Question

I have the following situation. I have a HashMap in Java with keys as strings.
Then in some stage , in runtime I create strings equal to those keys in order to retrieve the data from that map.The strings are created as follows within “for” loop:

 String keyToRetrive = "lights[" + Integer.toString(i) + "]" + ".Intensity";

The strange thing about it that when I iterate through the map to find the key that equals that string ,even when the match is found the search steps over.So in this search loop :

  while (iter.hasNext()) {
        Map.Entry entry = (Map.Entry) iter.next();
        if (name == entry.getKey()) {  ///name- "lights[0].Intesity"
            uniformOut = (ICleanable) entry.getValue();
            break;
        }
    }

The key with the name “lights[0].Intesity” never returns true even that the map contains one.How I solved it .I used hashCode() for both compared string values.So this version does work:

 while (iter.hasNext()) {
        Map.Entry entry = (Map.Entry) iter.next();
        if (name.hashCode() == entry.getKey().hashCode()) {
            uniformOut = (ICleanable) entry.getValue();
            break;
        }
    }

UPDATE: After being pointed to the fact that “==” doesn’t work good and “equals()” should be used I would like to narrow the question:Why “==” does work for strings which were not created from several concatenated blocks? I mean, if I defines key string to compare agains as a simple single string:

 String foo="foo";

Such a string is compared using “==” against HashMap key all right.

I am not an expert Java programmer so can anybody explain why it works this way?

Answer 1

You are comparing Strings using == operator. Use equals() instead:

name.equals(entry.getKey())

This is a common pitfall in Java, see How do I compare strings in Java? and Difference between Equals/equals and == operator?.

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BTW (unrelated to your problem) when concatenating strings you don’t need to call toString() explicitly so this:

"lights[" + Integer.toString(i) + "]" + ".Intensity"

can be replaced with:

"lights[" + i + "]" + ".Intensity"

It’ll work for i of any type, not only int.