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

1. Using iterator

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

   // 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]
   

2. 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 Predicate condition. After doing this, the original l is searched for this collection and all instances of it is removed.

   // 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:

   br>
   

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

3. 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 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]
   

4. Using removeIf()

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

   // 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]