CS 106A, Lecture 19 ArrayLists - web.stanford.edu · This document is copyright (C) ... CS 106A, Lecture 19 ArrayLists suggested reading: Java Ch. 11.8. 2 ArrayLists HashMaps

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CS106A,Lecture19ArrayLists

suggestedreading:JavaCh.11.8

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ArrayListsHashMaps

Wearehere

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Learning Goals• KnowhowtostoredatainandretrievedatafromanArrayList.

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Plan for today•Recap:Tic-Tac-Toe•ArrayLists•Example:reversiblewriting•Example:planner•ArrayLists vs.arrays•Recap

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Tic-Tac-ToeLet’suse2DarraystocreateaConsoleProgram versionofTic-Tac-Toe.

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Limitations of Arrays• Sizemustbespecifieduponcreation• Can’tadd/remove/insertelementslater• Nobuilt-inmethodsforsearching,etc.• Can’tprintarrayswithoutArrays.toString (orArrays.deepToString)

index 0 1 2 3 4 5 6 7 8 9

value 12 49 -2 26 5 17 -6 84 72 3

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Introducing… ArrayLists!•Avariabletypethatrepresentsalistofitems.•Youaccessindividualitemsbyindex.•Storeasingletypeofobject(String,GRect,etc.)•Resizable– canaddandremoveelements•Hashelpfulmethodsforsearchingforitems

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Our First ArrayList

ArrayList<String> myArrayList = new ArrayList<>();

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Our First ArrayList


import java.util.*;

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Our First ArrayList


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Our First ArrayList


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Our First ArrayList


Type of items your ArrayList will store.

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Our First ArrayList


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Our First ArrayList


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Our First ArrayList


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Our First ArrayList// Create an (initially empty) listArrayList<String> list = new ArrayList<>();

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// Add an element to the backlist.add("Hello"); // now size 1

“Hello”

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// Add an element to the backlist.add("Hello"); // now size 1

list.add("there!"); // now size 2

“Hello”

“Hello” “there!”

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Our First ArrayList// Add an element to the backlist.add("Hello"); // now size 1


// Access elements by index (starting at 0!)println(list.get(0)); // prints "Hello"println(list.get(1)); // prints "there!”println(list); // prints ["Hello", "there!"]

“Hello”


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// Access elements by index (starting at 0!)for (int i = 0; i < list.size(); i++) {println(list.get(i));

}

“Hello”


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// Access elements by index (starting at 0!)for (int i = 0; i < list.size(); i++) {println(list.get(i));

}

“Hello”


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// Access elements in order (also for arrays!)for (String str : list) {println(str);

}

“Hello”


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Iterating Over ArrayLists// Access elements in order (also for arrays!)for (String str : list) {println(str);

}

// equivalent to

for (int i = 0; i < list.size(); i++) {String str = list.get(i);println(str);

}

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Iterating Over ArrayLists// Access elements in order (also for arrays!)for (String str : list) {println(str);

}

// equivalent to

for (int i = 0; i < list.size(); i++) {String str = list.get(i);println(str);

}

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Bad Times with ArrayLists// Create an (initially empty) listArrayList<String> list = new ArrayList<>();

// Wrong type – bad times! Won’t compileGLabel label = new GLabel("Hello there!");list.add(label);

// Invalid index! IndexOutOfBounds Exceptionprintln(list.get(2));

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Example: Reversible Writing

IamnotapersonwhocontributesAndIrefusetobelievethat

Iwillbeuseful

Let’s write a program that reverses a text file.

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Example: Reversible Writing

IamnotapersonwhocontributesAndIrefusetobelievethat

Iwillbeuseful

IwillbeusefulAndIrefusetobelievethat

Iamnotapersonwhocontributes

Let’s write a program that reverses a text file.

"I Have a Dream" by Antonia Lee, Sara Fung, Christy Fung, Rachel Lamhttp://poets.spice.org.hk/index.php?option=com_content&view=article&id=45:my-family&catid=6:reverse-poem&Itemid=7

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Example: Reversible WritingLet’s write a program that reverses a text file.

