Is it possible to get all the distinct values in an arraylist or hashmap?
Let's say for example I have elements on arraylist like this :
123, 123, 456, 123, 456, 000
And in HashMap :
123, test1
123, test2
456, test3
123, test4
000, test5
5 Answers
Keys should be unique in HashMap. if you are choosing duplicate keys then existing key value will be replace with new value.
if you want to avoid duplicate then use Set.
4 Comments
tuturyokgaming
What will happen if I'm gonna read a text file and put it on HashMap, my text file has multiple data.
Ranjitsinh
In HashMap it will override last value so you will able to see last value you have added for the same key.
tuturyokgaming
So in short, the existing key will be replaced with another key that same as that key. Right?
Anil Reddy Yarragonda
Existing key is replaced with new one. That means you lost your existing key data.
If you are using a Map, by definition the keys will be unique. For implementations of List there are a few options. For Java 5 through Java 7
public <T> List<T> removeDuplicates(List<T> list){
Set<T> set = new LinkedHashSet<>(list);
return new ArrayList<>(set);
}
With Java 8
public <T> List<T> removeDuplicatesJava8(List<T> list){
return list.stream().distinct().collect(Collectors.toList());
}
4 Comments
tuturyokgaming
How about with Java 6?
MadConan
see the first example. "pre-java8" means anything before java 8. That would include java 6.
tuturyokgaming
Oh oops sorry, I didnt see that one.
tuturyokgaming
Thank you for explaining this @MadConan. So it's better to use Java 8 because it has distinct method.
HashMaps don't allow duplicate keys. If you enter a key which is already there then it replaces it with the new one.
Comments
To get a Collection of unique elements from a List you can do that
Set<Object> set = new HashSet<Object>(nonUniqueList);
answered Jun 17, 2015 at 12:51
Davide Lorenzo MARINO
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Comments
You could use a Map<K, List<V>> (or Map<K, V[]>) if you like to map multiple values to a key:
Code:
public static void main(String[] args) {
Map<Integer, List<String>> data = new HashMap<>();
add(data, 123, "test1");
add(data, 123, "test2");
add(data, 456, "test3");
add(data, 123, "test4");
add(data, 0, "test5");
data.forEach((k, v) -> System.out.println(k + " -> " + v));
}
static <K, V> void add(Map<K, List<V>> listMap, K key, V value) {
List<V> values = listMap.get(key);
if(values == null) {
values = new ArrayList<V>();
listMap.put(key, values);
}
values.add(value);
}
Output:
0 -> [test5]
456 -> [test3]
123 -> [test1, test2, test4]
2 Comments
tuturyokgaming
Awesome! Thank you for this one, I understand now how Set and HashMap works!
MadConan
@tuturyokgaming You should upvote any answer that helped you.
123again as a key in aMap