LinkedHashMap

In a HashMap, the order entries are added is not maintained—it is lost when they are put into the table. But with LinkedHashMap, a linked list maintains the order.

To ensure we can access elements in the order we add them, LinkedHashMap is a good alternative to HashMap. It still has constant time lookup, but just has some overhead.

An example program

Let us examine this program. We import the LinkedHashMap in the same way we would import HashMap. We use diamond inference syntax in the same way as well.

import java.util.LinkedHashMap;
import java.util.Map.Entry;

public class Program {
    public static void main(String[] args) {

        // Add 3 entries to a LinkedHashMap.
        LinkedHashMap<String, Integer> values = new LinkedHashMap<>();
        values.put("bird", 10);
        values.put("frog", 20);
        values.put("snake", 25);

        // For LinkedHashMap, we can loop over the items in the order they were added.
        // ... A linked list keeps track of the order as we add them.
        for (Entry<String, Integer> pair : values.entrySet()) {
            System.out.println(pair.getKey() + "/" + pair.getValue());
        }
    }
}
bird/10
frog/20
snake/25

HashMap loop

Consider this version of the program. It uses HashMap instead of LinkedHashMap. The items are added in a certain order, and then loops over the entries.

import java.util.HashMap;
import java.util.Map.Entry;

public class Program {
    public static void main(String[] args) {

        HashMap<String, Integer> values = new HashMap<>();
        values.put("bird", 10);
        values.put("frog", 20);
        values.put("snake", 25);

        for (Entry<String, Integer> pair : values.entrySet()) {
            System.out.println(pair.getKey() + "/" + pair.getValue());
        }
    }
}
frog/20
bird/10
snake/25

Some considerations

The LinkedHashMap is an ideal replacement for HashMap when we need to maintain the order of items we put into the table. If ordering is needed, prefer LinkedHashMap.

A simple review

The LinkedHashMap is a special kind of HashMap. It enforces a linked list data structure where the order is maintained. This helps if the time of an addition is important.

For most programs, HashMap is a better choice as it is simpler. It also has less conceptual complexity (it omits the "Linked" part). It is important to choose the correct collection.