17 March 2020

Java Compare Two Lists

development

The List interface in Java provides methods to be able to compare two Lists and find the common and missing items from the lists.

Compare two unsorted lists for equality

If you want to check that two lists are equal, i.e. contain the same items and and appear in the same index then we can use:

import java.util.Arrays;
import java.util.List;

public class CompareTwoLists {

    public static void main(String[] args) {
        List<String> listOne = Arrays.asList("a", "b", "c");
        List<String> listTwo = Arrays.asList("a", "b", "c");
        List<String> listThree = Arrays.asList("c", "a", "b");

        boolean isEqual = listOne.equals(listTwo);
        System.out.println(isEqual);

        isEqual = listOne.equals(listThree);
        System.out.println(isEqual);
    }
}

Output:

true
false

As you can see the equals() method compares the items and their location in the list.

Compare two sorted lists

Do two lists contain same items?

To compare two lists for equality just in terms of items regardless of their location, we need to use the sort() method from the Collections() class.

For example:

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CompareTwoLists {

    public static void main(String[] args) {
        List<String> listOne = Arrays.asList("b", "c", "a");
        List<String> listTwo = Arrays.asList("a", "c", "b");
        List<String> listThree = Arrays.asList("c", "a", "b");

        Collections.sort(listOne);
        Collections.sort(listTwo);
        Collections.sort(listThree);

        boolean isEqual = listOne.equals(listTwo);
        System.out.println(isEqual);

        isEqual = listOne.equals(listThree);
        System.out.println(isEqual);
    }
}

Output:

true
true

Compare two lists, find differences

The List interface also provides methods to find differences between two lists.

The removeAll() method compares two lists and removes all the common items. What’s left is the additional or missing items.

For example when we compare two lists, listOne and listTwo and we want to find out what items are missing from listTwo we use:

import java.util.ArrayList;
import java.util.Arrays;

public class CompareTwoArrayLists {

    public static void main(String[] args) {
        ArrayList<Integer> listOne = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

        ArrayList<Integer> listTwo = new ArrayList<>(Arrays.asList(1, 2, 4, 5, 6, 7));

        listOne.removeAll(listTwo);

        System.out.println("Missing items from listTwo " + listOne);
    }
}

Output:

Missing items from listTwo [3]

Likewise, if we used:

listTwo.removeAll(listOne);

System.out.println("Missing items from listOne " + listTwo);

We would get:

Missing items from listOne [6, 7]

Compare two lists, find common items

The retainAll() method only keeps the items that are common in both lists. For example:

public class CompareTwoArrayLists {

    public static void main(String[] args) {
        ArrayList<Integer> listOne = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

        ArrayList<Integer> listTwo = new ArrayList<>(Arrays.asList(1, 2, 4, 5, 6, 7));

        listOne.retainAll(listTwo);

        System.out.println("Common items in both lists " + listOne);
    }
}

Output:

Common items in both lists [1, 2, 4, 5]

17 March 2020