Remove all elements from the ArrayList in Java

Last Updated : 11 Dec, 2018

Prerequisite: ArrayList in Java Given an ArrayList, the task is to remove all elements of the ArrayList in Java. Examples:

Input: ArrayList = [1, 2, 3, 4] 
Output: ArrayList = [] 

Input: ArrayList = [12, 23, 34, 45, 57, 67, 89] 
Output: ArrayList = []

Using clear() method: Syntax:

collection_name.clear();

Code of clear() method:

public void clear() {
    for (int i = 0; i < size; i++)
        list[i] = null;

    size = 0;
}

Below is the implementation of the above approach:

Java

// Java Program for remove all elements ArrayList
// Using clear() method

// import  ArrayList package
import java.util.ArrayList;

public class GFG {

    // main method
    public static void main(String[] args)
    {
        // create empty ArrayList
        ArrayList<String> list = new ArrayList<>();

        // Adding elements of list
        list.add("Geeks");
        list.add("for");
        list.add("Geeks");
        list.add("Gaurav");

        // printing initial value ArrayList
        System.out.println("ArrayList: " + list);

        // print size of ArrayList
        System.out.println("Size of ArrayList = "
                           + list.size());

        // remove all elements using clear() method
        list.clear();

        // printing ArrayList
        System.out.println("\nAfter clear\n\n"
                         + "ArrayList: " + list);

        // print size of ArrayList after clear list
        System.out.println("Size of ArrayList = "
                           + list.size());
    }
}

Output:

ArrayList: [Geeks, for, Geeks, Gaurav]
Size of ArrayList = 4

After clear

ArrayList: []
Size of ArrayList = 0

Time Complexity: O(N)

Using removeAll() method Syntax:

collection_name.removeAll(collection_name);

Code of removeAll() method:

public boolean removeAll(Collection list) {
    boolean isModi = false;
    Iterator ite= iterator();
    while (ite.hasNext()) {
        if (list.contains(ite.next())) {
            ite.remove();
            isModi = true;
        }
    }
    return isModi;
}

Below is the implementation of the above approach:

Java

// Java Program for remove all elements ArrayList
// Using removeAll() method

// import  ArrayList package
import java.util.ArrayList;

public class GFG {

    // main method
    public static void main(String[] args)
    {
        // create empty ArrayList
        ArrayList<String> list = new ArrayList<>();

        // Adding elements of list
        list.add("Geeks");
        list.add("for");
        list.add("Geeks");
        list.add("Gaurav");

        // printing initial value ArrayList
        System.out.println("ArrayList: " + list);

        // print size of ArrayList
        System.out.println("Size of ArrayList = "
                           + list.size());

        // remove all elements using clear() method
        list.removeAll(list);

        // printing ArrayList
        System.out.println("\nAfter clear\n\n"
                         + "ArrayList: " + list);

        // print size of ArrayList after clear list
        System.out.println("Size of ArrayList = "
                           + list.size());
    }
}

Output:

ArrayList: [Geeks, for, Geeks, Gaurav]
Size of ArrayList = 4

After clear

ArrayList: []
Size of ArrayList = 0

Time Complexity: O(N^2)

Remove repeated elements from ArrayList in Java

Rajput-Ji

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