How to Find Matching Objects Between Two Lists in Java?

What is the best way to find matching objects between two lists in Java?

List<Object> list1 = Arrays.asList(new Object(1), new Object(2), new Object(3));
List<Object> list2 = Arrays.asList(new Object(2), new Object(3), new Object(4));

Finding matching objects between two lists in Java can be achieved using various techniques, depending upon the nature of the objects and the desired efficiency. Common approaches include using nested loops, Java 8 Streams, or Sets for more optimized performance.

import java.util.*;

class Object {
    int value;
    Object(int value) { this.value = value; }
    @Override
    public boolean equals(Object obj) { 
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Object object = (Object) obj;
        return value == object.value;
    }
    @Override
    public int hashCode() { return Objects.hash(value); }
}

public class MatchFinder {
    public static void main(String[] args) {
        List<Object> list1 = Arrays.asList(new Object(1), new Object(2), new Object(3));
        List<Object> list2 = Arrays.asList(new Object(2), new Object(3), new Object(4));
        Set<Object> set = new HashSet<>(list1);
        set.retainAll(list2);
        System.out.println("Matching Objects: " + set);
    }
}

Causes

• Different object properties or fields being compared.
• Using inappropriate data structures for comparison.
• Ignoring null values or duplicates in lists.

Solutions

• Using a nested loop to compare elements directly.
• Leveraging Java 8 Streams for a more functional approach.
• Utilizing HashSet for improved performance in lookups.

Mistake: Not overriding `equals()` and `hashCode()` methods in custom objects.

Solution: Ensure to override both methods to compare object fields accurately.

Mistake: Assuming the objects are unique without using a Set.

Solution: Use a Set for matching to avoid duplicates and improve lookup efficiency.