1. CS 200 Objects and ArrayList
Jim Williams, PhD

2. This Week Due Thursday Office Hours Team Lab: Multi-Dimensional Arrays
Chap 8 zyLabs, 3 parts
Code Refactor 1: Wumpus Caves
Office Hours
TA may just have a few minutes before rotating.
Team Lab: Multi-Dimensional Arrays
Bring paper and pencil to draw diagrams.
Lecture: Objects and ArrayList
comparison with arrays

3. Programming Paradigms
Procedural paradigm and using objects
Write static/class methods and pass data structures
Use predefined classes: Scanner, String, Integer
Focus of CS 200
Object-Oriented paradigm
Grouping of related data and methods
Interaction of objects
Create your own instantiable classes
Key part of CS 300

4. Object Concept Group related data and methods Scanner class
input and methods to access
String class
series of characters and related methods
Integer class
int value and methods to convert to/from that value

5. Object class java.lang.Object Special class in Java
Every class is a descendent of Object, either directly or indirectly.
e.g., Scanner, String, Integer
Primitives are not descendents of Object
Has methods such as equals() and toString() that are inherited by every class

6. See extends relationship in API

7. Class vs Instance/Object
template: field declarations and method definitions
Object,String,Random,Scanner,Integer
Instance/Object
allocation of memory for a specific one
new String("hello")
new Integer(3)
new Scanner( System.in)

8. ArrayList Is a Data Structure Similar to an array
holds elements of same data type
elements contiguous in memory
index starts at 0
Different from an array
grows automatically
only works with reference types
Essentially, manages an oversize array

9. Inside ArrayList package java.util;
In java.util package
Ultimately, extends from Object
package java.util;
class ArrayList<E> extends AbstractList<E> {
int size;
Object [] elementData;
boolean add(E e) { … }
void add(int index, E e) { … } }
Fields are variables declared in a class but outside methods
Methods access the fields

10. Java Source Code Usually within Java Development Kit (JDK).
C:\Program Files\AdoptOpenJDK\jdk hotspot
Look for src.zip
Since java.util.ArrayList
look in java\util\ for ArrayList.java
API created from source JavaDoc comments

11. TopHat: Path to Scanner
In source code folder, what path would you look to find Scanner.java?

12. Creating an ArrayList Import import java.util.ArrayList; Declaration
ArrayList<RefType> varName;
Allocation/Instantiation
varName = new ArrayList<RefType>();
Any reference type, but not primitive type.

13. Recall Primitive Wrapper Classes
Primitive Data Type
Primitive Wrapper class
int
Integer
double
Double
char
Character
etc.
Boxing: create an instance of wrapper class for primitive value.
Unboxing: get primitive value from instance of wrapper class.
Java Language Specification

14. TopHat: ArrayList Element Type
Which of the following Cannot be used as the datatype of an element in an ArrayList?
String
int
Random
Double
Object

15. Constructor
A special method only called once when initially creating an instance/object.
Called to initialize fields of instance/object, after memory for instance/object is allocated.

16. ArrayList Constructors
Special methods to initialize instances
ArrayList<RefType>() //default capacity of 10
ArrayList<RefType>( int initialCapacity)
If you know how many items will be added then set as the initialCapacity.
Otherwise, when growing the ArrayList has to create a new array, guessing at length, and copy elements over.

17. Declare and Allocate ArrayLists
Separate declaration from instantiation
ArrayList<String> names;
names = new ArrayList<String>(100);
ArrayList<Double> nums = new ArrayList<>(5);
ArrayList<int> nums2; //won't compile
Initial capacity
<> is abbreviation since in declaration statement
Use primitive wrapper class, Integer.

18. Memory: Internal View in main method ArrayList<Integer> nums;
nums = new ArrayList<Integer>(4);
Initial capacity
items added
size
elementData
null
nums
On heap, size and elementData are together
On stack, in main stack frame

19. Draw an Internal Diagram
What does internal view of memory look like?
ArrayList<Integer> nums;
nums = new ArrayList<Integer>(1);
nums.add( 5 ); nums.add( 4 );

20. Memory: Internal View in main method ArrayList<Integer> nums;
nums = new ArrayList<Integer>(1);
nums.add( 5 ); nums.add( 4 );
Grows automatically
size 2 5
elementData 4
nums

21. Memory: External View
Internally an array of size 20 is already allocated in anticipation of growth.
in main method
ArrayList<Integer> nums;
nums = new ArrayList<Integer>(20);
nums.add( 5 ); nums.add( 4 ); 5 4
nums

22. TopHat: How much memory?
ArrayList<Double> nums;
for reference
for reference to and instance of ArrayList
for reference, instance of ArrayList, and items in ArrayList
error
Just enough for a reference, there is not an instance of ArrayList allocated and there aren't any items allocated.

