Re: Updating an object in a HashMap
alacrite@gmail.com wrote:
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.
hm.get returns the Integer, but then you've done intValue of that, and
that's a plain int.
I would have assume a reference to the object would have been
returned and calling += would have unboxed the Integer and aggregated
the value.
Before you called intValue, you had a reference to the original Integer,
but Integer objects (just like Boolean, Byte, Short, Character, Long,
Float, Double, and String) are immutable. But after intValue, you had a
plain int, and you can't have a reference to a boolean, byte, short,
char, long, float, or double -- only objects.
Could someone explain to me what is going on? Also, what is the best
way to accomplish updating this value?
Since the Integer is immutable, you have to replace it with a new one.
hm.put("test1", hm.get("test1") + 5);
Autoboxing and unboxing translates this, in effect, to
hm.put("test1", new Integer(hm.get("test1").intValue + 5));
If the case were different -- say you were using an imaginary type
called "MutableInteger" -- you could do something like this:
hm.put("test1", new MutableInteger(0));
hm.get("test1").MutableInteger.add(5));
--
John W. Kennedy
"The grand art mastered the thudding hammer of Thor
And the heart of our lord Taliessin determined the war."
-- Charles Williams. "Mount Badon"