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List, Set, and Map at a glance
| Type | What it represents | Duplicates | Order and access | Typical use |
|---|---|---|---|---|
List |
An ordered sequence of elements. | Usually allowed; repeated values can appear at different positions. | Elements have integer indexes; the list contract lets the user control where elements are inserted. | Steps, ranked results, or records where position matters. |
Set |
A collection of distinct elements. | Not allowed: a set contains no duplicate elements. | Depends on implementation: iteration order may be unspecified, insertion-ordered, or sorted. | Unique names or IDs, membership checks, or removing duplicates. |
Map |
Associations between keys and values. | Keys are unique; values do not have to be. | Look up a value by key; iteration order depends on implementation. | ID-to-record lookup, word-to-definition associations, or grouping by key. |
List and Set are specialized Collection interfaces. A Map instead exposes key, value, and entry views; it does not extend Collection. See Oracle’s Java SE 21 Collections Framework overview.
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When to use a List
Choose a List when the sequence itself matters: for example, when you need to preserve the order of steps, show ranked results, access an element by its integer index, or keep repeated values as separate entries. Oracle defines a list as an ordered collection in which the user has precise control over where each element is inserted. The Java SE 26 List API documents that contract.
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For a general-purpose resizable list, Oracle’s framework overview identifies ArrayList as a general-purpose implementation. The interface choice does not by itself establish performance for a particular workload; weigh the operations your code actually performs rather than assuming one collection is always fastest.
When to use a Set
Choose a Set when each element should occur at most once, such as a collection of unique user IDs, or when you need to remove duplicate elements from a collection. Uniqueness does not mean every set has the same order: that depends on the implementation.
Choose the order you need
HashSet: use when uniqueness is the requirement and you do not need a defined iteration order. Its iteration order is not guaranteed.LinkedHashSet: use when you want iteration to follow insertion order.TreeSet: use when you want elements ordered by value.
These distinctions are documented in Oracle’s Java SE 26 Set API. Do not write code that depends on a HashSet yielding a particular iteration order.
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When to use a Map
Choose a Map when a key should identify or retrieve an associated value: an ID and its record, a word and its definition, or a category and its grouped items. A map cannot contain duplicate keys; each key can map to at most one value. Multiple keys may, however, map to equal or identical values.
Choose the map implementation by order
HashMap: Oracle identifies it as the general-purpose map. Do not rely on a particular iteration order.LinkedHashMap: use when insertion order is needed.TreeMap: use when entries should be ordered by key.
The map’s iteration behavior is an implementation detail, not a promise shared by every Map. Check the Java SE 26 Map API and the Java SE 21 framework overview when order matters.
Keep map keys stable
A key must remain consistent with the map’s equality-based lookup behavior while it is stored. Oracle’s Java SE 26 API warns that behavior is unspecified if a key changes in a way that affects equality while it is in the map. Prefer stable key fields, and do not mutate equality-relevant state on an object being used as a key.
A practical way to choose
- Ask what the code needs to represent. A sequence points to
List; distinct members point toSet; key-to-value associations point toMap. - Decide whether duplicates are meaningful. Lists can retain repeated elements. Sets prevent duplicate elements. Maps prevent duplicate keys, not duplicate values.
- Decide whether order is part of the requirement. If so, specify insertion order or sorted order and choose an implementation whose contract provides it. Do not infer order from a variable’s interface type.
- Match access to the task. Use a list’s integer position when position is meaningful, or a map’s key when lookup by identifier is the point. Use a set when the code needs distinct elements or membership.
- Evaluate performance for the actual workload. The framework overview names general-purpose implementations, but that is not a benchmark or a guarantee that one option is fastest for every operation or data size.
How Java’s sequenced interfaces fit in
Since JDK 21, the framework includes SequencedCollection, SequencedSet, and SequencedMap for types with defined encounter order and common operations for working with elements from either end. This adds shared vocabulary and operations for ordered collection types; it does not mean every set or map has a defined encounter order. See Oracle’s Java SE 21 java.util package documentation. If you target a release earlier than JDK 21, verify available interfaces against that release’s API.
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