# Collections ### Lists A `List` in Java is an interface that behaves very similarly to an array. * It's an ordered collection (also known as sequence). * The user of this interface has precise control over where each item is inserted in the list. * The user can access items by their integer index (position in the list). * The user can search for items in the list by looping over the items in it. Example: ```java // Database Schema /* CREATE TABLE Feedback ( feedbackId STRING(100) NOT NULL, email STRING(100), quiz STRING(20), feedback STRING(MAX), rating INT64, score FLOAT64, timestamp INT64 ) PRIMARY KEY (feedbackId); */ // https://google.qwiklabs.com/focuses/1128 List mutations = new ArrayList<>(); mutations.add( // TODO: Build a new insert mutation Mutation.newInsertBuilder("Feedback") .set("feedbackId") .to(feedback.getEmail() + '_' + feedback.getQuiz() + "_" + feedback.getTimestamp()) .set("email") .to(feedback.getEmail()) .set("quiz") .to(feedback.getQuiz()) .set("feedback") .to(feedback.getFeedback()) .set("rating") .to(feedback.getRating()) .set("score") .to( feedback.getSentimentScore()) .set("timestamp") .to(feedback.getTimestamp()) .build()); ``` ### ArrayList An `ArrayList` is a class that implements the interface `List` . It's simply a wrapper around an array, but provides really powerful methods that make dealing with the array much simpler. {% hint style="info" %} An item in an ArrayList is known as an element. {% endhint %} Let's have a look at some of the ArrayList's methods: * add(E element): Appends the specified element to the end of this list. * add(int index, E element): Appends the specified element to the specified index of this list. * get(int index): Returns the element at the specified position in this list. * contains(Object o): Returns true if this list contains the specified element. * remove(int index) * size() . To create and initialize an ArrayList: ``` ArrayList grades = new ArrayList(); ``` Then you can add elements by `add()` method: ``` grades.add(100); grades.add(97); // to access first item in the list grades.get(0); //100 //size of the array System.out.println(grades.size().toString()); //2 //also remove items by index grades.remove(0); //clear the entire list by clear() grades.clear(); ``` We do not need to specify the array size, unlike arrays. ```java ArrayList myArrayList = new ArrayList(); int[] myArray = new int[100]; ``` ### Generics For example: `ArrayList`s use Generics to allow you to specify the data type of the elements you're intending to add into that `ArrayList`. How ```java ArrayList listOfStrings = new ArrayList(); ``` Generics eliminate the need for casting ```java List list = new ArrayList(); list.add("hello"); String s = (String) list.get(0); ``` When rewritten above one using generics: ```java List list = new ArrayList(); list.add("hello"); String s = list.get(0); // no cast ``` Define own Generic Types . ### Collections and Polymorphism ```java ArrayList playlist = new ArrayList(); Video someVideo = new Video(); Audio someAudio = new Audio(); playlist.add(someVideo); playlist.add(someAudio); Media media = playlist.get(0); media.play(); ``` ### ArrayList methods ```java for(String city : cities) { if(city.equals("Sydney")) { return true; } } ``` The `indexOf` of method returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. ```java cities.indexOf("Sydney"); ``` If the above returned -1 then Sydney is not in the list, if it returned any positive value than that will be the index of the String "Sydney". . ### HashMap ```java public class Book { String title; String author; int numOfPages; int publishedYear; int edition; String ISBN; } ``` If you were to create a library class that will simulate a virtual library of all the books that exist in the world (\~130 Million) you can easily create an ArrayList of Books, and fill it up with all the book details that you may have. ```java public class Library { ArrayList allBooks; } ``` To find book by ISBN: ```java Book findBookByISBN(String isbn) { for(Book book : library.allBooks) { if(book.ISBN.equals(isbn)) { return book; } } } ``` A way more optimal solution is to use a HashMap instead of `ArrayList`s. ```java import java.util.HashMap public class Library { HashMap allBooks; allBooks = new HashMap(); } ``` Then, to add items to the HashMap: ```java Book taleOfTwoCities = new Book(); allBooks.put("98754236152364", taleOfTwoCities); //To search for book using ISBN Book findBookByISBN(String isbn) { Book book = allBooks.get(isbn); return book; } ``` ## Resources 1. 2. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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