Using HashMap put()

Question

I've been testing out HashMap recently and I've stumbled upon an interesting problem. I have an array, a3, that I have set some arbitrary values. I then take this array and put it into a HashMap instance map.

My problem is that when I assign an Object instance the value of put("a3", a3), I can't seem to print out the address of the instance.

For example:

HashMap<Object, Object> map = new HashMap<Object, Object>();
int[] a3 = new int[] {1, 2, 3, 4}; 

map.put("a3", a3); 

When I call System.out.println(map.put("a3", a3), I'm given an address in memory.

However, if I create an object instance and then try to print out that instance I get null.

Object copy = map.put("a3", a3); 
System.out.println(copy); 

Running the above segment gives me a value of null.

Any reason why both these statements, which appear identical, give me different results?

Answer 1

Read the API Documentation for a HashMap. Null is returned if no value existed in the map that matches what you are inserting.

http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html#put%28K,%20V%29

So if "a3" existed in the map already, the you would get a return, but since it doesn't then you get back null. "a3" has been put into the map but you didn't print the map, you printed the return from put().

Answer 2

As @James_Massey say, if you consult official documentation, you can see that the return value for put() method is:

the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)