How to Create a Unique List of Objects in Java?

How can I create a list of unique or distinct objects in Java?

HashSet<String> uniqueList = new HashSet<>();

In Java, maintaining a list of unique objects can be effectively accomplished using several data structures, including HashSet, ArrayList with Collections, and more. Each of these methods has its own use cases and advantages. Below, I'll outline effective techniques for ensuring you have a list that contains no duplicate entries.

// Using HashSet to maintain unique objects
Set<String> uniqueStrings = new HashSet<>();
uniqueStrings.add("Hello");
uniqueStrings.add("World");
uniqueStrings.add("Hello"); // Duplicate will not be added
System.out.println(uniqueStrings); // Output: [Hello, World]

Causes

• Using inappropriate data structures that allow duplicates, such as ArrayLists.
• Not implementing proper equality checks in custom objects.

Solutions

• Utilize a HashSet which inherently does not allow duplicate entries.
• Apply the Streams API in Java 8 or later to filter duplicates.
• Implement the equals() and hashCode() methods in your custom classes.

Mistake: Using ArrayList instead of HashSet for unique values.

Solution: Switch to using HashSet or TreeSet for automatic duplicate handling.

Mistake: Forgetting to override 'equals()' and 'hashCode()' in custom objects.

Solution: Ensure all custom objects implement 'equals()' and 'hashCode()' correctly to avoid duplicate entries.