1. CS Week 9
Jim Williams, PhD

2. This Week BP1: Milestone 2 due this week
Scanner examples labs 3, 4, 6, and 8
Team Lab: Multi-Dimensional Arrays
Bring paper and pencil to draw diagrams.
Lecture: Objects and ArrayList
comparison with arrays

3. What happens when
array grows automatically
array index out of bounds
error
who knows?
You try to add more elements than will fit into an array?
B and technically C since B is an error. (try it)

4. What happens when int [] list = new int[]{1,4,5}; list[1] = 2;
list array now contains 1,2,5
list array now contains 1,2,4,5
Error
int [] list = new int[]{1,4,5};
list[1] = 2;
A is the best answer.

5. How do we insert 2 into the array?
int [] list = new int[]{1,4,5};
list[1] = 2; //?

6. 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

7. Is result true or false? true Integer m = 5; Integer n = 5; false
boolean result = m == n;
true
false
error
A is correct. The fine print at the provided link says for Integer instances based on int literals between and 127 will have the same reference for the same int literal value. The intention of this example is to emphasize experimenting, discovering short illustrative examples and an introduction to the Java Language Specification.
Part of relevant details:
If the value p being boxed is an integer literal of type int between -128 and 127 inclusive (§3.10.1), or the boolean literal true or false (§3.10.3), or a character literal between '\u0000' and '\u007f' inclusive (§3.10.4), then let a and b be the results of any two boxing conversions of p. It is always the case that a == b.
Ideally, boxing a primitive value would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rule above is a pragmatic compromise, requiring that certain common values always be boxed into indistinguishable objects.
Java Language Specification

8. Printing out, toString method
Integer [] list2 = {1,2,3};
System.out.println( list2[1] );
System.out.println( list2 );
//java.util.Arrays.toString( list2)

9. Object Class
java.lang.Object
has methods such as toString()

10. Arrays vs ArrayList Arrays fixed length, contiguous in memory
automatically grows as elements are added by allocating new array and copying.

11. Java Source Code Usually within Java Development Kit (JDK).
On Windows typically found under:
C:\Program Files\Java
Look for src.zip

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

13. Lots of Elements ArrayList<Integer> list4;
list4 = new ArrayList<Integer>();
for ( int i = 0; i < 1000; i++) {
list4.add( i); }
System.out.println( list4);

14. Adding an array to ArrayList
ArrayList<Integer> list = new ArrayList<Integer>();
list.add(10);
Integer [] arr = new Integer[]{1,3,4,5};
list.addAll( java.util.Arrays.asList( arr));
System.out.println( list);

15. What happens when
you have to make sure there is enough room and move all the elements down and then insert it.
Insertion is handled automatically
You want to insert an element in an ArrayList at a specific index?
B is the best answer

16. How many elements in an Array vs ArrayList?
static void methodA(int [] arr) {
//how to find capacity of array? }
static void methodB(ArrayList list) {
//how to find the number of
//elements in list?
arr.length
list.size()
list.length()
arr.size
correct: A (try it)

17. 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

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

19. Write a method to print out array
public static void printArray(int [] arr) {
//use enhanced for loop }
public static void printArray(int [] arr) {
System.out.print("\n[");
boolean first = true;
for ( int num : arr) {
if ( first) {
first = false;
} else {
System.out.print(", "); }
System.out.print( num);
System.out.println("]");

20. 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;

21. 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;

22. Draw Picture
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

23. Sorting public static void bubbleSort(
ArrayList<Integer> list) { }
public static void bubbleSort(ArrayList<Integer> list) {
int temp = 0;
for (int i = 0; i < list.size(); i++) {
for (int j = 1; j < (list.size() - i); j++) {
if (list.get(j - 1) > list.get(j)) {
// swap
temp = list.get(j - 1);
list.set(j - 1, list.get(j));
list.set(j, temp); }