23. TopHat: How much memory?
ArrayList<Double> nums;
nums = new ArrayList<Double>(5);
for reference
reference and instance of ArrayList
reference, instance of ArrayList, and 5 items in ArrayList
error

24. TopHat: How much memory?
ArrayList<Double> nums;
nums = new ArrayList<Double>();
nums.add( 12.3 );
for reference
reference and instance of ArrayList
reference, instance of ArrayList, and 1 Double
error

25. Appending to an ArrayList
Initial capacity
ArrayList<Double> nums;
nums = new ArrayList<Double>(3);
nums.add(1.0);
nums.add(new Double(2.0));
nums.add(3.0);
nums.add(4.0);
System.out.print(nums.size());
Grows automatically
size is number of items in ArrayList, Not capacity
import java.util.ArrayList;
public class ClassNameHere {
public static void main(String[] args) {
ArrayList<Double> nums;
nums = new ArrayList<Double>(3);
nums.add(1.0);
nums.add(new Double(2.0));
nums.add(3.0);
nums.add(4.0);
System.out.print(nums.size()); }

26. TopHat: What will print out?
ArrayList<Double> nums;
nums = new ArrayList<Double>(2);
nums.add(1.0);
nums.add(new Double(2.0));
nums.add(4.0);
System.out.print(nums.size());

27. Create an ArrayList from an Array
Character [] charArray = {'a','b','c'};
ArrayList<Character> charList;
charList= new ArrayList<Character>();
chars.addAll(
java.util.Arrays.asList( charArray ));
Returns a List that is then added to ArrayList.

28. Printing an ArrayList ArrayList<Character> chars;
Populate an ArrayList from an array
ArrayList<Character> chars;
chars = new ArrayList<Character>();
chars.addAll( java.util.Arrays.asList(
new Character[]{'a','b','c'}));
System.out.print( chars );
System.out.print( chars.toString() );
Both prints have same result, since print(Object) calls the toString method

29. Inserting into an ArrayList
Default initial capacity is 10
ArrayList<Character> chars;
nums = new ArrayList<Character>();
nums.add('b');
nums.add(0, 'a');
nums.add(0, 'c');
System.out.print(nums.size());
System.out.print(nums);
at index, insert a new element

30. TopHat: What will print out?
ArrayList<Integer> list;
list = new ArrayList<Integer>();
list.add( 2);
list.add(0,3);
list.add(1,4);
System.out.println( list);

31. Setting an Element in an ArrayList
ArrayList<Character> chars;
nums = new ArrayList<Character>(3);
nums.add('b');
nums.set(0, 'a');
nums.set(0, 'c');
System.out.print(nums.size());
System.out.print(nums);
At index replace element

32. Removing an Element from ArrayList
ArrayList<Character> chars;
nums = new ArrayList<Character>(3);
nums.add('a'); nums.add('b'); nums.add('c');
nums.remove('a');
nums.remove( 1 );
System.out.print(nums.size());
System.out.print(nums);
Removing by element or index
Both shift elements down

33. TopHat: What will print out?
ArrayList<Integer> list;
list = new ArrayList<Integer>(8);
list.add(0,3);
list.set(0,5);
list.add(0,4);
list.remove( 1);
System.out.println( list);

34. (actual number of items)
Capacity vs Size
Capacity
(places available)
Size
(actual number of items)
array
.length attribute
(for perfect size array this is also the number of items)
separate variable of count of the items in array (e.g., for oversize array)
ArrayList
automatically increases when necessary
.size() method

35. TopHat: What are size & capacity?
ArrayList<Character> chars;
chars = new ArrayList<Character>();
chars.addAll( java.util.Arrays.asList(
new Character[]{'a','b','c'}));
size: 3
capacity: 10
size: 10
capacity: 3
size is the number of elements in the list from a user point of view
capacity is the number of places in the internal array within the ArrayList

36. TopHat: Which is fastest?
void methodA( ArrayList<Integer> nums ) {
nums.add( 0, 1); }
void methodB( ArrayList<Integer> nums ) {
nums.add( 1);
methodB since it adds it to the end.
methodA would have to move every element down one before inserting at the beginning

37. Other Common ArrayList Methods
get(int index) //return element at index
clear() //remove all elements
isEmpty() //whether it contains any elements

38. For Loop static void printList(ArrayList<Character> list) {
for ( int i = 0; i < list.size(); ++i) {
System.out.println( list.get( i )); }
import java.util.ArrayList;
public class ClassNameHere {
static void printList(ArrayList<Character> list) {
for ( char ch : list )
System.out.println( ch ); }
public static void main(String[] args) {
ArrayList<Character> chars = new ArrayList<>();
chars.addAll( java.util.Arrays.asList( new Character[]{'a','b','c'}));
printList( chars );

39. Enhanced For Loop Readable code to iterate through all items
Avoid out-of-bounds errors
Doesn't have position information
static void printList(ArrayList<Character> list) {
for ( char ch : list )
System.out.println( ch ); }
import java.util.ArrayList;
public class ClassNameHere {
static void printList(ArrayList<Character> list) {
for ( char ch : list )
System.out.println( ch ); }
public static void main(String[] args) {
ArrayList<Character> chars = new ArrayList<>();
chars.addAll( java.util.Arrays.asList( new Character[]{'a','b','c'}));
printList( chars );

40. Array vs ArrayList Array ArrayList Fixed size Yes No Grows dynamically
Primitive types
Reference types
Constant access time
Arrays have a simpler syntax.

