Create new Arrays from linkedHashMap with keys and values separately

Question

I have a LinkedHashMap and I am trying to split its keys and values and refer to specific key in the keyset or value in valueset. For ex. Lets say I have the folowing LinkedHashMap:

4 | 2

3 | 1

I want a function to return 1, which is a value at index 1 in the set with values or return 2 for index = 0. And with another function I want to get 3 as value with index 1 in key values. So basically from the LinkedHashMap create an Array with only values/ keys and then look for some given place in this array. My code looks as follow:

    public static Integer getLHMValue(Map<Integer, Long> lhm, int index) { 
        Set<Map.Entry<Integer, Long>> valueSet = lhm.entrySet();
        Integer[] valueArray = valueSet.toArray(new Integer[valueSet.size()]);
        Integer value = valueArray[index];
        return value;
    }

    public static Integer getLHMKey(Map<Integer, Long> lhm, int index) { 
        Set<Integer> keySet = lhm.keySet();
        Integer[] keyArray = keySet.toArray(new Integer[keySet.size()]);
        Integer key = keyArray[index];
        return key;
}

Though, I get java.lang.ArrayStoreException: java.util.LinkedHashMap$Entry in: Integer[] keyArray = valueSet.toArray(new Integer[valueSet.size()]);. Any ideas?

Answer 1

The problem is that the entrySet would be needed to first have an array of Map.Entrys. Keeping such an array for both methods would be faster.

List<Map.Entry<Integer, Long>> entries = new ArrayList<>(lhm.entrySet());

public static long getLHMValue(Map<Integer, Long> lhm, int index) { 
    return entries.get(index).getValue();
}

public static int getLHMKey(Map<Integer, Long> lhm, int index) { 
    return entries.get(index).getKey();
}

There is no efficient solution if you do it directly, and so it is not advisable to offer these functions to access key and value.

public static Long getLHMValue(Map<Integer, Long> lhm, int index) { 
    return lhm.entrySet().stream()
        .map(Map.Entry<Integer, Long>::getValue)
        .skip(index)
        .findFirst()
        .orElseThrow(IndexOutOfBoundsException::new);
}

public static Integer getLHMKey(Map<Integer, Long> lhm, int index) { 
    return lhm.entrySet().stream()
        .map(Map.Entry<Integer, Long>::getKey)
        .skip(index)
        .findFirst()
        .orElseThrow(IndexOutOfBoundsException::new);
}