Remove elements from a List that satisfy given predicate in Java

Below are the methods to efficiently remove elements from a List satisfying a Predicate condition:

p  ==> Predicate, specifying the condition
l  ==> List, from which element to be removed

Using iterator

Below program demonstrates the removal of null elements from the list, using the Predicate 

Java

// Java Program to remove nulls
// from a List using iterator and Predicate
import java.util.function.Predicate;
import java.util.*;

class GFG {

    // Generic function to remove Null Using Iterator
    public static <T> List<T>
    removeNullUsingIterator(List<T> l, Predicate<T> p)
    {

        // Create an iterator from the l
        Iterator<T> itr = l.iterator();

        // Find and remove all null
        while (itr.hasNext()) {

            // Fetching the next element
            T t = itr.next();

            // Checking for Predicate condition
            if (!p.test(t)) {

                // If the condition matches,
                // remove that element
                itr.remove();
            }
        }

        // Return the null
        return l;
    }

    public static void main(String[] args)
    {

        // Create the l with null values
        List<String> l = new ArrayList<>(
               Arrays.asList("Geeks",
                             null,
                             "forGeeks",
                             null,
                             "A computer portal"));

        // Print the list
        System.out.println("List with null values: " + l);

        // Creating a Predicate condition checking for null
        Predicate<String> isNull = item -> Objects.nonNull(item);

        // Removing nulls using iterator and Predicate
        l = removeNullUsingIterator(l, isNull);

        // Print the list
        System.out.println("List with null values removed: " + l);
    }
}

Output

List with null values: [Geeks, null, forGeeks, null, A computer portal]
List with null values removed: [Geeks, forGeeks, A computer portal]

Time complexity : O(N^2)

Space complexity: O(N)

Using List.removeAll()

In this method, a collection containing elements to be removed is used to remove those elements from the original l. It requires creating a collection with the required elements using a Predicate condition. After doing this, the original l is searched for this collection and all instances of it are removed.

Java

// Java Program to remove 10
// from a List using List.removeAll() and Predicate
import java.util.function.Predicate;
import java.util.*;

class GFG {

    // Generic function to remove elements using Predicate
    public static <T> List<T>
    removeElements(List<T> l, Predicate<T> p)
    {

        // Create collection using Predicate
        Collection<T> collection = new ArrayList<>();

        for (T t : l) {
            if (p.test(t)) {
                collection.add(t);
            }
        }

        // Print the list
        System.out.println("Collection to be removed: " + collection);

        // Removing 10 using List.removeAll()
        // passing a collection
        l.removeAll(collection);

        // Return the list
        return l;
    }

    public static void main(String[] args)
    {

        // Create a list with null values
        List<String> l = new ArrayList<>(
            Arrays.asList("1", "10", "15", "10", "12", "5", "10", "20"));

        // Print the list
        System.out.println("Original List: " + l);

        // Creating a Predicate condition checking for 10
        Predicate<String> is10 = i -> (i == "10");

        // Removing using Predicate
        l = removeElements(l, is10);

        // Print the list
        System.out.println("Updated List: " + l);
    }
}

Output

Original List: [1, 10, 15, 10, 12, 5, 10, 20]
Collection to be removed: [10, 10, 10]
Updated List: [1, 15, 12, 5, 20]

Using Lambdas (Java 8)

Stream.filter() method can be used in Java 8 that returns a stream consisting of the elements 
that match the given predicate condition.

Java

// Java Program to remove nulls
// from a List using Java 8
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.*;

class GFG {

    // Generic function to remove elements using Predicate
    public static <T> List<T>
    removeElements(List<T> l, Predicate<T> p)
    {

        // Removing nulls using Java Stream
        // using Predicate condition in lambda expression
        l = l.stream()
                   .filter(p)
                   .collect(Collectors.toList());

        // Return the list
        return l;
    }

    public static void main(String[] args)
    {

        // Create a list with null values
        List<String> l = new ArrayList<>(
            Arrays.asList("Geeks",
                          null,
                          "forGeeks",
                          null,
                          "A computer portal"));

        // Print the list
        System.out.println("List with null values: " + l);

        // Creating a Predicate condition checking for null
        Predicate<String> isNull = i -> (i != null);

        // Removing using Predicate
        l = removeElements(l, isNull);

        // Print the list
        System.out.println("List with null values removed: " + l);
    }
}

Output

List with null values: [Geeks, null, forGeeks, null, A computer portal]
List with null values removed: [Geeks, forGeeks, A computer portal]

Using removeIf()

As the name suggests, it is a direct method to remove all elements that satisfy the given predicate.

Java

// Java Program to remove nulls
// from a List using Java 8
import java.util.function.Predicate;
import java.util.*;

class GFG {

    // Generic function to remove elements using Predicate
    public static <T> List<T>
    removeElements(List<T> l, Predicate<T> p)
    {

        // Removing nulls using Java Stream
        // using Predicate condition in removeIf()
        l.removeIf(x -> p.test(x));

        // Return the list
        return l;
    }

    public static void main(String[] args)
    {

        // Create a list with null values
        List<String> l = new ArrayList<>(
            Arrays.asList("Geeks",
                          null,
                          "forGeeks",
                          null,
                          "A computer portal"));

        // Print the list
        System.out.println("List with null values: " + l);

        // Creating a Predicate condition checking for null
        Predicate<String> isNull = i -> (i == null);

        // Removing using Predicate
        l = removeElements(l, isNull);

        // Print the list
        System.out.println("List with null values removed: " + l);
    }
}

Output

List with null values: [Geeks, null, forGeeks, null, A computer portal]
List with null values removed: [Geeks, forGeeks, A computer portal]