When would you use HashMap, LinkedHashMap, or TreeMap?

Chooses among common Java map implementations by lookup behavior, iteration order, sorting, range operations, and key contracts.

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TL;DR

Use HashMap for general key lookup when no iteration order is required. Use LinkedHashMap when iteration must follow insertion order, or access order when configured for patterns such as an LRU-style cache. Use TreeMap when keys must stay sorted or the code needs range and nearest-key operations. TreeMap uses natural ordering or a comparator, so its key comparison must be consistent and meaningful.

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Map implementation guarantees and the ability to pay for ordering only when the requirement needs it.

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Overview
All three implement Map, but they make different promises about iteration and key ordering.
Use HashMap for general key lookup when no iteration order is required. Use LinkedHashMap when iteration must follow insertion order, or access order when configured for patterns such as an LRU-style cache. Use TreeMap when keys must stay sorted or the code needs range and nearest-key operations. TreeMap uses natural ordering or a comparator, so its key comparison must be consistent and meaningful.
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Details

Depending on HashMap iteration order.
Choosing TreeMap only because sorted output is needed once.
Assuming map implementation choice provides thread safety.
Using a comparator that disagrees with the intended key identity.

When would you use HashMap, LinkedHashMap, or TreeMap?

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