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Section 4
Boxing classesBoxing classes
Array initializationArray initialization
Lists: recursion and iterationLists: recursion and iteration
Boxing classes
Many library methods have Objects as Many library methods have Objects as parameters, i.e.parameters, i.e.
add(Object o)add(Object o)
However, primitive types, like int, However, primitive types, like int, double, float etc. are not objects, not double, float etc. are not objects, not to mention Objects.to mention Objects.
Solution: Boxing/wrapper classesSolution: Boxing/wrapper classes
Note: Cornell’s boxing classes: Note: Cornell’s boxing classes: http://www.pe.cornell.edu/physed/martial-http://www.pe.cornell.edu/physed/martial-ff03.html 03.html
Wrapper classes
For each primitive type there is For each primitive type there is a corresponding wrapper class:a corresponding wrapper class:
Primitive typePrimitive type Wrapper classWrapper class
intint IntegerInteger
floatfloat FloatFloat
charchar CharacterCharacter
...... ......
Wrapper classes
Usage:Usage:
int k = 23;int k = 23;Integer i = new Integer(k);Integer i = new Integer(k);k = i.intValue();k = i.intValue();
float f = 5.17;float f = 5.17;Float g = new Float(f);Float g = new Float(f);f = g.floatValue();f = g.floatValue();
See Java documentation for all See Java documentation for all methods!methods!
Wrapper classes
Every Java class inherits from Every Java class inherits from Object class.Object class.
We can therefore use Integer, We can therefore use Integer, Char (and any other) object Char (and any other) object whenever a method expects an whenever a method expects an object.object.
For example, we can call For example, we can call add(new Integer(10));add(new Integer(10));
Wrapper classes - usage
Java container classes store Java container classes store Objects.Objects.
TreeSet s = new TreeSet;TreeSet s = new TreeSet;
s.add(new Integer(7));s.add(new Integer(7));
We cannot store ints directly!We cannot store ints directly!
Array initialization
In Java (or C++), when an array of In Java (or C++), when an array of objects using new, array elements are objects using new, array elements are not initialized automatically!not initialized automatically!
Integer []arr = new Integer[100];Integer []arr = new Integer[100];
arr contains 100 nulls! arr contains 100 nulls!
for (int i = 0; i < 100; i++)for (int i = 0; i < 100; i++) arr[i] = new Integer(i);arr[i] = new Integer(i);
Now every element of an array is Now every element of an array is initialized.initialized.
Array initialization
The same thing happens during The same thing happens during creation of multidimensional creation of multidimensional arrays:arrays:
Object [][]arr = new Object[17][23];Object [][]arr = new Object[17][23];
for (int i = 0; i < 17; i++)for (int i = 0; i < 17; i++)
for (int j = 0; j < 23; j++)for (int j = 0; j < 23; j++)
arr[i][j] = new Integer(i*j);arr[i][j] = new Integer(i*j);
Lists
Cells containg Objects. Each Cell has a Cells containg Objects. Each Cell has a pointer to the next cell in the list.pointer to the next cell in the list.
class ListCellclass ListCell{{ ...... public ListCell getNext(); public ListCell getNext(); // returns the next element. // returns the next element. public void setNext(ListCell l);public void setNext(ListCell l); // sets the pointer to point to l// sets the pointer to point to l public Object getDatum();public Object getDatum(); // returns the object stored in the cell// returns the object stored in the cell}}
Lists: Iteration and recursion
class Courseclass Course{{ public Book textBook;public Book textBook; ......}}
class Studentclass Student{ { private ListCell courses;private ListCell courses;
public void readBook(Book b)public void readBook(Book b) {...}{...}
private void study() {???}private void study() {???} }}
Lists
Suppose that student studies by Suppose that student studies by reading all textbooks for all reading all textbooks for all his/her courses.his/her courses.
How does a study method look How does a study method look like?like?
Lists: Iteration
Iterative version:Iterative version:public void study()public void study()
{{
for (ListCell current = courses; current != null; for (ListCell current = courses; current != null;
current = current.getNext())current = current.getNext())
readBook(((Course)current.getDatum()).textBook;readBook(((Course)current.getDatum()).textBook;
}}
We simply go through a list doing We simply go through a list doing something to each element.something to each element.
current
CS 211 CS 212 CS 611 CS 711 null
Lists: Iteration
Iterative version:Iterative version:public void study()public void study()
{{
for (ListCell current = courses; current != null; for (ListCell current = courses; current != null;
current = current.getNext())current = current.getNext())
readBook(((Course)current.getDatum()).textBook;readBook(((Course)current.getDatum()).textBook;
}}
We simply go through a list doing We simply go through a list doing something to each element.something to each element.
current
CS 211 CS 212 CS 611 CS 711 null
Lists: Iteration
Iterative version:Iterative version:public void study()public void study()
{{
for (ListCell current = courses; current != null; for (ListCell current = courses; current != null;
current = current.getNext())current = current.getNext())
readBook(((Course)current.getDatum()).textBook;readBook(((Course)current.getDatum()).textBook;
}}
We simply go through a list doing We simply go through a list doing something to each element.something to each element.
current
CS 211 CS 212 CS 611 CS 711 null
Lists: Iteration
Iterative version:Iterative version:public void study()public void study()
{{
for (ListCell current = courses; current != null; for (ListCell current = courses; current != null;
current = current.getNext())current = current.getNext())
readBook(((Course)current.getDatum()).textBook;readBook(((Course)current.getDatum()).textBook;
}}
We simply go through a list doing We simply go through a list doing something to each element.something to each element.
