This section uses extracts from the -- Java Tutorial, with some adaptations.

A collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data.

The collections framework is a unified architecture for representing and manipulating collections. It contains the following:

• Interfaces: These are abstract data types that represent collections. Interfaces allow collections to be manipulated independently of the details of their representation.
  Example: the List<E> interface can be used to manipulate list-like collections which may be implemented in different ways such as ArrayList<E> or LinkedList<E>.
  

• Implementations: These are the concrete implementations of the collection interfaces. In essence, they are reusable data structures.
  Example: the ArrayList<E> class implements the List<E> interface while the HashMap<K, V> class implements the Map<K, V> interface.
  

• Algorithms: These are the methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces. The algorithms are said to be polymorphic: that is, the same method can be used on many different implementations of the appropriate collection interface.
  Example: the sort(List<E>) method can sort a collection that implements the List<E> interface.

A well-known example of collections frameworks is the C++ Standard Template Library (STL). Although both are collections frameworks and the syntax look similar, note that there are important philosophical and implementation differences between the two.

The following list describes the core collection interfaces:

• Collection — the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Collection interface is the least common denominator that all collections implement and is used to pass collections around and to manipulate them when maximum generality is desired. Some types of collections allow duplicate elements, and others do not. Some are ordered and others are unordered. The Java platform doesn't provide any direct implementations of this interface but provides implementations of more specific subinterfaces, such as Set and List. Also see the Collection API.

• Set — a collection that cannot contain duplicate elements. This interface models the mathematical set abstraction and is used to represent sets, such as the cards comprising a poker hand, the courses making up a student's schedule, or the processes running on a machine. Also see the Set API.

• List — an ordered collection (sometimes called a sequence). Lists can contain duplicate elements. The user of a List generally has precise control over where in the list each element is inserted and can access elements by their integer index (position). Also see the List API.

• Queue — a collection used to hold multiple elements prior to processing. Besides basic Collection operations, a Queue provides additional insertion, extraction, and inspection operations. Also see the Queue API.

• Map — an object that maps keys to values. A Map cannot contain duplicate keys; each key can map to at most one value. Also see the Map API.

• Others: Deque, SortedSet, SortedMap

The ArrayList class is a resizable-array implementation of the List interface. Unlike a normal array, an ArrayList can grow in size as you add more items to it. The example below illustrates some of the useful methods of the ArrayList class using an ArrayList of String objects.

import java.util.ArrayList;

public class ArrayListDemo {

    public static void main(String args[]) {
        ArrayList<String> items = new ArrayList<>();

        System.out.println("Before adding any items:" + items);

        items.add("Apple");
        items.add("Box");
        items.add("Cup");
        items.add("Dart");
        print("After adding four items: " + items);

        items.remove("Box"); // remove item "Box"
        print("After removing Box: " + items);

        items.add(1, "Banana"); // add "Banana" at index 1
        print("After adding Banana: " + items);

        items.add("Egg"); // add "Egg", will be added to the end
        items.add("Cup"); // add another "Cup"
        print("After adding Egg: " + items);

        print("Number of items: " + items.size());

        print("Index of Cup: " + items.indexOf("Cup"));
        print("Index of Zebra: " + items.indexOf("Zebra"));

        print("Item at index 3 is: " + items.get(2));

        print("Do we have a Box?: " + items.contains("Box"));
        print("Do we have an Apple?: " + items.contains("Apple"));

        items.clear();
        print("After clearing: " + items);
    }

    private static void print(String text) {
        System.out.println(text);
    }
}

Before adding any items:[]
After adding four items: [Apple, Box, Cup, Dart]
After removing Box: [Apple, Cup, Dart]
After adding Banana: [Apple, Banana, Cup, Dart]
After adding Egg: [Apple, Banana, Cup, Dart, Egg, Cup]
Number of items: 6
Index of Cup: 2
Index of Zebra: -1
Item at index 3 is: Cup
Do we have a Box?: false
Do we have an Apple?: true
After clearing: []

[Try the above code on Repl.it]

HashMap is an implementation of the Map interface. It allows you to store a collection of key-value pairs. The example below illustrates how to use a HashMap<String, Point> to maintain a list of coordinates and their identifiers e.g., the identifier x1 is used to identify the point 0,0 where x1 is the key and 0,0 is the value.

import java.awt.Point;
import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {
    public static void main(String[] args) {
        HashMap<String, Point> points = new HashMap<>();

        // put the key-value pairs in the HashMap
        points.put("x1", new Point(0, 0));
        points.put("x2", new Point(0, 5));
        points.put("x3", new Point(5, 5));
        points.put("x4", new Point(5, 0));

        // retrieve a value for a key using the get method
        print("Coordinates of x1: " + pointAsString(points.get("x1")));

        // check if a key or a value exists
        print("Key x1 exists? " + points.containsKey("x1"));
        print("Key x1 exists? " + points.containsKey("y1"));
        print("Value (0,0) exists? " + points.containsValue(new Point(0, 0)));
        print("Value (1,2) exists? " + points.containsValue(new Point(1, 2)));

        // update the value of a key to a new value
        points.put("x1", new Point(-1,-1));

        // iterate over the entries
        for (Map.Entry<String, Point> entry : points.entrySet()) {
            print(entry.getKey() + " = " + pointAsString(entry.getValue()));
        }

        print("Number of keys: " + points.size());
        points.clear();
        print("Number of keys after clearing: " + points.size());

    }

    public static String pointAsString(Point p) {
        return "[" + p.x + "," + p.y + "]";
    }

    public static void print(String s) {
        System.out.println(s);
    }
}

Coordinates of x1: [0,0]
Key x1 exists? true
Key x1 exists? false
Value (0,0) exists? true
Value (1,2) exists? false
x1 = [-1,-1]
x2 = [0,5]
x3 = [5,5]
x4 = [5,0]
Number of keys: 4
Number of keys after clearing: 0

[Try the above code on Repl.it]