How to append a string to a HashMap element?

Question

I have a hashmap in java and I need to append a string to one specific key. Is this code correct ? Or it is not a good practice to invoke .get method to retrieve the original content ?

myMap.put("key", myMap.get("key") + "new content") ;

thanks

Solution 1

If you mean you want to replace the current value with a new value, that's absolutely fine.

Just be aware that if the key doesn't exist, you'll end up with "nullnew content" as the new value, which may not be what you wanted. You may want to do:

String existing = myMap.get("key");
String extraContent = "new content";
myMap.put("key", existing == null ? extraContent : existing + extraContent);

Solution 2

I have a hashmap in java and I need to append a string to one specific key.

You will need to remove the mapping, and add a new mapping with the updated key. This can be done in one line as follows.

myMap.put(keyToUpdate + toAppend, myMap.remove(keyToUpdate));

The Map.remove method removes a mapping, and returns the previously mapped value

Solution 3

If you do this often you may wish to use StringBuilder.

Map<String, StringBuilder> map = new LinkedHashMap<String, StringBuilder>();

StringBuilder sb = map.get(key);
if (sb == null)
   map.put(key, new StringBuilder(toAppend));
else
   sb.append(toAppend);

Comments

[aioobe]

"I need to append a string to one specific key." Your solution appends to a value in the map.

[Jon Skeet]

@aioobe: I interpreted that as "I need to append a string to the value associated with one specific key". Maybe the OP will clarify...