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I need to add object to new ArrayList:

private Map<String,List<MyObject>> populateMapFromList2(List<MyObject> dataSourceList){
Map<String,List<MyObject>> moMap = new HashMap<String, List<MyObject>>();
for (MyObject mo : dataSourceList){
    String firstLetter = (mo.getName().substring(0,1)).toUpperCase();
    if (!moMap.containsKey(firstLetter)) {
        moMap.put(firstLetter,new ArrayList<MyObject>());
    }
    moMap.get(firstLetter).add(mo);
}
return moMap;

}

in the line:new ArrayList<MyObject>() I want to add mo Like: new ArrayList<DataSource>().add(mo)

but it is a boolean value (Go figure). is there another way to add value for the first time to not parametrized list? or the only way is this? List<MyObject> initList = new ArrayList<MyObject>(); initList.add(mo); moMap.put(firstLetter,initList);

Thanks,

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2 Answers

up vote -1 down vote

If using google-guava is an option, you can do Lists.newArrayList(mo).

The documentation is available on the google-guava API page for Lists.

EDIT Non google-guava option:

If google-guava is not an option, you can implement it yourself like this:

public static <T> List<T> newArrayList(T... a) {
    return new ArrayList<T>(Arrays.asList(a));
}

This uses a combination of varargs and generics to create the array list. Varargs allows you to specify a variable number of parameters to this function. The generic type allows the type of the parameters to be captured and a list of corresponding type to be returned.

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Thanks, But no it is not an option. – Netanel Jan 23 '12 at 12:07
I have added a non-guava option in the answer. – laixer Jan 23 '12 at 12:12
up vote 0 down vote

You can still use the following syntax:

List<MyObject> initList = new ArrayList<MyObject>() {{ add(mo); }}

Or using Google Guava, you can use Lists.newArrayList(mo).

You should consider using a MultiMap for what you are trying to achieve (a Map whose values are Lists). Note that Apache Common has a MultiMap implementation too, you can also take a look at the DefaultedMap class.

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+1 for MultiMap – nd. Jan 23 '12 at 12:11
I think this is an inner class? I still have to parametrize "initList" and it is still another line. – Netanel Jan 23 '12 at 12:15
No this is an instance initialization. You should consider using a MultiMap, or create your own implementation of a DefaultedMap that will return you an empty List when calling get() on a non existing key. Now if you really want a one liner, make all your variables final, and use it: moMap.put(firstLetter, moMap.containsKey(firstLetter) ? new ArrayList<Object>() {{ addAll(moMap.get(firstLetter)); add(mo); }} : new ArrayList<Object>() {{ add(mo); }}); – Alex Jan 23 '12 at 12:38

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