Java Collection

  • 19/11/2022
  • 2-minute read

In this post, I will talk about Java collection. Some well-known containers such as ArrayList, TreeSet are implementing classes of their respective interfaces which inherit from the collection interface. Therefore, it is a good idea to learn methods from the collection interface because all of its derived implementing classes will acquire the same methods.

Collection is a kind of container provided by Java. It stores objects and can change its length.

List Interface (Vector / ArrayList / LinkedList)

  • Elements are ordered
  • Allow duplicated values
  • Can use index to traversal elements

Set Interface (TreeSet / HashSet / LinkedHashSet)

  • Elements in TreeSet and HashSet are not ordered
  • Does not allow duplicated values
  • Can not use index to traversal elements

Collection Interface

  • Can not use index to traversal elements

Commonly Used Methods in Collection Interface

// add() method: add an element to a collection
private static void demo01() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    coll.add("Hello");
    coll.add("World");
    System.out.println(coll); // [Hello, World]
}

// remove() method: delete an element from a collection
private static void demo02() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    coll.add("Hello");
    coll.add("World");
    coll.remove("Hello");
    System.out.println(coll); // [World]
}

// contains(): check if the collection contains the element
private static void demo03() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    coll.add("Hello");
    coll.add("World");
    System.out.println(coll.contains("Hello")); // true
    System.out.println(coll.contains("Hey"));   // false
}

// isEmpty(): check if the collection is empty or not
private static void demo04() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    System.out.println(coll.isEmpty()); // true
}

// size(): returns the number of elements in the collection
private static void demo05() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    coll.add("Hello");
    coll.add("World");
    System.out.println(coll.size()); // 2
}

// toArray(): Store elements of the collection into an array
private static void demo06() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    coll.add("Hello");
    coll.add("World");
    Object[] arr = coll.toArray();
    for (int i = 0; i < arr.length; i++) {
        System.out.println(arr[i]);
    }
}

// clear(): clear all elements of the collection
private static void demo07() {
    Collection<String> coll = new ArrayList<>(); // Polymorphism
    coll.add("Hello");
    coll.add("World");
    coll.clear();
    System.out.println(coll); // []
}
JavaData Structure