Collections: Lists, ArrayLists, Sets

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What we’ll cover

What is a List?

The List Interface

ArrayList

Set Interface

Types of Sets

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What is a Collection?

• Collection is an interface which ensures a class has the ability to hold a series of objects.
• List is an interface which extends Collection.

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Collection Interface

• Fundamental interface for Collection classes in java.

public interface Collection<E> extends Iterable<E> {
    boolean add(E element);
    boolean addAll(Collection<? extends E> collection);
    void clear();
    boolean contains(Object object);
    boolean containsAll(Collection<?> collection);
    void foreach(Consumer<E> consumer);
    boolean isEmpty();
    Iterator<E> iterator();
    boolean remove(Object object);
    boolean removeAll(Collection<?> collection);
    boolean retainAll(Collection<?> collection);
    int size();
    Object[] toArray();
    <T> T[] toArray(T[] array);
}

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What is a List?

• An ordered collection of elements.
• Users of List objects can:
  • access elements by their integer index (position in List)
  • insert elements into List
  • remove elements from the List
  • search for the presence of element in the List

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List Interface

public interface List<E> extends Collection<E> {
  int lastIndexOf(Object object);
  int indexOf(Object object);
  E set(int index, E object);
  List<E> sub(int startingIndex, int endingIndex);
}

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ArrayList

• java.util.ArrayList
• Dynamic in capacity (size)
  • size increases upon insertion, decreases upon removal
• Quicker than LinkedList with random access to elements.
• Can only handle non-primitive types.

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Unmodifiable List

• java.util.Arrays.ArrayList
  • An unmodifiable list, with a stupid name.
  • Should have been named UnmodifiableList.

public void demo() {
  String[] array = {"The", "Quick", "Brown", "Fox"};
  List<String> unmodifiableList = Arrays.asList(array);
  unmodifiableList.add("Jumps"); // throws Exception
}

Output

java.lang.UnsupportedOperationException

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Converting Array to ArrayList

• populates ArrayList upon construction

  public void demo() {
  String[] array = {"The", "Quick", "Brown", "Fox"};
  List<String> unmodifiableList = Arrays.asList(array);
  List<String> arrayList = new ArrayList<>(unmodifiableList);
  System.out.println(arrayList);
  }
  

Output

[The, Quick, Brown, Fox]

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• populates ArrayList with for each loop (behavior of Iterable)

public void demo() {
  String[] phrase = {"The", "Quick", "Brown", "Fox"};
  List<String> list = new ArrayList<>();
  for(String word : phrase) {
    list.add(word);
  }
}

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• populates ArrayList with inherited method Collection.foreach

public void demo() {
  String[] phrase = {"The", "Quick", "Brown", "Fox"};
  List<String> list = new ArrayList<>(Arrays.asList(phrase));
  list.forEach(word -> list.add(word));
}

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Set Interface

• Very similar to List interface, except
  • the add method should reject duplicates.
  • the equals method should be defined so that two sets are identical if they have the same elements, but not necessarily in the same order.
  • the hashCode method should be defined so that two sets with the same elements yield the same hash code.

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Sets

|||| | ————— | ———— | | HashSet | sorts elements in hash order (fast, not predictable) | TreeSet | sorts elements in ascending sorted order | LinkedHashSet | provides insertion-order access to elements | EnumSet | Optimized set for holding enum instances | BitSet | Optimized for storing sequences of bits ||||

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HashSet

• Does not maintain insertion-order of elements
• Elements are positioned by their hash value.
• Can retrieve elements quickly due to efficient hash-sorting

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HashSet example

public void test() {
    String[] words = {"John", "Charles", "Cutler", "Tuskegee"};
    Set<String> set = new HashSet<>(Arrays.asList(words));
    System.out.println(set);
}

Output

[Charles, Tuskegee, John, Cutler]

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TreeSet

• Similar to HashSet
• Sorts elements after each insertion
  • Sort is dependent on implementation of compareTo or toString if elements not Comparable

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TreeSet example

public void test() {
    String[] words = {"John", "Charles", "Cutler", "Tuskegee"};
    Set<String> set = new TreeSet<>(Arrays.asList(words));
    System.out.println(set);
}

Output

[Charles, Cutler, John, Tuskegee]

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LinkedHashSet

• provides insertion-order access to element

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LinkedHashSet example

public void test() {
    String[] words = {"John", "Charles", "Cutler", "Tuskegee"};
    Set<String> set = new LinkedHashSet<>(Arrays.asList(words));
    System.out.println(set);
}

Output

[John, Charles, Cutler, Tuskegee]

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