Sorting a HashMap by date

Question

In a Java class I have a method to reOrder an existing HashMap by date. The HashMap is of a type where the Object contains a field called exp

Answer 1

For simplicity I am assuming that type of your map is something more like Map map where MyClass has method like getDate() which returns expPayDate.

My issue is what is the best way to determine the item with the newest date.

If you want to find single map entry which value contains max date you don't need to sort entire map which at best would give you O(n*logn). What you need is simple iteration of all elements in map and comparing them with current max, which will be O(n) operation.

You can use stream() (functionality added in Java 8) and its max method. This method needs Comparator and you can easily create one by using comparing method and passing lambda expression which will return value which should be used when comparing.

So your code can look like

//import java.util.Map.Entry;

Optional> max = map.entrySet().stream()
        .max(Comparator.comparing(e -> e.getValue().getDate()));

Entry entry = max.get();
MyClass maxMC = entry.getValue();

If you can't use Java 8 you can write your own method which will iterate over elements and find max. Such method can look like

public static  T max(Iterable iterable, Comparator comp) {
    Iterator it = iterable.iterator();
    T max = null;
    if (it.hasNext()) {
        max = it.next();
    }
    while (it.hasNext()) {
        T tmp = it.next();
        if (comp.compare(max, tmp) < 0)
            max = tmp;
    }
    return max;
}

and you can use it like

Comparator> myComparator = new Comparator>() {
    @Override
    public int compare(Entry o1, Entry o2) {
        return o1.getValue().getDate().compareTo(o2.getValue().getDate());
    }
};
Entry maxEntry = max(map.entrySet(), myComparator);
MyClass max = maxEntry.getValue();