ArrayList removeAll() – remove all occurrences from list

ArrayList removeAll() removes all of matching elements that are contained in the specified method argument collection. It removes all occurrences of matching elements, not only first occurrence.

1. ArrayList removeAll() method

Internally, the removeAll() method iterate over all elements of arraylist. For each element, it pass element to contains() method of argument collection.

If element is found in argument collection, it re-arranges the index. If element is not found, it retains the element inside backing array.

public boolean removeAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, false);
}

private boolean batchRemove(Collection<?> c, boolean complement) {
    final Object[] elementData = this.elementData;
    int r = 0, w = 0;
    boolean modified = false;
    try {
        for (; r < size; r++)
            if (c.contains(elementData[r]) == complement)
                elementData[w++] = elementData[r];
    } finally {
        // Preserve behavioral compatibility with AbstractCollection,
        // even if c.contains() throws.
        if (r != size) {
            System.arraycopy(elementData, r,
                             elementData, w,
                             size - r);
            w += size - r;
        }
        if (w != size) {
            // clear to let GC do its work
            for (int i = w; i < size; i++)
                elementData[i] = null;
            modCount += size - w;
            size = w;
            modified = true;
        }
    }
    return modified;
}

Method parameter – collection containing elements to be removed from this list.
Method returns – true if this list changed as a result of the call.
Method throws – ClassCastException – if the class of an element of this list is incompatible with the specified collection. It may also throw NullPointerException if this list contains a null element and the specified collection does not permit null elements.

2. ArrayList removeAll() example

Java program to remove all occurrences of an object from the arraylist using removeAll() method.

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

public class ArrayListExample 
{
    public static void main(String[] args) throws CloneNotSupportedException 
    {
        ArrayList<String> alphabets = new ArrayList<>(Arrays.asList("A", "B", "A", "D", "A"));
        
        System.out.println(alphabets);
        
        alphabets.removeAll(Collections.singleton("A"));
        
        System.out.println(alphabets);
    }
}

Program output.

[A, B, A, D, A]
[B, D]

That’s all for the ArrayList removeAll() method in Java.

Happy Learning !!

Read More:

A Guide to Java ArrayList
ArrayList Java Docs