A+ Computer Science LISTS · Primitive variables carry their values with them like color coded sticky notes int age = 18; Every time I declare a variable, a new sticky note is created

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A+ Computer Science

LISTS

What isa List?

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Problems with Arrays:What are the limitations of Arrays?

Problems with Arrays:● Hard to add elements (need to existing bump elements up)● Hard to remove elements (need to bump existing elements down)● Hard to replace an element● Hard to find an element● Have to import and use Arrays.toString to print the array values

So JAVA creators made the ArrayList class

● It builds on basic arrays by adding helpful methods ● It’s a fancy class of Arrays that stores Objects● They wrote the code so you don’t have to!

If Arrays and ArrayLists were Fords

Arrays are a manual Ford Fiesta...they get you where you need to be, but you need to do a lot of the work...roll up the windows, dial the radio, shift the gears etc.

ArrayLists are an automatic Ford GT. They also get you where you need to be, but have a lot of nice, built-in features that you can rely on.

What isa List?


Stacey Got a Little Ahead of Himself.Let’s do a quick preview lesson on:

REFERENCE VARIABLES(Sometimes called “Pointers”)

Primitive variables carry their values with them like color coded sticky notes

int age = 18;

Every time I declare a variable, a new stickynote is created with its name and value all togetherat the same memory location

age 18

We are about to learn that sometimes the name is separated from the value. The name POINTS to the value

age

Integer age = 18;

18Ox8d65a

● age “points” to the memory location where the value is stored.

● Age acts like a POINTER● Age does not know it is 18● Age only knows where to POINT

Wrapper Class

ArrayLists MUST store Objects, or Pointer variables. They cannot store primitive types. For this reason, Java writers made ‘Wrapper Classes” to emulate primitive variables.

age

age

Wrapper Class for ArrayList Primitive Type

Integer Int

Double Double

Boolean boolan


● Arraylist is a class that houses anarray.● An ArrayList can store any type.● All ArrayLists store the first

reference at spot /index position 0.

Now - ArrayList


Array vsArrayList

● When you initialize, you tell the CPU how much memory you will require. For example:

○ Enough for 7 ints○ Enough for 10 Strings○ Enough for {19.0, 21.5, -6.4}

● All of the data is stored TOGETHER.

● You can’t ever store more or less

● When you initialize, you make ONE reference pointing to NOTHING. You tell it what TYPE of Object it expects to point to next.

● When you add a new object, it becomes part of the list

● Then it expects to POINT to the next object

● This forms a LINKED list● The values are stored all over the

computer’s memory


0 1 2 3 4 5 6 7 8 9

nums 0 0 0 0 0 0 0 0 0 0

int[] nums = new int[10]; //Java int array

An array is a group of items all of the same type which are accessed through a single identifier.

Array - fixed in memory


aplus

nothing

null

null

ArrayList aplus;

aplus is a reference to an ArrayList.

ArrayList - Dynamic memory

Generics



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

List<Double> decNums;decNums = new ArrayList<Double>();

ArrayList - Declaration

Note that ArrayLists must contain OBJECTS because of their ability to “point” to other obejcts. So instead of double, we use Double. Instead of int, we use Integer. These are called “wrapper classes.”


ArrayList<Long> bigStuff;bigStuff = new ArrayList<Long>();

List<It> itList;itList = new ArrayList<It>();

ArrayList

We are working towards being able to build our own OBJECT Types to store in ArrayLists. In this example, Stacey built his own “It” class of Objects


List<String> vals;vals = new ArrayList<String>();vals.add("aplus");vals.add("compsci");vals.add("contests");out.println(vals.get(0).charAt(0));out.println(vals.get(2).charAt(0));

ray stores String references.

OUTPUTac

Hands’ On Learning Time! To Do now and for Homework #41) Learn About Array Lists by looking at the Oracle JAVA

doc.2) Work through the white worksheets about ArrayLists3) Start Netflix! Open up a new class in Labs 16_ArrayLists

Answers to ArrayLists

Hobbits: Gandalf, Bilbo, Fili, Thorin, KiliVillains: Smaug, Sauron

false0

Elves: LegolasThranduilArwen

there is no element at index 8Nums: 975313579

975313579nums.removeAll(9)

Answers to Collections

Ages: 11, 12, 13, 16 3

0Dates: 25,4,9

4,9,259,25,44,25,9

Numeros: 0, 0, 022,22,220true

generics.java

ArrayListMethods



ArrayListfrequently used methods

Name Use

add(item) adds item to the end of the list

add(spot,item) adds item at spot – shifts items up->

set(spot,item) put item at spot z[spot]=item

get(spot) returns the item at spot return z[spot]

size() returns the # of items in the list

remove( int spot ) removes item at spot from the list

import java.util.ArrayList;


ArrayList<String> vals;vals = new ArrayList<String>();vals.add("aplus");vals.add("rocks");vals.add(0, "comp");vals.add(1, "sci");out.println(vals);

OUTPUT

[comp, sci, aplus, rocks]

add( )


ArrayList<Integer> nums;nums = new ArrayList<Integer>();nums.add(34);nums.add(0,99);nums.add(21);nums.add(1,7);nums.add(3);nums.add(11);out.println(nums);

