You should always use remove(index) to delete object, if you are not iterating, otherwise always use Iterator's remove() method for removing object from ArrayList.

• Using remove()

import java.util.ArrayList;
import java.util.List;
public class Example {
    public static void main(String[] args) { 
        List<Integer> numbers = new ArrayList<Integer>(); 
        numbers.add(1); 
        numbers.add(2); 
        numbers.add(3); 
        System.out.println("ArrayList contains : " + numbers); 
        // Calling remove(index) 
        numbers.remove(1); //removing object at index 1 i.e. 2nd Object, which is 2 
        //Calling remove(object) 
        numbers.remove(new Integer(3)); 
        System.out.println("ArrayList After : " + numbers); 
    }
}
ArrayList contains : [1, 2, 3]
ArrayList After : [1]

• Using iterator

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class Example {

public static void main(String[] args) { 
    List<Integer> numbers = new ArrayList<Integer>(); 
    numbers.add(100);
    numbers.add(201); 
    numbers.add(300);
    numbers.add(401); 
    System.out.println("ArrayList Before : " + numbers); 
    Iterator<Integer> itr = numbers.iterator(); // remove all even numbers 
    while (itr.hasNext()) { 
        Integer number = itr.next(); 
        if (number % 2 == 0) { 
        itr.remove();
/* not numbers.remove(number); 
 Otherwise this will create ConcurrentModificationException*/
    } 
} 
System.out.println("ArrayList After : " + numbers); 
}

ArrayList Before : [100, 201, 300, 401]
ArrayList After : [201, 401]