Remove all elements from the ArrayList in Java

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 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 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)