Why integer key do not work in HashMap?

You're putting a key of 1L (Long) and getting a key of 1 (Integer).

They're not the same thing, so be careful.

Either remove the L from the put, or add the L to the get. Or even better, don't write them out as primitives and rely on autoboxing.

It's because you're not passing a Long type of key when retrieving the value, but an integer instead. These get autoboxed as different objects and therefore do not represent the same key.

Integers and Longs are not the same.

Integer#equals

public boolean equals(Object obj) {
    if (obj instanceof Integer) {
        return value == ((Integer)obj).intValue();
    }
    return false;
}

Long#equals

public boolean equals(Object obj) {
    if (obj instanceof Long) {
        return value == ((Long)obj).longValue();
    }
    return false;
}

There is a type mismatch between putting and getting.

What happens here is AutoBoxing, Java will perform conversion between primitives to its Object equivalents and vice-versa for you automatically rather than via the new(), So became Integer and another is Long.

Try the below ways:

public static void main (String[] args) {
        Map<Number, String> map = new HashMap<Number, String>();
        map.put(1L, "test");
        System.out.println(map.get(1L));
    }

Or

public static void main (String[] args) {
        Map<Number, String> map = new HashMap<Number, String>();
        map.put(1, "test");
        System.out.println(map.get(1));
    }

Though both have same value(1) but of different objects they are not equal,

i.e. Integer(1) != Long(1).

That is the problem here, which is being happened by using Auto-boxing.

A quick example

        Long l = new Long(1);
        Integer i = new Integer(1);
        System.out.println(i.equals(l)); //false -->Hashmap get() failed here
        System.out.println(i.intValue() ==l.intValue());//true

I spot the problem, see java.util.HashMap#get source code, it contains:

public V get(Object key) {
        if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }

So it check key with equals before returning the value.