Grouping objects: Collections and loops

Object-oriented programming with Java
Ahmed Al-Ajeli
Grouping objects
Collections and loops
Ahmed Al-Ajeli
Lecture 8
2
Main concepts to be covered
•Collections and iteration
•Loops: the for-each loop
•Searching collections
•Identity versus equality
Object-oriented programming with Java
Ahmed Al-Ajeli
3
Collections and iteration
•With a collection, we often want to repeat the actions: exactly once for every object in the collection.
•Java has several sorts of loop statement to deal with collections.
–We will start with its for-each loop.
4
For-each loop pseudo code
for(ElementType element : collection) {
loop body
}
for each element in collection do: {
loop body
}
loop header
for keyword
Action(s) to be repeated
Pseudo-code expression of the operation of a for-each loop
General form of the for-each loop
loop variable
Object-oriented programming with Java
Ahmed Al-Ajeli
5
A Java example
// List all file names in the organizer.
public void listAllFiles()
{
for(String filename : files) {
System.out.println(filename);
}
}
Using each filename in files in order, print filename
NB: if the length is zero then loop is never executed.
6
Selective processing
•Statements can be nested, giving greater selectivity to the actions:
public void listMatching (String searchString)
{
for(String filename : files) {
if(filename.contains(searchString)) { System.out.println(filename); }
}
}
contains gives a partial match of the filename;
contains method of the String class returns boolean type.
Object-oriented programming with Java
Ahmed Al-Ajeli
7
Critique of for-each
•Easy to write.
•Termination happens naturally.
•The collection cannot be changed by the actions.
•There is no index provided.
–Not all collections are index-based.
•We can’t stop part way through;
–e.g., if we only want to find the first match.
•It provides ‘definite iteration’ – aka ‘bounded iteration’.
8
The while loop
•A for-each loop repeats the loop body for every object in a collection.
–Sometimes we require more flexibility than this.
–The while loop supports flexibility.
•We use a boolean condition to decide whether or not to keep iterating.
•This is a very flexible approach.
•Not tied to collections.
Object-oriented programming with Java
Ahmed Al-Ajeli
9
For-each loop equivalent
// List all file names in the organizer.
public void listAllFiles()
{
int index = 0;
while(index < files.size()) { String filename = files.get(index);
System.out.println(filename);
index++;
}
}
Increment index by 1
while the value of index is less than the size of the collection, get and print the next file name, and then increment index
10
Search tasks are indefinite
•Consider: searching for your keys.
•You cannot predict, in advance, how many places you will have to look.
•Although, there may well be an absolute limit – i.e., checking every possible location.
•You will stop when you find them.
•‘Infinite loops’ are also possible.
–Through error or the nature of the task.
Object-oriented programming with Java
Ahmed Al-Ajeli
11
Looking for your keys
while(the keys are missing) {
look in the next place;
}
Or:
while(not (the keys have been found)) {
look in the next place;
}
12
Looking for your keys
boolean searching = true;
while(searching) {
if(the keys are in the next place){
searching = false;
}
}
?The other way around:
boolean found = false;
while(! found) {
if(they are in the next place) {
found = true;
}
}
Suppose we don’t find them?
Object-oriented programming with Java
Ahmed Al-Ajeli
13
for-each versus while
•for-each:
–easier to write.
–safer: it is guaranteed to stop.
•while:
–we don’t have to process the whole collection.
–doesn’t even have to be used with a collection.
–take care: could create an infinite loop.
14
Searching
•A fundamental activity.
•Applicable beyond collections.
•Necessarily indefinite.
•We must code for both success and failure – nowhere else to look.
•Both must make the loop’s condition false, in order to stop the iteration.
•A collection might be empty to start with.
Object-oriented programming with Java
Ahmed Al-Ajeli
15
Finishing a search
•How do we finish a search?
•Either there are no more items to check: index >= files.size()
•Or the item has been found: found == true found ! searching
16
Continuing a search
•We need to state the condition for continuing:
•So the loop’s condition will be the opposite of that for finishing: index < files.size() && ! found index < files.size() && searching
•NB: ‘or’ becomes ‘and’ when inverting everything.
Object-oriented programming with Java
Ahmed Al-Ajeli
17
Searching a collection
public int findFirst(String searchString)
{
int index = 0;
boolean searching = true;
while(index < files.size() && searching) {
String filename = files.get(index);
if(filename.equals(searchString)) {
searching = false;
}
else {
index++;
}
}
if(searching) { // We didn t find it.
return -1;
}
else { // Return where it was found.
return index;
}
}
18
Indefinite iteration
•Does the search still work if the collection is empty?
•Yes! The loop’s body won’t be entered in that case.
•Important feature of while:
–The body can be executed zero or more times.
Object-oriented programming with Java
Ahmed Al-Ajeli
19
Side note: The String class
•The String class is defined in the java.lang package.
•It has some special features that need a little care.
•In particular, comparison of String objects can be tricky.
20
Side note: The problem
•The compiler merges identical String literals in the program code.
–The result is reference equality for apparently distinct String objects.
•But this cannot be done for identical String objects that arise outside the program’s code;
–e.g., from user input.
Object-oriented programming with Java
Ahmed Al-Ajeli
21
Side note: String equality
if(input == "bye") {
...
}
if(input.equals("bye")) {
...
}
Important: Always use .equals for testing String equality!
tests identity
tests equality
Do not use!!
22
Identity vs equality (Strings)
"bye"
:String
input
"bye"
:String
Scanner scanner = new Scanner (…);
String input = scanner.nextLine();
if(input == "bye") {
...
}
==
?
false!
== tests identity
Object-oriented programming with Java
Ahmed Al-Ajeli
23
Identity vs equality (Strings)
"bye"
:String
input
"bye"
:String
Scanner scanner = new Scanner (…);
String input = scanner.nextLine();
if(input.equals("bye")) {
...
}
equals
?
true!
equals tests equality