1. ArrayLists
22-Feb-19

2. Arrays in Java
Java has built in arrays as well as more complicated classes to automate many array tasks (the ArrayList class)
arrays hold elements of the same type
primitive data types or classes
space for array must be dynamically allocated with new operator. (Size is any integer expression. Due to dynamic allocation does not have to be a constant.)
AP Computer Science

3. public void arrayExamples()
{ int[] intList = new int[10];
for(int i = 0; i < intList.length; i++)
{ assert 0 >= i && i < intList.length;
intList[i] = i * i * i; }
intList[3] = intList[4] * intList[3];

4. Array Details all arrays must be dynamically allocated
arrays have a public, final field called length
built in size field, no separate variable needed
don't confuse length (capacity) with elements in use
elements start with an index of zero, last index is length - 1
AP Computer Science

5. Array Initialization Array variables are object variables
They hold the memory address of an array object
The array must be dynamically allocated
All values in the array are initialized (0, 0.0, char 0, false, or null)
Arrays of primitives and Strings may be initialized with an initializer list:
int[] intList = {2, 3, 5, 7, 11, 13};
double[] dList = {12.12, 0.12, 45.3};
String[] sList = {"Olivia", "Kelly", "Isabelle"};
AP Computer Science

6. Arrays of objects
A native array of objects is actually a native array of object variables
all object variables in Java are really what?
Pointers!
public void objectArrayExamples()
{ Rectangle[] rectList = new Rectangle[10];
// How many Rectangle objects exist?
rectList[5].setSize(5,10);
//uh oh!
for(int i = 0; i < rectList.length; i++)
{ rectList[i] = new Rectangle(); }
rectList[3].setSize(100,200);
AP Computer Science

7. ArrayLists and arrays A ArrayList is like an array of Objects
Differences between arrays and ArrayLists:
Arrays have special convenience syntax; ArrayLists don’t
An array is a fixed size, but a ArrayList expands as you add things to it
This means you don’t need to know the size beforehand
Arrays can hold primitives or objects, but ArrayLists can only hold objects
However, autoboxing can make it appear that an ArrayList can hold primitives

8. Arrays in Java
The ArrayList Class
A class that is part of the Java Standard Library and a class that is part of the AP subset
a kind of automated array
not all methods are part of the ap subset
AP Computer Science

9. About Lists (in general)
Arrays in Java
About Lists (in general)
A list is an ordered collection or a sequence.
ArrayList implements the List interface
The user of this interface will have control over where in the list each element is inserted.
The user can access elements by their integer index (position in the list), and search for elements in the list.
Items can be added, removed, and accessed from the list
AP Computer Science

10. Methods ArrayList() //constructor void add(int index, Object x)
Arrays in Java
Methods
ArrayList() //constructor
void add(int index, Object x)
boolean add(Object x)
Object set(int index, Object x)              
Object remove(int index)
int size ()
Object get(int index)  
Iterator iterator()                 
AP Computer Science

11. How the methods work add: Arrays in Java
boolean add(Object x) – inserts the Object x at the end of the list (size increases by 1), returns true
void add(int index, Object x) – inserts the Object x at the given index position (elements will be shifted to make room and size increases by 1)
AP Computer Science

12. How the methods work get: Arrays in Java
returns the Object at the specified index
should cast when using value returned
throws IndexOutOfBoundsException if index<0 or index>=size
AP Computer Science

13. How the methods work set Arrays in Java
replaces value of Object parameter at the given index
size is not changed
AP Computer Science

14. How the methods work remove Arrays in Java
removes the element at the specified index
throws IndexOutOfBoundsException if index<0 or index>=size
size will be decreased by 1
returns Object removed
AP Computer Science

15. Creating a ArrayList the new way
Specify, in angle brackets after the name, the type of object that the class will hold
Examples:
ArrayList<String> vec1 = new ArrayList<String>();
ArrayList<String> vec2 = new ArrayList<String>(10);
To get the old behavior, but without the warning messages, use the <?> wildcard
Example: ArrayList<?> vec1 = new ArrayList<?>();

16. Adding elements to a ArrayList
boolean add(Object obj)
Appends the object obj to the end of this ArrayList
With generics, the obj must be of the correct type, or you get a compile-time (syntax) error
Always returns true
This is for consistency with other, similar classes
void add(int index, Object element)
Inserts the element at position index in this ArrayList
The index must be greater than or equal to zero and less than or equal to the number of elements in the ArrayList

17. Removing elements boolean remove(Object obj) void remove(int index)
Removes the first occurrence of obj from this ArrayList
Returns true if an element was removed
Uses equals to test if it has found the correct element
void remove(int index)
Removes the element at position index from this ArrayList
void clear()
Removes all elements

18. Accessing with and without generics
Object get(int index)
Returns the component at position index
Using get the old way:
ArrayList myList = new ArrayList(); myList.add("Some string"); String s = (String)myList.get(0);
Using get the new way:
ArrayList<String> myList = new ArrayList<String>(); myList.add("Some string"); String s = myList.get(0);
Notice that casting is no longer necessary when we retrieve an element from a “genericized” ArrayList

19. Searching a ArrayList boolean contains(Object element)
Tests if element is a component of this ArrayList
Uses equals to test if it has found the correct element
int indexOf(Object element)
Returns the index of the first occurrence of element in this ArrayList
Returns -1 if element was not found in this ArrayList
int lastIndexOf(Object element)
Returns the index of the last occurrence of element in this ArrayList

20. Getting information boolean isEmpty() int size() Object[ ] toArray()
Returns true if this ArrayList has no elements
int size()
Returns the number of elements currently in this ArrayList
Object[ ] toArray()
Returns an array containing all the elements of this ArrayList in the correct order

21. More about equals There are many different notions of equality
Example: two sets are equal if they contain the same elements; order of elements is irrelevant
Java defines public boolean equals(Object) in the Object class, but
equals is defined to be the same as ==
It’s often a good idea to override equals for your own objects
If you do this, note that the argument should be a general Object
The String class (and some others) override equals

22. Conclusion A ArrayList is like an array of Objects
The advantage of a ArrayList is that you don’t need to know beforehand how big to make it
The disadvantage of a ArrayList is that you can’t use the special syntax for arrays
You should never use an array that you hope is “big enough”—use a ArrayList instead