ContainersDhrubojyoti Kayal

Collections◦ a sequence of individual elements with one or

more rules applied to them. ◦ List, Queue, Set

Map◦ a group of key-value object pairs, allowing you

to look up a value using a key.

Basics

Lists promise to maintain elements in a particular sequence.

ArrayList - excels at randomly accessing elements, but is slower when inserting and removing elements in the middle of a List.

LinkedList - which provides optimal sequential access, with inexpensive insertions and deletions from the middle of the List.

List

List list = new ArrayList(); list.add(1); Integer val = (Integer) list.get(0); System.out.println(list.size()); list.add(2); list.remove(0); System.out.println(list.size());

List in Action

Declare an ArrayList and fill 100 integers in it.

Now loop through the list to print the list.

Exercise

List list = new ArrayList(); list.add(1); list.add("john"); Integer val2 = (Integer) list.get(1);

Paving way for Generics

Allow only a specific type to be added in a list

Runtime system takes care of the type casts under the hood.

List<Integer> list = new ArrayList<Integer> list.add(1); list.add(“hi”);

Generics

Declare an ArrayList and fill it with 100 integers

Now iterate the list using for each loop

Exercise

An iterator is an object whose job is to move through a sequence and select each object in that sequence without the client programmer knowing or caring about the underlying structure of that sequence.

Java Iterator can move in only one direction Typical workflow

◦ Get iterator object - list.iterator()◦ Get the next object in the sequence with next( ).◦ See if there are any more objects in the sequence

with hasNext( ).

Iterator

Repeat the previous example with an Iterator

Excercise

Concrete implementation of Set interface Duplicate free collection Opmtized for rapid lookup Set set = new HashSet set.add(1); set.contains(1) boolean

HashSet

Declare an integer array with 6 elements with elements – {1,5,3,5,6,3}

Now find the duplicates in that array.

Excercise

Implements the basic List interface Insertion and removal in the middle of the List is more

efficient than ArrayList It is less efficient for random-access operations. Adds methods that allow it to be used as a stack, a

Queue or a double-ended queue (deque). add(Object o) addFirst(Object o) addLast(Object o) removeFirst() removeLast() getFirst(); getLast();

LinkedList

Write a Java program to implement a Queue with a LinkedList

Write a Java program to implement a Stack with a LinkedList

Excercise

Implementation of Map Key Value pair Useful for cache and lookup Map<String, Integer> map = new

HashMap<String, Integer> map.get(key) map.put(key,value);

HashMap

Q&A