How to remove all null elements from a ArrayList or String Array?

Question

Try:

tourists.removeAll(Collections.singleton(null));

Read the Java API. The code will throw java.lang.UnsupportedOperationException for immutable lists (such as created with Arrays.asList); see this answer for more details.

As of 2015, this is the best way (Java 8):

tourists.removeIf(Objects::isNull);

Note: This code will throw java.lang.UnsupportedOperationException for fixed-size lists (such as created with Arrays.asList), including immutable lists.

list.removeAll(Collections.singleton(null));

It will Throws UnsupportedException if you use it on Arrays.asList because it give you Immutable copy so it can not be modified. See below the code. It creates Mutable copy and will not throw any exception.

public static String[] clean(final String[] v) {
    List<String> list = new ArrayList<String>(Arrays.asList(v));
    list.removeAll(Collections.singleton(null));
    return list.toArray(new String[list.size()]);
}

Not efficient, but short

while(tourists.remove(null));

If you prefer immutable data objects, or if you just dont want to be destructive to the input list, you can use Guava's predicates.

ImmutableList.copyOf(Iterables.filter(tourists, Predicates.notNull()))

 for (Iterator<Tourist> itr = tourists.iterator(); itr.hasNext();) {
      if (itr.next() == null) { itr.remove(); }
 }

Pre-Java 8 you should use:

tourists.removeAll(Collections.singleton(null));

Post-Java 8 use:

tourists.removeIf(Objects::isNull);

---

The reason here is time complexity. The problem with arrays is that a remove operation can take O(n) time to complete. Really in Java this is an array copy of the remaining elements being moved to replace the empty spot. Many other solutions offered here will trigger this issue. The former is technically O(n*m) where m is 1 because it's a singleton null: so O(n)

You should removeAll of the singleton, internally it does a batchRemove() which has a read position and a write position. And iterates the list. When it hits a null, it simply iterates the read position by 1. When they are the same it passes, when they are different it keeps moving along copying the values. Then at the end it trims to size.

It effectively does this internally:

public static <E> void removeNulls(ArrayList<E> list) {
    int size = list.size();
    int read = 0;
    int write = 0;
    for (; read < size; read++) {
        E element = list.get(read);
        if (element == null) continue;
        if (read != write) list.set(write, element);
        write++;
    }
    if (write != size) {
        list.subList(write, size).clear();
    }
}

Which you can explicitly see is an O(n) operation.

The only thing that could ever be faster is if you iterated the list from both ends, and when you found a null, you set its value equal to the value you found at the end, and decremented that value. And iterated until the two values matched. You'd mess up the order, but would vastly reduce the number of values you set vs. ones you left alone. Which is a good method to know but won't help much here as .set() is basically free, but that form of delete is a useful tool for your belt.

---

for (Iterator<Tourist> itr = tourists.iterator(); itr.hasNext();) {
      if (itr.next() == null) { itr.remove(); }
 }

While this seems reasonable enough, the .remove() on the iterator internally calls:

ArrayList.this.remove(lastRet);

Which is again the O(n) operation within the remove. It does an System.arraycopy() which is again not what you want, if you care about speed. This makes it n^2.

There's also:

while(tourists.remove(null));

Which is O(m*n^2). Here we not only iterate the list. We reiterate the entire list, each time we match the null. Then we do n/2 (average) operations to do the System.arraycopy() to perform the remove. You could quite literally, sort the entire collection between items with values and items with null values and trim the ending in less time. In fact, that's true for all the broken ones. At least in theory, the actual system.arraycopy isn't actually an N operation in practice. In theory, theory and practice are the same thing; in practice they aren't.