41. Is an Array or ArrayList better?
For a fixed size data structure such as a cave or game board?
For reading in an unknown number of values from a user or a file?
Array for a fixed size

42. Today Hours Spent in Last Week Due Chap 8 zyLabs
Code Refactor 1: Wumpus Caves

43. Multidimensional ArrayLists
ArrayLists of ArrayLists conceptually similar to arrays of arrays
ArrayList<ArrayList<Integer>> table;
table = new ArrayList<ArrayList<Integer>>();
table.add( new ArrayList<Integer>());

44. Draw Multidimensional ArrayLists
ArrayList<ArrayList<Integer>> table;
table = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> row = new ArrayList<>();
row.add(4); row.add(5);
table.add( row );
row.add(6);

45. Draw a Diagram ArrayList<int[]> nums = new ArrayList<>();
nums.add(new int[] {1, 2});
nums.add(new int[] {4, 5, 6});
nums.add(new int[] {7, 8, 9});
nums.add(nums.get(1));
nums.remove(1);
nums.get(1)[0] = 3;
import java.util.ArrayList;
public class Week9 {
public static void main(String[] args) {
ArrayList<int[]> nums = new ArrayList<>();
nums.add(new int[] {1, 2});
nums.add(new int[] {4, 5, 6});
nums.add(new int[] {7, 8, 9});
nums.add(nums.get(1));
nums.remove(1);
nums.get(1)[0] = 3;
for (int[] arr : nums) {
System.out.println(java.util.Arrays.toString(arr)); }

46. Draw a Diagram & TopHat [[],[],[]]
ArrayList<ArrayList<String>> list = new ArrayList<>();
list.add( new ArrayList<String>());
list.add(1, new ArrayList<String>());
list.get(1).add("D");
list.set(0, list.get(1));
list.get(2).clear();
System.out.println( list);
[[],[],[]]
[[], [D], []]
[[D],[D],[]]
error or other
try it.

47. TopHat: What is printed out?
ArrayList<Integer> list4;
list4 = new ArrayList<Integer>(100);
for ( int i = 0; i < 1000; i++) {
list4.add( i); }
System.out.println( list4.size());

48. TopHat: Write the code.
With the goal of minimizing the running time of your program, how would you declare and instantiate an ArrayList, called temps, to store a sequence of at least 100,000 temperature measurements?

49. TopHat: What size and elements?
ArrayList<String> list;
list = new ArrayList<String>();
list.add("A");
list.add(0,"B");
list.add("C");
list.set(2,"D");
list.add("E");
System.out.println( list); 5
[A,B,C,D,E] 4
[B, A, D, E] 3
[B,D,E]
error or other

50. Common Algorithms
Searching
Sorting
Palindrome?

51. Linear Search //Returns the index of where the element x was
//found or -1 if not found.
public static int linearSearch(
ArrayList<Integer> list, int x) { }
/**
* Returns the index of where the element x was found
* or -1 if not found.
arr x */
public static int linearSearch( int arr[], int x) {
for ( int i = 0; i < arr.length; i++) {
if ( arr[i] == x) {
return i; }
return -1;

52. Binary Search // Return index of where x is in list if found,
// otherwise returns -1;
public static int binarySearch(ArrayList<Integer> list, int x) { } /*
* Return index of where x is in array if found, otherwise
* returns -1; */
public static int binarySearch(int arr[], int x) {
int left = 0, right = arr.length - 1;
while (left <= right) {
//determine mid point
int mid = left + (right-left)/2;
//check mid point
if (arr[mid] == x)
return mid;
//if x is greater than mid point then change left
if (arr[mid] < x)
left = mid + 1;
//if x is less than mid point then change right
else
right = mid - 1; }
//not found, return -1
return -1;

53. Sorting Demo 9, 6, 4, 5, 8, 2 import java.util.Arrays;
public class BubbleSort {
public static void swapInts(int a, int b) {
int temp = a;
a = b;
b = temp; }
public static void main(String[] args) {
int x = 1;
int y = 2;
System.out.println("x = " + x + ", y = " + y);
swapInts(x, y);
int[] list = new int[]{10, 5, 3, 7, 9};
printArray(list);
swapInts(list[0], list[4]);
swapElements(list, 0, 4);
bubbleSort(list);
addElementAtEnd(list, 12);
list = addElementAtEnd(list, 4);
public static void printArray( int [] arr) {
System.out.println( Arrays.toString( arr));
public static void swapElements(int[] arr, int i, int j) {
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
public static void bubbleSort(int[] arr) {
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr.length -1; j++) {
if (arr[j] > arr[j+1]) {
swapElements(arr, j, j+1);
public static int[] addElementAtEnd( int[] arr, int elem) {
int newLength = arr.length + 1;
int[] newArray = new int[newLength];
newArray[i] = arr[i];
newArray[newArray.length - 1] = elem;
return newArray;