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‎POLICIES.md

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@@ -51,7 +51,7 @@ References: [[1](https://github.com/exercism/java/issues/177#issuecomment-261291
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> throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
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> ```
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>
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> - Exercises of difficulty 5 or higher: copy the StubTemplate.java file (provided [here](https://github.com/exercism/java/tree/main/_template/src/main/java)) and rename it to fit the exercise. For example, for the exercise linked-list the file could be named LinkedList.java. Then either (1) add hints to the hints.md file (which gets merged into the README.md for the exercise) or (2) provide stubs as above for exercises that demand complicated method signatures.
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> - Exercises of difficulty 5 or higher: copy the StubTemplate.java file (provided [here](https://github.com/exercism/java/blob/main/resources/exercise-template/src/main/java/ExerciseName.java)) and rename it to fit the exercise. For example, for the exercise linked-list the file could be named LinkedList.java. Then either (1) add hints to the hints.md file (which gets merged into the README.md for the exercise) or (2) provide stubs as above for exercises that demand complicated method signatures.
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References: [[1](https://github.com/exercism/java/issues/178)], [[2](https://github.com/exercism/java/pull/683#discussion_r125506930)], [[3](https://github.com/exercism/java/issues/977)], [[4](https://github.com/exercism/java/issues/1721)]
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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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"slug": "tim-from-marketing-2",
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"name": "tim-from-marketing-2",
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"uuid": "a6cfc286-8c62-4f5b-8e59-f6bfc4374092",
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"concepts": [],
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"prerequisites": []
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"concepts": [
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"optional type"
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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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"lists"
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]
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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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