** Java 8 *

public static int minimumDistances(List a) { Map map = new HashMap<>(); int minDistance = 9999;

for (int i = 0; i < a.size(); i++) {
    int currentKey = a.get(i);
    if (map.containsKey(currentKey)) {
        minDistance = Math.min(minDistance, Math.abs(i - map.get(currentKey)));
    }
    map.put(currentKey, i);
}
return minDistance != 9999 ? minDistance : -1;

}