2.4.3 LinkedHashMap Explained

The LinkedHashMap class in Java is a part of the Java Collections Framework and extends the HashMap class. It maintains a doubly-linked list of the entries in the map, which preserves the insertion order of elements. This makes LinkedHashMap suitable for scenarios where the order of elements is important.

Key Concepts

1. Insertion Order

Unlike HashMap, which does not guarantee any order, LinkedHashMap maintains the order in which elements are inserted. This means that when you iterate over a LinkedHashMap, the elements will be returned in the order they were added.

Example

        LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put(3, "Cherry");
        linkedHashMap.put(1, "Apple");
        linkedHashMap.put(2, "Banana");
        System.out.println(linkedHashMap); // Output: {3=Cherry, 1=Apple, 2=Banana}
    

2. Access Order

LinkedHashMap can also be configured to maintain the order of elements based on access order. This means that the most recently accessed entry will be the last element in the iteration order. This feature is useful for implementing LRU (Least Recently Used) caches.

Example

        LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>(16, 0.75f, true);
        linkedHashMap.put(3, "Cherry");
        linkedHashMap.put(1, "Apple");
        linkedHashMap.put(2, "Banana");
        linkedHashMap.get(3); // Accessing the element
        System.out.println(linkedHashMap); // Output: {1=Apple, 2=Banana, 3=Cherry}
    

3. Performance

LinkedHashMap provides similar performance characteristics to HashMap for basic operations like put, get, and remove. The additional overhead of maintaining the linked list is minimal, making LinkedHashMap efficient for most use cases.

Example

        LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put(1, "Apple");
        linkedHashMap.put(2, "Banana");
        linkedHashMap.put(3, "Cherry");
        System.out.println(linkedHashMap.get(2)); // Output: Banana
    

4. Null Keys and Values

Like HashMap, LinkedHashMap allows one null key and multiple null values. This can be useful in scenarios where optional data needs to be stored.

Example

        LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put(null, "Null Key");
        linkedHashMap.put(1, null);
        System.out.println(linkedHashMap); // Output: {null=Null Key, 1=null}
    

Examples and Analogies

Think of a LinkedHashMap as a shopping list where items are added in the order you need them. When you go through the list, you see the items in the exact order you added them. If you mark an item as "used" (accessed), it moves to the end of the list, similar to how you might prioritize items in a to-do list.

By mastering LinkedHashMap, you can efficiently manage and manipulate collections of data where the order of insertion or access is important, making your Java SE 11 applications more organized and performant.