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 Predicateimport 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 Predicateimport 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 8import 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 8import 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]
