Add to List value on HashMap

Fundhor :

I'm populating a hashmap to have my objects grouped by one of their attributes. I find it “ugly” to check whether the list (the value) exists before adding my object to it.

An example will be more explicit:

// Need a map to group Person by age.
// Person = {age: int}
Map<Integer, List<Person>> myHashMap = new HashMap();

for (Person person : persons) {
   int age = person.getAge();
   List<Person> personsOfSameAge = myHashMap.get(age);
   if (personsOfSameAge != null) {
       personsOfSameAge.add(person);
   } else {
       personsOfSameAge = new ArrayList();
       personsOfSameAge.add(person);
       myHashMap.put(age, personsOfSameAge);
   }
}

Is there a better way to code this?

myHashMap.addToValueListOrCreateValueList(myObject);

davidxxx :

In Java 8, your whole code could be written like that (as other fine answers proposed):

Map<Integer, List<Person>> myHashMap = new HashMap();

for (Person person : persons) {
   myHashMap.computeIfAbsent(age,age->new ArrayList<Person>()).add(person);
}

But you could be even shorter by using a stream that collects into a Map with Collectors.groupingBy() :

Map<Integer, List<Person>> myMap = persons.stream().collect(Collectors.groupingBy(Person:getAge));

As a side note, your actual Java 7 code could also be improved. Of course, not as much as with Java 8 but if you cannot use Java 8, this may be interesting.
In your actual code, this is duplicated :

personsOfSameAge.add(person);

And you use two conditional statements (if and else) while only if would be enough if you handle first the special case : no value in the Map .
Here is a modified version :

Map<Integer, List<Person>> myHashMap = new HashMap<>();

for (Person person : persons) {
   int age = person.getAge();
   List<Person> personsOfSameAge = myHashMap.get(age);
   if (personsOfSameAge == null) {
       personsOfSameAge = new ArrayList();
       myHashMap.put(age, personsOfSameAge);
   }          
   personsOfSameAge.add(person);                
}