In your case, there's no need to iterate through the list, because you know which object to delete. You have several options. First you can remove the object by index (so if you know, that the object is the second list element):

 a.remove(1);       // indexes are zero-based

Or, you can remove the first occurence of your string:

 a.remove("acbd");  // removes the first String object that is equal to the
                    // String represented by this literal

Or, remove all strings with a certain value:

 while(a.remove("acbd")) {}

It's a bit more complicated, if you have more complex objects in your collection and want to remove instances, that have a certain property. So that you can't remove them by using remove with an object that is equal to the one you want to delete.

In those case, I usually use a second list to collect all instances that I want to delete and remove them in a second pass:

 List<MyBean> deleteCandidates = new ArrayList<>();
 List<MyBean> myBeans = getThemFromSomewhere();

 // Pass 1 - collect delete candidates
 for (MyBean myBean : myBeans) {
    if (shallBeDeleted(myBean)) {
       deleteCandidates.add(myBean);
    }
 }

 // Pass 2 - delete
 for (MyBean deleteCandidate : deleteCandidates) {
    myBeans.remove(deleteCandidate);
 }

The problem you have right now is you're just removing an element from the list's index based on user input while you should make sure list is of that size. Second thing, You can't remove an element from the list while browsing it via the forEach loop, you would get a ConcurrentModificationException , read concurrentModificationException. The way to do it would be using an iterator, check

Iterator<Passenger> itr = passengerList.iterator();
while (itr.hasNext()){
    Passenger p = itr.next();
    if (id == p.getId()) {
        itr.remove();
        break;
    }
}

ArrayList removes objects based on the equals(Object obj) method. So you should implement properly this method. Something like:

public boolean equals(Object obj) {
    if (obj == null) return false;
    if (obj == this) return true;
    if (!(obj instanceof ArrayTest)) return false;
    ArrayTest o = (ArrayTest) obj;
    return o.i == this.i;
}

Or

public boolean equals(Object obj) {
    if (obj instanceof ArrayTest) {
        ArrayTest o = (ArrayTest) obj;
        return o.i == this.i;
    }
    return false;
}