1. EKT472: Object Oriented Programming

2. 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

3. Generics
In Java versions 1.4 and earlier, an ArrayList just held Objects, which could be of any (non-primitive) type
For compatibility reasons you can still do this, but you will get a lot of warning messages
In Java 5 you should specify the class of objects that the ArrayList will hold
This is done with the new generics syntax

4. Creating a ArrayList the old way
The syntax for creating ArrayLists has changed between Java 1.4 and Java 5
For compatibility reasons, the old way still works, but will give you warning messages
Here are the (old) constructors:
import java.util.ArrayList;
ArrayList vec1 = new ArrayList();
Constructs an ArrayList with an initial capacity of 10
ArrayList vec2 = new ArrayList(initialCapacity);

5. Using an ArrayList the old way
boolean add(Object obj)
Appends the object obj to the end of this ArrayList
Example: myArrayList.add("A string is an object");
Object get(int index)
Returns the component at position index
Note that this is returned as an Object; to use it as a String, you must cast it
Example: String s = (String)myArrayList.get(5);

6. 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<?>();

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. Iterators ArrayList is part of the Java Collections Framework
Collection is an interface that specifies the basic operations every collection (data structure) should have
Some Collections don’t have a definite order
Sets, Maps, Graphs
How to access all the items in a Collection with no specified order?

14. Iterator Interface An iterator object is a “one shot” object
it is designed to go through all the elements of a Collection once
if you want to go through the elements of a Collection again you have to get another iterator object
Iterators are obtained by calling a method from the Collection

15. Iterator Iterface Methods
The Iterator interface specifies 3 methods:
boolean hasNext()
//returns true if this iteration has more elements
E next()
//returns the next element in this iteration
//pre: hastNext()
void remove()
/*Removes from the underlying collection the last element returned by the iterator.
pre: This method can be called only once per call to next. After calling, must call next again before calling remove again. */

16. Clicker Question 1 ArrayList<String> list
Which of the following produces a syntax error?
ArrayList<String> list
= new ArrayList<String>();
Iterator<String> it1 = new Iterator(); // I
Iterator<String> it2 = new Iterator(list); // II
Iterator<String> it3 = list.iterator(); // III
A. I
B. II
C. III
D. I and II
E. II and III

17. Iterator rails fenceposts
Imagine a fence made up of fence posts and rail sections
fenceposts

18. Fence Analogy iterator object The iterator lives on the fence posts
The data in the collection are the rails
Iterator created at the far left post
As long as a rail exists to the right of the Iterator, hasNext() is true
iterator object

19. Fence Analogy ArrayList<String> names =
new ArrayList<String>();
names.add(“Jan”);
names.add(“Levi”);
names.add(“Tom”);
names.add(“Jose”);
Iterator<String> it = names.iterator();
int i = 0;
“Jan” “Levi” “Tom” “Jose”

20. Fence Analogy
while( it.hasNext() ) {
i++;
System.out.println( it.next() ); }
// when i == 1, prints out Jan
first call to next moves iterator to next post and returns “Jan”
“Jan” “Levi” “Tom” “Jose”

21. Fence Analogy “Jan” “Levi” “Tom” “Jose” while( it.hasNext() ) { i++;
System.out.println( it.next() ); }
// when i == 2, prints out Levi
“Jan” “Levi” “Tom” “Jose”

22. Fence Analogy “Jan” “Levi” “Tom” “Jose” while( it.hasNext() ) { i++;
System.out.println( it.next() ); }
// when i == 3, prints out Tom
“Jan” “Levi” “Tom” “Jose”

23. Fence Analogy “Jan” “Levi” “Tom” “Jose” while( it.hasNext() ) { i++;
System.out.println( it.next() ); }
// when i == 4, prints out Jose
“Jan” “Levi” “Tom” “Jose”

24. Fence Analogy “Jan” “Levi” “Tom” “Jose” while( it.hasNext() ) { i++;
System.out.println( it.next() ); }
// call to hasNext returns false
// while loop stops
“Jan” “Levi” “Tom” “Jose”

25. Typical Iterator Pattern
public void printAll(Collection<String> list) {
Iterator<String> it = list.iterator(); while( it.hasNext() ) {
String temp = it.next();
System.out.println( temp ); }

26. Clicker Question 2 ArrayList<Integer> list;
What is output by the following code?
ArrayList<Integer> list;
list = new ArrayList<Integer>();
list.add(3);
list.add(5);
Iterator<Integer> it = list.iterator();
System.out.print(it.next() + " ");
System.out.print(it.next());
3 B C
D E. 3 3 then a runtime error

27. remove method
AnIterator can be used to remove things from the Collection
Can only be called once per call to next()
public void removeWordsOfLength(int len) { Iterator<String> it = myList.iterator
while( it.hasNext() ) { String temp = it.next();
if(temp.length() == len) it.remove(); }
// original list = [“dog”, “cat”, “hat”, “sat”]
// resulting list after removeWordsOfLength(3) ?

28. Clicker Question 3 No output due to syntax error
public void printTarget(ArrayList<String> names,
int len) {
Iterator<String> it = names.iterator();
while( it.hasNext() )
if( it.next().length() == len )
System.out.println( it.next() ); }
Given names = [“Jan”, “Ivan”, “Tom”, “George”] and len = 3 what is output by the printTarget method?
Jan Ivan Tom George
Jan Tom
Ivan George
No output due to syntax error
No output due to runtime error