How to Prevent Item Reordering in a Java HashMap

How can I avoid items being reordered when I put them in a Java HashMap?

Map<String, String> map = new HashMap<>();
map.put("one", "1");
map.put("two", "2");
map.put("three", "3");

In Java, a HashMap does not guarantee any specific order of its entries, as it is designed to provide constant-time performance for the basic operations (get and put). If you need to maintain the insertion order, consider using a LinkedHashMap instead, which maintains a hidden, doubly-linked list running through its entries.

Map<String, String> orderedMap = new LinkedHashMap<>();
orderedMap.put("one", "1");
orderedMap.put("two", "2");
orderedMap.put("three", "3");
// The elements will maintain the order they were inserted.

Causes

• The nature of HashMap is based on hashing mechanism and hashing functions, which can lead to a different order of elements as the size dynamically changes.
• HashMap uses an array of buckets and the natural order of keys is not preserved, leading to potential reordering upon rehashing.

Solutions

• Use a LinkedHashMap instead of HashMap to maintain the order of insertion.
• If you need to retain entry order and key order together, consider using a TreeMap with a custom comparator.

Mistake: Using HashMap when you need order preservation.

Solution: Switch to LinkedHashMap for maintaining order.

Mistake: Assuming that rehashing or resizing a HashMap maintains existing order.

Solution: Understand that during resizing, the order might change; use a stable collection instead.