“Iamnotapersonwhocontributes”

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“Iamnotapersonwhocontributes”"AndIrefusetobelievethat"

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“Iwillbeuseful”

Key idea: fill an ArrayList with each line in the file

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“Iwillbeuseful”

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“Iwillbeuseful”

Key idea: print the ArrayList items in reverse order

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Example: Reversible WritingString filename = promptUserForFile("Filename: ", "res");try {Scanner s = new Scanner(new File(filename));ArrayList<String> lines = new ArrayList<>();

// Read all lines and store in our ArrayListwhile (scanner.hasNextLine()) {

lines.add(scanner.nextLine());}

// Output the lines from back to frontfor (int i = lines.size() - 1; i >= 0; i--) {

println(lines.get(i));}

} catch (IOException ex) {println("Could not read file.");

}

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}

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}

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}

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}

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}

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}

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}

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ArrayList Methodslist.add(value); appendsvalueatendoflistlist.add(index, value); insertsgivenvaluejustbeforethegivenindex,

shiftingsubsequentvaluestotherightlist.clear(); removesallelementsofthelistlist.get(index) returnsthevalueatgivenindexlist.indexOf(value) returnsfirstindexwheregivenvalueisfoundinlist

(-1ifnotfound)list.isEmpty() returnstrue ifthelistcontainsnoelementslist.remove(index); removes/returnsvalueatgivenindex,shifting

subsequentvaluestotheleftlist.remove(value); removesthefirstoccurrenceofthevalue,ifanylist.set(index, value); replacesvalueatgivenindexwithgivenvaluelist.size() returnsthenumberofelementsinthelistlist.toString() returnsastringrepresentationofthelist

suchas"[3, 42, -7, 15]"

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Insert/remove• Ifyouinsert/removeinthefrontormiddleofalist,elementsshift tofit.

list.add(2, 42);• shiftelementsrighttomakeroomforthenewelement

list.remove(1);• shiftelementslefttocoverthespaceleftbytheremovedelement

index 0 1 2 3 4value 3 8 9 7 5

index 0 1 2 3 4 5value 3 8 42 9 7 5

index 0 1 2 3 4 5value 3 8 42 9 7 5

index 0 1 2 3 4value 3 42 9 7 5

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Example: Planner

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Example: Planner• Let’swriteaprogramtohelpplanoutourday

– Theprogramfirstpromptsforthingsyouwanttodotoday– Then,itaskstheusertore-inputtheminorderofcompletion– Finally,itoutputstheordertheuserhaschosenfortheirtasks

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Planner: Approach

“Walk Daisy”

Todos:

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Planner: Approach

“Walk Daisy”

“Play Zelda”

Todos:

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Planner: Approach

“Walk Daisy”

“Play Zelda”

“Lunch with Avi”

Todos:

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Planner: Approach

“Walk Daisy”

“Play Zelda”


Todos:

Order: “WalkDaisy”

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Planner: Approach

“Play Zelda”


Todos:


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Planner: Approach

“Play Zelda”

Todos:



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Planner: Approach

Todos:



“Play Zelda”

DONE!

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ArrayLists + Primitives = 💔// Doesn’t compile LArrayList<int> list = new ArrayList<>();

Unlike arrays, ArrayLists can only store objects!

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ArrayLists + Primitives = 💔

Primitive “Wrapper”Classint Integerdouble Doubleboolean Booleanchar Character

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ArrayLists + Wrappers = ❤// Use wrapper classes when making an ArrayListArrayList<Integer> list = new ArrayList<>();

// Java converts Integer <-> int automatically!int num = 123;list.add(num);

int first = list.get(0); // 123

Conversion happens automatically!

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Array vs. ArrayListArrayList

ArrayList<Integer> list = new ArrayList<>();

list.add(1); // [1]list.add(2); // [1, 2]

list.set(0, 3); // [3, 2]int x = list.get(0); // 3

list.add(4); // [3, 2, 4]list.contains(2); // true

Array

int[] arr = new int[2]; // [0, 0]

arr[0] = 1; // [1, 0]arr[1] = 2; // [1, 2]

arr[0] = 3; // [3, 2]int x = arr[0]; // 3

[no equivalent]

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Array vs. ArrayList

Whydobothoftheseexistinthelanguage?– ArraysareJava'sfundamentaldatastorage– ArrayList isalibrarybuiltontopofanarray

WhenwouldyouchooseanarrayoveranArrayList?– Whenyouneedafixedsizethatyouknowaheadoftime

–Simplersyntaxforgetting/setting–Moreefficient

– Multi-dimensional arrays(e.g.images)– Histograms/tallying

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Recap• ArrayLists areavariabletyperepresentingalistofitems• Unlikearrays,ArrayLists have:

– Theabilitytoresizedynamically– Usefulmethodsyoucancallonthem

• UnlikeArrayLists,arrayshave:– Theabilitytostoreanytypeofitem,notjustobjects

NextTime:HashMaps

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CS 106A, Lecture 19 ArrayLists - web.stanford.edu · This document is copyright (C) ... CS 106A, Lecture 19 ArrayLists suggested reading: Java Ch. 11.8. 2 ArrayLists HashMaps