOfStrings.add(1);
+        listOfStrings.add(true);
+        listOfStrings.add("hello");
+        listOfStrings.add(Double.valueOf(273773.98));
+        listOfStrings.add(null);
+        listOfStrings.add(new Node(2, 4));
+        listOfStrings.add(new User("John Doe"));
+        return listOfStrings;
+    }
+
+    private List<String> expectedStringListWithoutNull() {
+        return List.of("1", "true", "hello", Double.toString(273773.98), new Node(2, 4).toString(), new User("John Doe").toString());
+    }
+
+    private List<Object> objectListWithoutNull() {
+        return List.of(1, true, "hello", Double.valueOf(273773.98), new Node(2, 4), new User("John Doe"));
+    }
+}

core-java-modules/pom.xml

@@ -31,7 +31,6 @@
         <module>core-java-collections-2</module>
         <module>core-java-collections-3</module>
         <module>core-java-collections-4</module>
-        <module>core-java-collections-array-list</module>
         <module>core-java-collections-conversions</module>
         <module>core-java-collections-conversions-2</module>
         <module>core-java-collections-set-2</module>

pom.xml

@@ -1133,6 +1133,7 @@
                  <!-- <module>core-java-modules/core-java-19</module> --> <!-- uses preview features, to be decided how to handle -->
                  <module>core-java-modules/core-java-collections-set</module>
                  <module>core-java-modules/core-java-collections-list-4</module>
+                 <module>core-java-modules/core-java-collections-array-list</module>
                  <module>core-java-modules/core-java-collections-maps-4</module>
                  <module>core-java-modules/core-java-collections-maps-5</module>
                  <module>core-java-modules/core-java-concurrency-simple</module>