Autoboxing & Generics Caveat

[Ustad Hanif]

Could someone kindly tell me why the following piece of code behaves the way it does? Basically I've defined a hashmap where the key is a Long object. I populate the map with an entry and then try and retrieve that entry with an int primitive. I understand the int I pass into get() is autoboxed into an Integer. The compile-time check asserts that myint can be promoted from int to long, but what is happening at run-time that causes a value of 'null' to be returned? I prostrate before the Ranch and beseech its apt members for guidance!!

public class TestStuff {

    public static void main(String args[]) {
        TestStuff testStuff = new TestStuff();
        
        testStuff.testMap();

    }
	
	public void testMap() {
		Map<Long, String> myMap = new HashMap<Long, String>();
		
		int myint = 1;
		long mylong = 1L;

		myMap.put(mylong, "hello");
		
		System.out.println("my value for int key is: " + myMap.get(myint));
		System.out.println("my value for long key is: " + myMap.get(mylong));		
		
		
	}
}

results:
my value for int key is: null
my value for long key is: hello

[Jeff Verdegan]

int x = 1;
long y = 1;
boolean z = (y == x); // true, because int is promoted to long for ==

Integer xx = 1; // int 1 is autoboxed to Integer
Long yy = 1L; // long 1L is autoboxed to Long

// false, because Long and Integer equals() are written to only be true for the same class. Long.equals(Integer) and vv. are always false.
boolean zz = (y.equals(x)); 

That last line of code is what the Map's key matching will be doing (if the hashCodes even match in the first place, which they may or may not).

[Ustad Hanif]

Concise, right to the point and beautifully answered! Thanks also for the lightning speed response time Jeff!

[Jeff Verdegan]

You're quite welcome. Glad I could help.