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Add maps concept (#2914)
--------- Co-authored-by: Manuel Maxera <[email protected]>
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‎concepts/maps/.meta/config.json

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{
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"blurb": "Maps are a collection of key value pairs.",
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"authors": [
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"kahgoh"
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],
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"contributors": []
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}

‎concepts/maps/about.md

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# About Maps
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A **Map** is a data structure for storing key value pairs.
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It is similar to dictionaries in other programming languages.
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The [Map][map-javadoc] interface defines the operations you can make with a map.
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## HashMap
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Java has a number of different Map implementations.
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[HashMap][hashmap-javadoc] is a commonly used one.
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```java
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// Make an instance
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Map<String, Integer> fruitPrices = new HashMap<>();
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```
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~~~~exercism/note
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When defining a `Map` variable, it is recommended to define the variable as a `Map` type rather than the specific type, as in the above example.
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This practice makes it easy to change the `Map` implementation later.
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~~~~
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`HashMap` also has a copy constructor.
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```java
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// Make a copy of a map
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Map<String, Integer> copy = new HashMap<>(fruitPrices);
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```
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Add entries to the map using [put][map-put-javadoc].
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```java
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fruitPrices.put("apple", 100);
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fruitPrices.put("pear", 80);
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// => { "apple" => 100, "pear" => 80 }
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```
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Only one value can be associated with each key.
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Calling `put` with the same key will update the key's value.
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```java
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fruitPrices.put("pear", 40);
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// => { "apple" => 100, "pear" => 40 }
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```
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Use [get][map-get-javadoc] to get the value for a key.
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```java
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fruitPrices.get("apple"); // => 100
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```
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Use [containsKey][map-containskey-javadoc] to see if the map contains a particular key.
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```java
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fruitPrices.containsKey("apple"); // => true
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fruitPrices.containsKey("orange"); // => false
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```
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Remove entries with [remove][map-remove-javadoc].
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```java
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fruitPrices.put("plum", 90); // Add plum to map
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fruitPrices.remove("plum"); // Removes plum from map
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```
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The [size][map-size-javadoc] method returns the number of entries.
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```java
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fruitPrices.size(); // Returns 2
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```
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You can use the [keys] or [values] methods to obtain the keys or the values in a Map as a Set or collection respectively.
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```java
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fruitPrices.keys(); // Returns "apple" and "pear" in a set
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fruitPrices.values(); // Returns 100 and 80, in a Collection
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```
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## HashMap uses `hashCode` and `equals`
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HashMaps uses the object's [hashCode][object-hashcode-javadoc] and [equals][object-equals-javadoc] method to work out where to store and how to retrieve the values for a key.
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For this reason, it is important that their return values do not change between storing and getting them, otherwise the HashMap may not be able to find the value.
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For example, lets say we have the following class that will be used as the key to a map:
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```java
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public class Stock {
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private String name;
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public void setName(String name) {
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this.name = name;
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}
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@Override
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public int hashCode() {
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return Objects.hash(name);
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}
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@Override
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public boolean equals(Object obj) {
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if (obj == this) {
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return true;
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}
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if (Objects.equals(Stock.class, obj.getClass()) && obj instanceof Stock other) {
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return Objects.equals(name, other.name);
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}
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return false;
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}
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}
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```
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The `hashCode` and `equals` depend on the `name` field, which can be changed via `setName`.
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Altering the `hashCode` can produce surprising results:
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```java
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Stock stock = new Stock();
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stock.setName("Beanies");
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Map<Stock, Integer> stockCount = new HashMap<>();
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stockCount.put(stock, 80);
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stockCount.get(stock); // Returns 80
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Stock other = new Stock();
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other.setName("Beanies");
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stockCount.get(other); // Returns 80 because "other" and "stock" are equal
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stock.setName("Choccies");
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stockCount.get(stock); // Returns null because hashCode value has changed
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stockCount.get(other); // Also returns null because "other" and "stock" are not equal
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stock.setName("Beanies");
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stockCount.get(stock); // HashCode restored, so returns 80 again
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stockCount.get(other); // Also returns 80 again because "other" and "stock" are back to equal
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```
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## Map.of and Map.copyOf
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Another common way to create maps is to use [Map.of][map-of-javadoc] or [Map.ofEntries][map-ofentries-javadoc].
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```java
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// Using Map.of
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Map<String, Integer> temperatures = Map.of("Mon", 30, "Tue", 28, "Wed", 32);
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// or using Map.ofEntries
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Map<String, Integer> temperatures2 = Map.ofEntries(Map.entry("Mon", 30, "Tue", 28, "Wed", 32));
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```
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Unlike `HashMap`, they populate the map upfront and become read-only once created.
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[Map.copyOf][map-copyof-javadoc] makes a read-only copy of a map.
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```java
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Map<String, Integer> readOnlyFruitPrices = Map.copyOf(fruitPrices);
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```
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Calling methods like `put`, `remove` or `clear` results in an `UnsupportedOperationException`.
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[map-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html
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[hashmap-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/HashMap.html
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[map-put-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#put(K,V)
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[map-get-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#get(java.lang.Object)
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[map-containskey-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#containsKey(java.lang.Object)
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[map-remove-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#remove(java.lang.Object)
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[map-size-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#size()
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[map-of-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#of()
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[map-ofentries-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#ofEntries(java.util.Map.Entry...)
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[map-copyof-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#copyOf(java.util.Map)
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[object-hashcode-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Object.html#hashCode()
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[object-equals-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)

