1. CS 200 Objects and ArrayList
Jim Williams, PhD

2. This Week Consulting Hours BP1: Milestone 2 due this week
TA may just have a few minutes before moving on.
BP1: Milestone 2 due this week
Team Lab: Multi-Dimensional Arrays
Bring paper and pencil to draw diagrams.
Style and Commenting Standards
Lecture: Objects and ArrayList
comparison with arrays

3. Object Concept
A way to group together related variables and methods.

4. Object class java.lang.Object
Every class is a descendent of Object, either directly or indirectly.
Primitives are not descendents of Object.
has methods such as toString() that are inherited by every class.

5. class vs instance/object
Object,String,Random,Scanner,Integer
instance/object instantiated with:
new String("hello")
new Integer(3)
new Random()
new Scanner( System.in)

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

7. Recall Wrapper Classes
Primitive Data Type
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

8. Printing out, toString method
Integer [] list = {1,2,3};
System.out.println( list[1] );
System.out.println( list );
//java.util.Arrays.toString( list)

9. Motivation for ArrayList
Add 7 to array
int [] list = new int[]{1,4,5};
list[3] = 7; //?error
Write a method
public static int [] addElement(int [] list, int elem) {
//? }
public static int [] addElementAtEnd(int [] list, int elem) {
int newLength = list.length + 1;
int [] newList = new int[newLength];
for ( int i = 0; i < list.length; i++) {
newList[i] = list[i]; }
newList[newList.length -1] = elem;
return newList;

10. Arrays vs ArrayList Arrays fixed length, constant access time
contents any type (primitive or reference)
ArrayList
contents only Reference types
automatically grows as elements are added by allocating a new array and copying.

11. Creating an ArrayList Declaration
The variable now exists and is for holding a reference to an ArrayList for reference type.
ArrayList< ReferenceType > varName
Instantiation
allocates memory and calls a constructor
new ArrayList< ReferenceType >()
ArrayList<String> list = new ArrayList<String>();

12. Question
String
int
Random
Scanner
Object
Which of the following Cannot be used as the datatype of an element in an ArrayList?

13. Java Source Code Usually within Java Development Kit (JDK).
On Windows typically found under:
C:\Program Files\Java
Look for src.zip
Since java.util.ArrayList
look in java\util\ for ArrayList.java
API created from source JavaDoc comments

14. Question
What path in source code folder would you look to find Scanner.java?

15. ArrayList Fields (variables in each instance) int size
Object [] elementData
Constructors
ArrayList() //default capacity of 10
ArrayList( int initialCapacity)

16. ArrayList Diagram ArrayList<Integer> list;
list = new ArrayList<Integer>();
list.add( 2);
list.add(0,3);
System.out.println( list);

17. Today
Hours Spent in Last Week

18. Draw a Diagram ArrayList<Integer> list;
list = new ArrayList<Integer>(20);
list.add(0,3);
list.add(5);
list.add(0,4);
list.remove( 1);
System.out.println( list);

19. How many elements? ArrayList<Integer> list4;
list4 = new ArrayList<Integer>(100);
for ( int i = 0; i < 1000; i++) {
list4.add( i); }
System.out.println( list4.size());
1000
100
error
(after 100 added)

20. Capacity vs Size Capacity Size array .length attribute
programmer keeps separate count of elements inserted in array
ArrayList
automatically changes as needed
.size() method

21. How to retrieve the number of elements added to an Array vs ArrayList?
A arr.length
B list.size()
B list.length()
A arr.size
Other
static int numElements(int [] arr) {
return A; }
static int numElements(ArrayList list) {
return B;
correct: A (try it)

22. Question
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?

23. What size and elements? 5 [A,B,C,D,E] 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.size());
System.out.println( list); 5
[A,B,C,D,E] 4
[B, A, D, E] 3
[B,D,E]
error or other

24. Enhanced For Loop
ArrayList<String> names = new ArrayList<>();
names.add("spot");
names.add("fido");
for ( String name : names) {
System.out.println( name); }

25. 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)); }

26. Draw a Diagram [[],[],[]]
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.

27. Common Algorithms
Searching
Sorting
Palindrome?

28. 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;

29. 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;

30. 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;