CS 211 CS 212 CS 611 CS 711
current
null
Lists: Iteration
Iterative version:Iterative version:public void study()public void study()
{{
for (ListCell current = courses; current != null; for (ListCell current = courses; current != null;
current = current.getNext())current = current.getNext())
readBook(((Course)current.getDatum()).textBook;readBook(((Course)current.getDatum()).textBook;
}}
We simply go through a list doing We simply go through a list doing something to each element.something to each element.
current
CS 211 CS 212 CS 611 CS 711 null
Lists: recursion
private void study1(ListCell c)private void study1(ListCell c)
{{
if (c == null)if (c == null)
return;return;
readBook(((Course)c.getDatum()).textBook;readBook(((Course)c.getDatum()).textBook;
study1(c.next());study1(c.next());
}}
CS 211 CS 212 CS 611 CS 711 null
Lists: recursion
private void study1(ListCell c)private void study1(ListCell c)
{{
if (c == null)if (c == null)
return;return;
readBook(((Course)c.getDatum()).textBook;readBook(((Course)c.getDatum()).textBook;
study1(c.next());study1(c.next());
}}
CS 212 CS 611 CS 711 null
Lists: recursion
private void study1(ListCell c)private void study1(ListCell c)
{{
if (c == null)if (c == null)
return;return;
readBook(((Course)c.getDatum()).textBook;readBook(((Course)c.getDatum()).textBook;
study1(c.next());study1(c.next());
}}
CS 611 CS 711 null
Lists: recursion
private void study1(ListCell c)private void study1(ListCell c)
{{
if (c == null)if (c == null)
return;return;
readBook(((Course)c.getDatum()).textBook;readBook(((Course)c.getDatum()).textBook;
study1(c.next());study1(c.next());
}}
CS 711 null
Lists: recursion
private void study1(ListCell c)private void study1(ListCell c)
{{
if (c == null)if (c == null)
return;return;
readBook(((Course)c.getDatum()).textBook;readBook(((Course)c.getDatum()).textBook;
study1(c.next());study1(c.next());
}}
null
Lists: recursion
private void study1(ListCell c)private void study1(ListCell c)
{{
if (c == null)if (c == null)
return;return;
readBook(((Course)c.getDatum()).textBook;readBook(((Course)c.getDatum()).textBook;
study1(c.next());study1(c.next());
}}
Note that if we wanted to print Note that if we wanted to print out the name of each course the out the name of each course the code would look almost exactly code would look almost exactly the same, but we need to write it the same, but we need to write it anyway. anyway.
Functional programming ad:
study = mapM_ readBook courses
printCourses = mapM_ print courses
We can abstract the idea of iteration and actually write a function mapM_!
Deleting from lists
We want delete the first We want delete the first occurence of the Object o from occurence of the Object o from the List l and return the the List l and return the modified list:modified list:
ListCell delete(ListCell l, Object ListCell delete(ListCell l, Object o)o)
{ ? ? ? }{ ? ? ? }
Lists: Iteration
Deleting element: iterative Deleting element: iterative version.version.
Intuition: We look for o in the Intuition: We look for o in the list in a loop, once we found it list in a loop, once we found it we update the list and return.we update the list and return.
Lists: Iteration
ListCell delete(ListCell l, Object o)ListCell delete(ListCell l, Object o){{ ListCell current = l, previous = null;ListCell current = l, previous = null; while (current != null)while (current != null) { { if (current.getDatum().equals(o)) // found the objectif (current.getDatum().equals(o)) // found the object { { if (previous == null) return l.getNext() ; if (previous == null) return l.getNext() ; // it was the first one// it was the first one elseelse { previous.setNext(current.getNext()); return l; }{ previous.setNext(current.getNext()); return l; } } } elseelse previous = current;previous = current; current = current.getNext();current = current.getNext(); }} return l;return l;}}
Difficult to understand!Difficult to understand!
Lists: Recursion
Deleting element: recursive Deleting element: recursive way:way:
Intuition:Intuition: – If list l is empty, return null. – If first element of l is o, return
rest of list l. – Otherwise, return list
consisting of first element of l, and the list that results from deleting o from the rest of list l.
Lists: Recursion
Deleting an element from list:Deleting an element from list:ListCell delete(ListCell l, Object o)ListCell delete(ListCell l, Object o)
{{
if (l == null) return l;if (l == null) return l;
if (l.getDatum().equals(o)) return l.getNext();if (l.getDatum().equals(o)) return l.getNext();
l.setNext(delete(l.getNext(), o);l.setNext(delete(l.getNext(), o);
return l;return l;
}}
Functional programming ad:
delete [] o = []
delete (x:xs) o = if x == o then xs else (x: (delete xs o ))