How to Compare Two Array Lists in Java

Java offers a straightforward method to compare two ArrayList objects: the equals() method. This method determines equality based on two criteria: both lists must have the same size, and each element at a corresponding index in both lists must be equal.

Understanding ArrayList.equals()

The equals() method in Java’s ArrayList class provides a robust way to compare two ArrayList objects for equality. Let’s delve into its functionality:

Functionality

The equals() method compares two ArrayLists element by element, ensuring that:

1. Size Equality: Both ArrayLists must contain the same number of elements.
2. Element-wise Equality: Each element in the first ArrayList must be equal to the element at the corresponding index in the second ArrayList. This comparison utilizes the equals() method of the elements themselves. Therefore, if your ArrayList contains custom objects, ensure you have correctly overridden the equals() method in your custom class.

Syntax

boolean equals(Object o)

Parameters:

• o: The object to be compared with the current ArrayList. While it accepts any Object, it will only return true if o is also an ArrayList with matching content.

Return Value:

• true: If the ArrayLists are equal based on the criteria mentioned above.
• false: Otherwise.

Example: Comparing Two ArrayLists

Let’s illustrate how to use the equals() method with a practical example:

import java.util.ArrayList;

public class CompareArrayLists {

    public static void main(String[] args) {

        // Create two ArrayLists
        ArrayList<String> list1 = new ArrayList<>();
        ArrayList<String> list2 = new ArrayList<>();

        // Populate list1
        list1.add("apple");
        list1.add("banana");
        list1.add("orange");

        // Populate list2 with the same elements
        list2.add("apple");
        list2.add("banana");
        list2.add("orange");

        // Compare the two lists
        boolean areEqual = list1.equals(list2);

        // Print the result
        System.out.println("Are the lists equal? " + areEqual); // Output: true


        // Modify list2
        list2.add("grape");

        // Compare again
        areEqual = list1.equals(list2);


        // Print the result
        System.out.println("Are the lists equal now? " + areEqual);  // Output: false
    }
}

In this example, we first create two identical ArrayLists. The initial comparison using equals() returns true. Then, we modify list2, causing the subsequent comparison to return false.

Time Complexity

The time complexity of the equals() method for ArrayList is O(n), where n is the number of elements in the ArrayList. In the worst-case scenario, it needs to compare all elements in both lists. If the sizes of the lists are different, the comparison finishes in O(1) time.

Conclusion

The equals() method offers a simple yet effective way to compare two ArrayLists in Java. Remember that equality depends on both the size and the order of elements within the lists. By understanding its functionality and limitations, you can leverage this method to perform accurate comparisons in your Java applications.