Treemap get throws NullPointerException

Question

Follwing is my java class TestEntry.java

private void initializemapTest()
    {
        eventMap = new TreeMap<String,String>();

        //Put some value into eventMap

        mapTest = new TreeMap<String, String>( new Comparator<String>()
        {
            public int compare( String key1, String key2 )
            {
               if( key1 == null )
                {
                    if( key2 == null )
                    {
                        return 0;
                    }
                    else
                    {
                        return 1;
                    }
                }
                else
                {
                    if( key2 == null )
                    {
                        return -1;
                    }
                    else
                    {
                        return key1.compareTo( key2 );
                    }
                }
            }
        } );

        for( String s : eventMap.keySet() )
        {
            mapTest.put( eventMap.get( s ), s );   //Error at this line
        }

    }

As per my knowledge eventMap doesnot allow null values, hence keyset of eventMap does not have any null values, if value of any key in eventMap is null, while i try to put it in mapTest, it shoukd not throw any null pointer exception, because its respective comparator allows null values

But why am i getting this exception

       java.lang.NullPointerException
        at java.util.TreeMap.cmp(TreeMap.java:1911)
        at java.util.TreeMap.get(TreeMap.java:1835)
        at kidiho.sa.client.reports.ReportEntry.initializemapTest(TestEntry.java:22)

Answer 1

It will throw NullPointerException because in TreeMap api get() method is throwing NullPointerException deliberately if that is null.

final Entry<K,V> getEntry(Object key) {
        // Offload comparator-based version for sake of performance
        if (comparator != null)
            return getEntryUsingComparator(key);
        if (key == null)
            throw new NullPointerException();
        Comparable<? super K> k = (Comparable<? super K>) key;
        Entry<K,V> p = root;
        while (p != null) {
            int cmp = k.compareTo(p.key);
            if (cmp < 0)
                p = p.left;
            else if (cmp > 0)
                p = p.right;
            else
                return p;
        }
        return null;
    }

Answer 2

From TreeMap:

    final Entry<K,V> getEntry(Object key) {
    // Offload comparator-based version for sake of performance
    if (comparator != null)
        return getEntryUsingComparator(key);
    if (key == null)
        throw new NullPointerException();
    Comparable<? super K> k = (Comparable<? super K>) key;
    Entry<K,V> p = root;
    while (p != null) {
        int cmp = k.compareTo(p.key);
        if (cmp < 0)
            p = p.left;
        else if (cmp > 0)
            p = p.right;
        else
            return p;
    }
    return null;
}

That is: TreeMap doesn't allow null keys, so you cannot do:

tm.put(null, something)

And subsequently, you cannot do

tm.get(null)

As according to the TreeMap behaviour, those operations actually don't make sense

Answer 3

As other said, you can't use a null value as a TreeMap key, it will throw a NullPointerException.

You're not getting the NullPointerException from the same place probably because your first map has a registered comparator and the second has none.