Collection Implementation to Block null elements being added

Question

The HashSet Set implementation allows the addition of null. Is there any Collection implementation that will not allow null? I know I can do a remove for the null on the HashSet, but I was wondering if I can restrict it.

public static void main(String[] args) {
    Set<String> testing = new HashSet<String>();
    testing.add(null);
    testing.add("test");

    for(String str : testing){

    }

  }


//TreeSet allows null as well
 public static void main(String[] args) {
    TreeSet<String> testing = new TreeSet<String>();
    testing.add(null);

    for(String str : testing){
      System.out.println("testing");
    }

  }

Are you looking for any `Collection` or just `Set` implementation? — Sotirios Delimanolis, Jan 06 '14 at 19:25
@SotiriosDelimanolis TreeSet allows nulls, see updated question. — c12, Jan 06 '14 at 19:27
It'll fail if you try to run it, with a `NullPointerException`. — Sotirios Delimanolis, Jan 06 '14 at 19:28
@SotiriosDelimanolis I did run it and its prints out testing — c12, Jan 06 '14 at 19:29
Every implementation for SET has different purposes and pros/cons. If your starting with HashSet it means you want a list of Unique Values. If you want a HashMap it means you want key/value pairs. And a TreeMap means you want your key/value pairs to be ordered by Key. Each Data Structure has a specific purpose. — Menelaos, Jan 06 '14 at 19:31
What Java version are you using? [It fails for Java 7.](http://ideone.com/Gnqbty) — Sotirios Delimanolis, Jan 06 '14 at 19:31
@SotiriosDelimanolis you are right. I also got NullPointerException while running the program using TreeSet. — Qadir Hussain, Jan 06 '14 at 19:34
It shouldn't. The javadoc says it will throw a `NullPointerException`. Please double check your code. — Sotirios Delimanolis, Jan 06 '14 at 19:40
@SotiriosDelimanolis if you only have one element and its null in a TreeSet and don't add anything to the TreeSet after it will not throw a NPE — c12, Jan 06 '14 at 19:56
@c12 How are you adding that first `null` element? You can't add a `null` to a `TreeSet`. The underlying `TreeMap` will try to compare the `key` to itself, throwing a `NullPointerException` when trying to invoke the `compareTo` method on a `null` reference. — Sotirios Delimanolis, Jan 06 '14 at 20:01
@c12 [It seems you are correct.](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/TreeMap.java#TreeMap.put%28java.lang.Object%2Cjava.lang.Object%29) The `put` method `null` check is commented out. I would either upgrade or try the other collection types in my answer. — Sotirios Delimanolis, Jan 06 '14 at 20:10

score 2 · Accepted Answer · edited May 23 '17 at 12:05

2

A TreeSet is appropriate for your requirement. Its add(Object) method javadoc states

Throws:

NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

in both Java 6 and Java 7.

Also, if you are looking for Collection implementations (rather than Set implements), there are others.

edited May 23 '17 at 12:05

Community

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answered Jan 06 '14 at 19:35

Sotirios Delimanolis

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+1 This does cover the requirements about a set not accepting null. – Menelaos Jan 06 '14 at 19:37

Menelaos · Answer 2 · 2014-01-06T19:28:53.143

If you want to use HashSet, but with that restriction for no null, you could also create a class extending HashSet.

Example:

public class HashSetNullLess<E> extends HashSet<E> {

    public boolean add(E e) {
            if(e==null)
            return false;  //Or throw unsupported exception message

            return super.add(e);
    }
}

This should pretty much take care of the problem as I checked the code of HashSet.java and even the constructors accepting collections lead to the add method via addAll in AbstractCollection.java

See:

public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }

score 0 · Answer 3 · edited Feb 14 '15 at 02:19

0

I agree with SotiriosDelimanolis. I tried to run this code in JDK 7.

public class Test {

    public static void main(String arg[]) {

        TreeSet<String> set = new TreeSet<String>();
        set.add(null);
        set.add("Test");
        System.out.println(set);
    }
}

But got the following error:

Exception in thread "main" java.lang.NullPointerException

I think using your requirement will be fulfilled.

edited Feb 14 '15 at 02:19

Pang

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answered Jan 06 '14 at 19:52

Qadir Hussain

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@c12 without adding any element still getting same exception. Check it carefully. – Qadir Hussain Jan 06 '14 at 19:58
with jdk version 1.6? – c12 Jan 06 '14 at 19:59

score -1 · Answer 4 · answered Jan 06 '14 at 19:33

Consider to wrap Hashtable class. It does not allow nulls, however it implements Map interface. You can do this in a way as HasSet wraps HashMap. Take into account Hashtable is synchronized.

Use next, but it is a really implicit and could make code reading harder

    HashSet<String> set = new HashSet<String>() {
    @Override
    public boolean add(String s) {
        if(s == null)
            throw new NullPointerException();
        return super.add(s);
    }
};

Use TreeMap, but before actual using it add just one element, because it accepts null only as initial element

Collection Implementation to Block null elements being added

4 Answers4