OUTPUT

[99, 7, 34, 21, 3, 11]

add( )

addone.javaaddtwo.java


ArrayList<Integer> ray;ray = new ArrayList<Integer>();ray.add(23);ray.add(11);ray.set(1,5);ray.add(4);ray.set(0,7);ray.add(53);out.println(ray);

OUTPUT

[7, 5, 4, 53]

set( )


ArrayList<Integer> ray;ray = new ArrayList<Integer>();ray.add(23);ray.add(11);ray.set(1,5);ray.add(4);ray.set(0,7);ray.add(53);out.println(ray.set(1,93) );

OUTPUT

5

set( )


List<Integer> ray;ray = new ArrayList<Integer>();ray.add(23);ray.add(0, 11);ray.set(5,66);out.println(ray);

OUTPUT

Runtime exception

set( )


ArrayList<Integer> ray;ray = new ArrayList<Integer>();ray.add(23);ray.add(11);ray.add(12);ray.add(65);

out.println(ray.get(0));out.println(ray.get(1));out.println(ray.get(2));

OUTPUT231112

.get(spot) returns the reference stored at spot!

get( )


List<Integer> ray;ray = new ArrayList<Integer>();ray.add(23);ray.add(11);ray.add(12);ray.add(65);

out.println(ray.get(3));out.println(ray.get(4));

OUTPUT65

Runtime exception

.get(spot) returns the reference stored at spot!

get( )

set.javaget.java

ProcessingLists with

Loops



for (int i=0; i<ray.size(); i++){ out.println(ray.get(i)); }

.size( ) returns the number of elements/items/spots/boxes or whatever you want to call them.

Standard For Loop


List<Integer> ray;ray = new ArrayList<Integer>();

ray.add(23);ray.add(11);

for( int i = 0; i < ray.size(); i++){ out.println( ray.get( i ) );}

OUTPUT

2311

Standard For Loop


for ( int item : bunchOfNums ){ out.println( item ); }

The for each loop will access each reference in the list, starting with the first and ending with the last.

For Each Loop


for ( Integer num : primeList ){ out.println( num ); }

The for each loop will access each reference in the list, starting with the first and ending with the last.

For Each Loop



ray.add(23);ray.add(11);ray.add(53);

for(int num : ray){ out.println(num);}

OUTPUT

231153

For Each Loop



ray.add(23);ray.add(11);ray.add(53);

for(Integer num : ray){ out.println(num);}

OUTPUT

231153

For Each Loop

forloopone.javaforeachloopone.java

RemovingItems



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

ray.add("a");ray.add("b");ray.remove(0);ray.add("c");ray.add("d");ray.remove(0);out.println(ray);

OUTPUT

[c, d]

remove( )



ray.add("a");ray.add("b");out.println(ray.remove(0) );

OUTPUT

a

remove( )

removeone.java

BasicJava

Output



spot = size – 1while( spot is greater than

or equal to 0 ){ if ( this item is a match ) remove this item from the list subtract 1 from spot}

removing multiple values


int spot = list.size() – 1;while( spot >= 0 ){ if ( list.get(spot).equals( value ) ) list.remove( spot ); spot = spot – 1;}

removing multiple values

removeall.java



ray.add("a");ray.add("x");ray.clear();ray.add("t");ray.add("w");out.println(ray);

OUTPUT

[t, w]

clear( )

clear.java

CollectionsClass



Collectionsfrequently used methods

Name Use

sort(x) puts all items in x in ascending order

binarySearch(x,y) checks x for the location of y

fill(x,y) fills all spots in x with value y

rotate(x,y) shifts items in x left or right y locations

reverse(x) reverses the order of the items in x

import java.util.Collections;


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

ray.add(23);ray.add(11);ray.add(66);ray.add(53);Collections.sort(ray);out.println(ray);out.println(Collections.binarySearch(ray,677));out.println(Collections.binarySearch(ray,66));

OUTPUT[11, 23, 53, 66]-53

Collections


ArrayList<Integer> ray;ray = ArrayList<Integer>();

ray.add(23);ray.add(11);ray.add(53);out.println(ray);rotate(ray,2);out.println(ray);rotate(ray,2);reverse(ray);out.println(ray);

OUTPUT[23, 11, 53][11, 53, 23][11, 23, 53]

Collections


ArrayList<Integer> ray;ray = new ArrayList<Integer>();ray.add(0);ray.add(0);ray.add(0);out.println(ray);

Collections.fill(ray,33);out.println(ray);

OUTPUT[0, 0, 0][33, 33, 33]

Collections

binarysearch.javarotate.java

fill.java

SearchMethods



ArrayListfrequently used methods

Name Use

contains(x) checks if the list contains x

indexOf(x) checks the list for the location of x


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

ray.add(23);ray.add(11);ray.add(66);ray.add(53);

out.println(ray);out.println(ray.indexOf(21));out.println(ray.indexOf(66));

out.println(ray);out.println(ray.contains(21));out.println(ray.contains(66));

OUTPUT[23, 11, 66, 53]-12[23, 11, 66, 53]falsetrue

search.java


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A+ Computer Science LISTS · Primitive variables carry their values with them like color coded sticky notes int age = 18; Every time I declare a variable, a new sticky note is created