[BAEL-5840] Sort a collection of Objects on multiple fields (#12950)

* [BAEL-5840] Sort a collection of Objects on multiple fields

* Fix maven plugin version

* Fix pom again

* Fix again

* Move to core-java-collections-4 module

* Remove unused files

* Test renaming

* Handle NPE

* Use Nonnull annotation

* remove redundant dependency

* remove redundant scope

* Update pom.xml

core-java-modules/core-java-collections-4/pom.xml

@@ -20,10 +20,16 @@
             <artifactId>commons-lang</artifactId>
             <version>${commons-lang.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons-lang3.version}</version>
+        </dependency>
     </dependencies>
 
     <properties>
         <commons-lang.version>2.2</commons-lang.version>
+        <commons-lang3.version>3.12.0</commons-lang3.version>
     </properties>
 
-</project>
+</project>

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/sorting/Employee.java

@@ -2,8 +2,11 @@ package com.baeldung.collections.sorting;
 
 import java.util.Date;
 
+import javax.annotation.Nonnull;
+
 public class Employee implements Comparable<Employee>{
 
+    @Nonnull
     private String name;
     private Date joiningDate;
 

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/sorting/multiple/CheckFieldsOneByOne.java

@@ -0,0 +1,15 @@
+package com.baeldung.collections.sorting.multiple;
+
+import java.util.Comparator;
+
+
+public class CheckFieldsOneByOne implements Comparator<Person> {
+    @Override
+    public int compare(Person o1, Person o2) {
+        int nameCompare = o1.getName().compareTo(o2.getName());
+        if(nameCompare != 0) {
+            return nameCompare;
+        }
+        return Integer.compare(o1.getAge(), o2.getAge());
+    }
+}

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/sorting/multiple/ComparatorLambda.java

@@ -0,0 +1,10 @@
+package com.baeldung.collections.sorting.multiple;
+
+import java.util.Comparator;
+
+public class ComparatorLambda {
+    public static Comparator<Person> createEmployeeComparator() {
+        return Comparator.comparing(Person::getName)
+          .thenComparing(Person::getAge);
+    }
+}

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/sorting/multiple/CompareToBuilderExample.java

@@ -0,0 +1,15 @@
+package com.baeldung.collections.sorting.multiple;
+
+import java.util.Comparator;
+
+import org.apache.commons.lang3.builder.CompareToBuilder;
+
+public class CompareToBuilderExample implements Comparator<Person> {
+    @Override
+    public int compare(Person o1, Person o2) {
+        return new CompareToBuilder()
+          .append(o1.getName(), o2.getName())
+          .append(o1.getAge(), o2.getAge())
+          .build();
+    }
+}

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/sorting/multiple/ComparisonChainExample.java

@@ -0,0 +1,15 @@
+package com.baeldung.collections.sorting.multiple;
+
+import java.util.Comparator;
+
+import com.google.common.collect.ComparisonChain;
+import com.google.common.collect.Ordering;
+
+public class ComparisonChainExample implements Comparator<Person> {
+    @Override
+    public int compare(Person o1, Person o2) {
+        return ComparisonChain.start()
+          .compare(o1.getName(), o2.getName(), Ordering.natural().nullsFirst())
+          .compare(o1.getAge(), o2.getAge()).result();
+    }
+}

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/sorting/multiple/Person.java

@@ -0,0 +1,29 @@
+package com.baeldung.collections.sorting.multiple;
+
+public class Person {
+    private String name;
+    private int age;
+
+    public Person(String name, int age) {
+        this.name = name;
+        this.age = age;
+    }
+
+    // getters and setters
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getAge() {
+        return age;
+    }
+
+    public void setAge(int age) {
+        this.age = age;
+    }
+}

core-java-modules/core-java-collections-4/src/test/java/com/baeldung/collections/sorting/multiple/ComparatorsUnitTest.java

@@ -0,0 +1,34 @@
+package com.baeldung.collections.sorting.multiple;
+
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+
+public class ComparatorsUnitTest {
+    @Test
+    public void givenPersonsList_whenComparedOnTwoFields_thenReturnCorrectOrder() {
+        Person person1 = new Person("John", 21);
+        Person person2 = new Person("Tom", 20);
+        // Another employee named John
+        Person person3 = new Person("John", 22);
+
+        List<Comparator<Person>> comparators =
+          Arrays.asList(new CheckFieldsOneByOne(),
+            new ComparisonChainExample(),
+            new CompareToBuilderExample(),
+            ComparatorLambda.createEmployeeComparator());
+        // All comparators should produce the same result
+        for(Comparator<Person> comparator : comparators) {
+            Assertions.assertIterableEquals(
+              Arrays.asList(person1, person2, person3)
+                .stream()
+                .sorted(comparator)
+                .collect(Collectors.toList()),
+              Arrays.asList(person1, person3, person2));
+        }
+    }
+}