Java ArrayList removeIf() method
Syntax and Examples

Introduction

The `removeIf()` method in Java's `ArrayList` is a powerful tool for removing elements that meet a specific condition. Instead of manually iterating through the list and removing elements one by one (which can be tricky due to index shifting), `removeIf()` provides a concise and efficient way to achieve this using a predicate.

Syntax

Here's the syntax for the `removeIf()` method:

public boolean removeIf(Predicate<? super E> filter)

Parameters

Let's break down what that means with a table.

Parameter	Description
filter	A predicate (a functional interface) that tests each element of the list. It takes an element as input and returns a boolean value: true if the element should be removed, and false otherwise.

Return Value

The `removeIf()` method returns a `boolean` value. It returns true if at least one element was removed from the list; otherwise, it returns false.

Examples

Removing Even Numbers

This example demonstrates how to remove all even numbers from an ArrayList.

import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

public class RemoveIfExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);

        System.out.println("Original list: " + numbers);

        boolean removed = numbers.removeIf(n -> n % 2 == 0); //Predicate to check if a number is even

        System.out.println("List after removing even numbers: " + numbers);
        System.out.println("Elements were removed? " + removed);
    }
}

Original list: [1, 2, 3, 4, 5, 6]
List after removing even numbers: [1, 3, 5]
Elements were removed? true

Explanation:

• We create an `ArrayList` of integers.
• The `removeIf()` method is called with a lambda expression (n -> n % 2 == 0) as the predicate. This lambda checks if a number is even by using the modulo operator (`%`). If the remainder when divided by 2 is 0, it's an even number and will be removed.
• The `removed` variable stores whether any elements were actually deleted.

Removing Strings Shorter Than Length 5

This example shows how to remove strings from an ArrayList that have a length less than 5.

import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

public class RemoveIfStringExample {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");
        names.add("Dave");
        names.add("Eve");

        System.out.println("Original list: " + names);

        boolean removed = names.removeIf(s -> s.length() < 5); //Predicate to check string length

        System.out.println("List after removing strings shorter than 5 characters: " + names);
        System.out.println("Elements were removed? " + removed);
    }
}

Original list: [Alice, Bob, Charlie, Dave, Eve]
List after removing strings shorter than 5 characters: [Alice, Charlie, Dave]
Elements were removed? true

Explanation:

• We create an `ArrayList` of strings.
• The `removeIf()` method is called with a lambda expression (s -> s.length() < 5) as the predicate. This lambda checks if the length of a string is less than 5. If it is, the string is removed.

No Elements Removed

This example shows what happens when no elements satisfy the removal condition.

import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

public class RemoveIfNoMatchExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(3);
        numbers.add(5);

        System.out.println("Original list: " + numbers);

        boolean removed = numbers.removeIf(n -> n % 2 == 0); //Predicate to check if a number is even, but none are

        System.out.println("List after attempting to remove even numbers: " + numbers);
        System.out.println("Elements were removed? " + removed);
    }
}

Original list: [1, 3, 5]
List after attempting to remove even numbers: [1, 3, 5]
Elements were removed? false

Explanation:

• We create an `ArrayList` of integers.
• The `removeIf()` method is called with a lambda expression (n -> n % 2 == 0) as the predicate. However, none of the numbers in the list are even.
• Because no elements matched the condition, the list remains unchanged and the `removed` variable is set to false.