Java删除重复的数据

Question

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

Answer 1

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 .

Answer 2

HashMaps don't allow duplicate keys. If you enter a key which is already there then it replaces it with the new one.

Answer 3

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());
}

Answer 4

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]

Answer 5

为了得到一个Collection ，从一个独特的元素的List ，你可以这样做

Set<Object> set = new HashSet<Object>(nonUniqueList);