Collections

It’s not simple like JavaScript. There are a bunch of different ways to do collections of objects in Java. The basic Array in Java has a set length once instantiated, and cannot change. There are a bunch of things in java.util.* for managing collections and lists of things instead.

The very base of them all is the Collection class. The Collection is basically a high-level interface providing guideline for how to implement a collection of stuff, making different data structures relatively predictable and uniform — they all have a common set of methods, like add, remove and whatnot.

The ArrayList - The most basic collection, an array that automatically expands itself.

public class ApplesAndOrangesWithoutGenerics { 
  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    ArrayList apples = new ArrayList(); 
    for(int i = 0; i < 3; i++) {
      apples.add(new Apple());
    }
    // No problem adding an Orange to apples: apples.add(new Orange());
      for(Object apple : apples) {
            ((Apple) apple).id();
            // Orange is detected only at run time
      } 
  }
}

There are also these:

• Set
• HashSet
• Map
• HashMap
• ArrayList
• LinkedList
• Stack
• Queue
• PriorityQueue

Autoboxing

Collections can’t hold primitives, only objects. Autoboxing is a term that refers to a nice thing that happens when you make a collection of primitives — it converts it to a wrapper object for you on the way in and out.

Iterator

In addition to the Collection interface, there is also the Iterator interface. This is an interface that all Collections implement as a nested/related object, used as a cursor to a given member/item in the collection. You can ask a collection to hand you an iterator by calling .iterator() on the collection. It returns the first item in the collection. You can then call .next() on the iterator, and it will return you the next item. Iterator also has other methods, like .hasNext(), .prev(), .remove(), etc.

Use this to move through (iterate through) a collection. More here -> docs.