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get string value from HashMap depending on key name

Posted by: admin November 30, 2017 Leave a comment

Questions:

I have a HashMap with various keys and values, how can I get one value out?

I have a key in the map called my_code, it should contain a string, how can I just get that without having to iterate through the map?

So far I’ve got..

   HashMap newMap = new HashMap(paramMap);
   String s = newMap.get("my_code").toString();

I’m expecting to see a String, such as “ABC” or “DEF” as that is what I put in there initially, but if I do a System.out.println() I get something like java.lang.string#F0454

Sorry, I’m not too familiar with maps as you can probably guess 😉

Answers:

Just use Map#get(key) ?

Object value = map.get(myCode);

Here’s a tutorial about maps, you may find it useful: http://java.sun.com/docs/books/tutorial/collections/interfaces/map.html.

Edit: you edited your question with the following:

I’m expecting to see a String, such as “ABC” or “DEF” as that is what I put in there initially, but if I do a System.out.println() I get something like java.lang.string#F0454

Sorry, I’m not too familiar with maps as you can probably guess 😉

You’re seeing the outcome of Object#toString(). But the java.lang.String should already have one implemented, unless you created a custom implementation with a lowercase s in the name: java.lang.string. If it is actually a custom object, then you need to override Object#toString() to get a “human readable string” whenever you do a System.out.println() or toString() on the desired object. For example:

@Override
public String toString() {
    return "This is Object X with a property value " + value;
}

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If you are storing keys/values as strings, then this will work:

HashMap<String, String> newMap = new HashMap<String, String>();
newMap.put("my_code", "shhh_secret");
String value = (String) newMap.get("my_code");

The question is what gets populated in the HashMap (key & value)

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Your question isn’t at all clear I’m afraid. A key doesn’t have a “name”; it’s not “called” anything as far as the map is concerned – it’s just a key, and will be compared with other keys. If you have lots of different kinds of keys, I strongly suggest you put them in different maps for the sake of sanity.

If this doesn’t help, please clarify the question – preferrably with some code to show what you mean.

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If you will use Generics and define your map as

Map<String,String> map = new HashMap<String,String>();

then fetching value as

 String s = map.get("keyStr"); 

you wont be required to typecast the map.get() or call toString method to get String value

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map.get(myCode)

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An important point to be noted here is that if your key is an object of user-defined class in java then make it a point to override the equals method. Because the HashMap.get(Object key) method uses the equals method for matching the key value. If you do not override the equals method then it will try to find the value simply based on the reference of the key and not the actual value of key in which case it will always return a null.

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Answers:

You can use the get(Object key) method from the HashMap.
Be aware that i many cases your Key Class should override the equals method, to be a useful class for a Map key.