‎concepts/maps/introduction.md

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# Introduction
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A **Map** is a data structure for storing key value pairs.
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It is similar to dictionaries in other programming languages.
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The [Map][map-javadoc] interface defines operations on a map.
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Java has a number of different Map implementations.
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[HashMap][hashmap-javadoc] is a commonly used one.
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```java
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// Make an instance
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Map<String, Integer> fruitPrices = new HashMap<>();
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```
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Add entries to the map using [put][map-put-javadoc].
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```java
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fruitPrices.put("apple", 100);
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fruitPrices.put("pear", 80);
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// => { "apple" => 100, "pear" => 80 }
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```
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Only one value can be associated with each key.
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Calling `put` with the same key will update the key's value.
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```java
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fruitPrices.put("pear", 40);
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// => { "apple" => 100, "pear" => 40 }
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```
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Use [get][map-get-javadoc] to get the value for a key.
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```java
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fruitPrices.get("apple"); // => 100
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```
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Use [containsKey][map-containskey-javadoc] to see if the map contains a particular key.
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```java
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fruitPrices.containsKey("apple"); // => true
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fruitPrices.containsKey("orange"); // => false
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```
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Remove entries with [remove][map-remove-javadoc].
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```java
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fruitPrices.put("plum", 90); // Add plum to map
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fruitPrices.remove("plum"); // Removes plum from map
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```
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The [size][map-size-javadoc] method returns the number of entries.
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```java
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fruitPrices.size(); // Returns 2
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```
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You can use the [keys] or [values] methods to obtain the keys or the values in a Map as a Set or collection respectively.studentScores
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```java
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fruitPrices.keys(); // Returns "apple" and "pear" in a set
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fruitPrices.values(); // Returns 100 and 80, in a Collection
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```
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[map-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html
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[hashmap-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/HashMap.html
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[map-put-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#put(K,V)
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[map-get-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#get(java.lang.Object)
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[map-containskey-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#containsKey(java.lang.Object)
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[map-remove-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#remove(java.lang.Object)
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[map-size-javadoc]: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#size()

‎concepts/maps/links.json

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[
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{
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"url": "https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html",
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"description": "Interface Map documentation"
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},
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{
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"url": "https://dev.java/learn/api/collections-framework/maps/",
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"description": "Using Maps to Store Key Value Pairs"
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}
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]

‎config.json

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"generic-types"
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],
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"status": "beta"
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},
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{
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"slug": "international-calling-connoisseur",
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"name": "International Calling Connoisseur",
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"uuid": "03506c5a-601a-42cd-b037-c310208de84d",
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"concepts": [
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"maps"
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],
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"prerequisites": [
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"classes",
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"foreach-loops",
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"generic-types"
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]
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}
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],
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"practice": [
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"practices": [],
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"prerequisites": [
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"if-else-statements",
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"for-loops"
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"for-loops",
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"maps"
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],
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"difficulty": 5
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},
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"practices": [],
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"prerequisites": [
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"for-loops",
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"arrays"
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"arrays",
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"maps"
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],
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"difficulty": 5
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},
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"practices": [],
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"prerequisites": [
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"chars",
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"exceptions"
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"exceptions",
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"maps"
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],
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"difficulty": 6
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},
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"practices": [],
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"prerequisites": [
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"foreach-loops",
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"maps",
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"strings"
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],
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"difficulty": 6
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"uuid": "38a405e8-619d-400f-b53c-2f06461fdf9d",
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"practices": [],
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"prerequisites": [
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"maps",
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"strings"
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],
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"difficulty": 6
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"uuid": "2e760ae2-fadd-4d31-9639-c4554e2826e9",
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"practices": [],
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"prerequisites": [
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"enums"
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"enums",
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"maps"
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],
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"difficulty": 7
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},
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"chars",
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"if-else-statements",
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"lists",
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"for-loops"
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"for-loops",
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"maps"
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],
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"difficulty": 7
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},
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"prerequisites": [
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"arrays",
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"strings",
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"if-else-statements"
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"if-else-statements",
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"maps"
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],
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"difficulty": 7
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},
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"exceptions",
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"for-loops",
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"if-else-statements",
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"maps",
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"numbers"
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],
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"difficulty": 8
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"slug": "lists",
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"name": "Lists"
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},
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{
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"uuid": "2f6fdedb-a0ac-4bab-92d6-3be61520b9bc",
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"slug": "maps",
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"name": "Maps"
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},
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{
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"uuid": "54118389-9c01-431b-a850-f47da498f845",
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"slug": "method-overloading",

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