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Lists, sets, dictionaries

The three families of collections: the list (order, duplicates, index), the set (uniqueness) and the dictionary (key to value). Adding, accessing, removing, testing, and choosing the right collection for a need.

  • 24 cards
  • 5 topics
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  • Collection: what is it?

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    An object that holds several elements and offers operations to add, find, remove and traverse them.

    In Java, List, Set and Map are the three basic interfaces: they describe what you can do, not how the elements are stored. ArrayList, HashSet or HashMap are classes that implement them.

  • What are the three sizes printed?

    ```java
    List<Integer> liste = new ArrayList<>();
    Set<Integer> ensemble = new HashSet<>();
    Map<String, Integer> dico = new HashMap<>();
    for (int n : new int[] {3, 5, 3}) {
    liste.add(n);
    ensemble.add(n);
    }
    dico.put("a", 1);
    dico.put("a", 2);
    System.out.println(liste.size() + " " + ensemble.size() + " " + dico.size());
    `

    • 3 2 1
    • 3 3 2
    • 2 2 1
    • 3 2 2
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    3 2 1

    The list keeps duplicates (3 elements), the set drops them (3 and 5), and the dictionary has a single entry for the key "a" whose value went from 1 to 2.

  • In a Java list, an element is read with get(i), where i is its …; the first element has index ….

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    In a Java list, an element is read with get(i), where i is its index; the first element has index 0.

    An index outside the list, such as get(3) on three elements, raises IndexOutOfBoundsException: valid indices go from 0 to size - 1.

  • List: what is it?

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    An ordered collection where each element has a position numbered from 0 and the same value may appear several times.

    It keeps insertion order and accepts duplicates: after add(3), add(5), add(3), a List holds [3, 5, 3] and get(0) returns 3.

  • A shop must instantly find a product from its reference. Which structure should it use?

    • A list of products, traversed until the reference is found
    • A dictionary keyed by the reference
    • A set of products, sorted by reference
    • A list of references, the position standing for the product
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    A dictionary keyed by the reference

    Finding data by an identifier is the dictionary's job: products.get(ref). A list would force comparing everything, and a set attaches no data to a reference.

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