HashMap put()

[Allen Bandela]

Hello:

I've stumbled across an unexpected behaviour (atleast for me) on using the HashMap.put(Object, Object) method.

//Java 5 or >
Integer i = new Integer(1);
i++;
HashMap c = new HashMap();
Object o = new Object();
c.put(o, i++);
System.out.println("i in c = "+c.get(o));

Would you expect the value assigned to key 'o' to be 3? I did. But, its 2. passing values to method in Java is pass by value, correct? in other words isn't the JVM passing the address in 'i' to put(..) above?

Well, basically, I was going through past code and found this. I thought, hey , there's a bug, 'i' was already incremented . But, it was working as intended.

Thanks.

[John de Michele]

Allen,

On your line 6, you have

c.put(o, i++);

What this says is "Associate the value of 'i' with the key 'o' in HashMap 'c', then increment 'i'". If you want i to be 3, you need do this:

c.put(o, ++i);

John.

[Henry Wong]

Would you expect the value assigned to key 'o' to be 3? I did. But, its 2. passing values to method in Java is pass by value, correct? in other words isn't the JVM passing the address in 'i' to put(..) above?

Integer objects are immutable -- so i++ is really unboxing, incrementing, and reboxing (yielding a different object).

Henry