Find Duplicate Objects in a List using Java

In this post, I will show you how to find duplicate objects in a List using Java’s Comparator interface implementation based on multiple fields in a POJO.


The following configurations are required in order to run the application

JDK 1.8
Have maven installed and configured
Junit, Mockito, PowerMockito dependency in pom.xml

Now we will see the below steps how to create a maven based Java project in Eclipse
Step 1. Create a standalone maven project in Eclipse

Go to File -> New -> Other. On popup window under Maven select Maven Project. Then click on Next. Select the workspace location – either default or browse the location. Click on Next. Now in next window select the row as highlighted from the below list of archtypes and click on Next button.


Now enter the required fields (Group Id, Artifact Id) as shown below

Group Id : com.roytuts
Artifact Id : junit

Step 2. Modify the pom.xml file as shown below.

Step 3. If you see JRE System Library[J2SE-1.5] then change the version by below process

Do right-click on the project and go to Build -> Configure build path, under Libraries tab click on JRE System Library[J2SE-1.5], click on Edit button and select the appropriate jdk 1.8 from the next window. Click on Finish then Ok.

Step 4. Create a Model class called Car. So we will determine duplicate car is found based on make, model and year fields.

Step 5. Create CarComparator class to compare each fields of the Car model class

Step 6. Create CarUtils class to create utility method for finding duplicates

Step 7. Create Junit class called CarTest which will test for duplicate Cars in a List of Cars

Step 8. Now run the above Junit test class, you will see the below output

Thanks for reading.

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