1. CSE 214 – Computer Science I More on Linked Lists

2. Eclipse Video Tutorials
Accessible online via schedule page
Setting up Projects
Debugging
Learn to use the debugger ASAP
I’ll repeat this all day if I have to

3. What’s an Iterator? Common approach to abstracting out iteration
Essential to using data structures in Java & C++
especially c++
&
especially lists
Iterators are more important for lists than for arrays. Why?

4. Accessing elements in a list
Philosophy of some list-managers:
if you need to access a list element by index then you shouldn’t really be using a list
Iterators are typically used instead:
boolean hasNext()           Returns true if the iteration has more elements.  
E next()           Returns the next element in the iteration.  
void remove()           Removes from the underlying collection the last element returned by the iterator (optional operation).

5. Using an Iterator Ex: to print all the contents of a list
Iterator it = linkedList.listIterator();
while(it.hasNext()) {
Object obj = it.next();
System.out.println(obj); }

6. Defining an Iterator - Polymorphism
public Iterator listIterator() { return new ListIterator(); } private class ListIterator implements Iterator { … public boolean hasNext(){ public Object next(){ public void remove(){}

7. All objects have 2 types Apparent Type Actual Type
the declared object variable type
what it appears to be
determines what methods can be called on an object
enforced by the compiler
Actual Type
the constructed variable type
what it actually is in memory
determines which version of a method to use

8. Example The toString method:
in the Object class, it prints the memory address
it’s overridden in the string class, where it prints the text
public static void main(String[] args) {
String s = "Hello";
printObject(s);
Object o = new Object();
printObject(o); }
public static void printObject(Object obj) {
System.out.println("toString output: "
+ obj.toString());
What’s the Output?

9. Why is this important? Look at how we’re using Iterators: For it:
Iterator it = linkedList.listIterator();
while(it.hasNext()) {
Object obj = it.next();
System.out.println(obj); }
For it:
What’s the Apparent type?
What’s the Actual type?
In what class are the next & hasNext methods defined?

10. What type is E? A generic type What does that mean?
it can be anything
Why is this useful?
we can build classes that process many different types of elements (more on this in HW 2)

11. Call-by-reference vs. Call-by-value
NOTE: Java only uses call-by-value
even when objects are passed as parameters
Ex, in C++ we can use either

12. C++ Primitives Example
int main() { int a = 5; int b = 5; changeNums(a, b); cout << a; cout << endl; cout << b; } void changeNums(int x, int &y) x = 0; y = 0;
Output? 5

13. Java Primitives Example
public static void main(String[] args) { int a = 5; int b = 5; changeNums(a, b); System.out.println(a); System.out.println(b); } public static void changeNums(int x, int y) x = 0; y = 0;
Output? 5

14. Java Objects Example Output? AAA DDD
public static void main(String[] args) {
CityNode a = new CityNode("AAA", null);
CityNode b = new CityNode("BBB", null);
changeNodes(a, b);
System.out.println(a.city);
System.out.println(b.city); }
public static void changeNodes(CityNode x, CityNode y)
x = new CityNode("CCC", null);
y.city = "DDD";
NOTE: When you pass an object to a method, you are passing a copy of the object’s address

15. Assigning values to objects vs. instance variables
city:
null
next: x:
value: 3
count:
['A', 'A', 'A']
CityNode a = new CityNode("AAA", null); …
changeNodes(a, b);
void changeNodes(CityNode x, CityNode y) {
x = new CityNode("CCC", null); }
city:
null
next:
value: 3
count:
['C','C','C']

16. Assigning values to objects vs. instance variables
city:
null
null
next: y:
value: 3
count:
['B','B','B'] …
CityNode b = new CityNode("BBB", null);
changeNodes(a, b);
void changeNodes(CityNode x, CityNode y) {
y.city = "DDD"; }
value: 3
count:
['D','D','D']

17. In Java, how do methods make new objects?
Output?
Hello
Hell
They return them
Ex, in the String class:
substring method returns a new String
String s = "Hello";
s.substring(0, 4);
System.out.println(s);
s = s.substring(0, 4);

18. Ex: a replaceAll method
public class CityListManager { private CityNode head = null; void replaceAll(String oldCity, String newCity) } …
CityNode traveller = head;
while (traveller != null) {
if (traveller.city.equals(oldCity))
traveller.city = newCity;
traveller = traveller.next; }

19. DoublyLinkedList
Next lecture:
generic doubly linked lists