Transcript Week 10 presentation - Computing Sciences

Week 10
Introduction to Computer Science
and Object-Oriented Programming
COMP 111
George Basham
Week 10 Topics
10.1.1 Wrappers and Auto-boxing
10.1.2 The Enhanced for Loop
10.1.3
10.1.4
10.1.5
10.1.6
Partially Filled Arrays
Common Array Algorithms
Regression Testing
Two-dimensional Arrays
10.1.1 Wrappers and Auto-boxing
• Because numbers are not objects in Java,
you cannot directly enter them in array
lists
• For example, this will not compile:
• ArrayList<double> data = new
ArrayList<double>(); // No
• To store sequences of numbers in an
array list, you must turn them into wrapper
classes
10.1.1 Wrappers and Auto-boxing Cont.
Primitive Type Wrapper Class
byte
Byte
boolean
Boolean
char
Character
double
Double
float
Float
int
Integer
long
Long
short
Short
10.1.1 Wrappers and Auto-boxing Cont.
• Beginning with Java 5.0, if you assign a primitive
type to a wrapper class, the conversion is
automatic (called auto-boxing):
• Double d = 29.95;
• Conversely, beginning with Java 5.0, wrapper
classes are automatically converted to primitive
types (called auto-unboxing):
• double x = d;
10.1.1 Wrappers and Auto-boxing Cont.
• This will work to store simple number
types in an array list:
• ArrayList<Double> data = new
ArrayList<Double>(); // Yes
• data.add(29.95);
• double x = data.get(0);
10.1.2 The Enhanced for Loop
• Java version 5.0 introduces a very
convenient shortcut for a common loop
type, when you need to iterate through an
sequence of elements
• The new loop construct is know as the “for
each” loop
• The “for each” loop has a very specific
purpose, traversing the elements of a
collection from the beginning to the end
10.1.2 The Enhanced for Loop
Cont.
double[] data = new double[100];
… assume that array is then populated
double sum = 0;
for (double e : data)
{
sum = sum + e;
}
10.1.2 The Enhanced for Loop
Cont.
ArrayList<BankAccount> accounts = new
ArrayList<BankAccount>();
… assume that array list is then
populated
double sum = 0;
for (BankAccount a : accounts)
{
sum = sum + a.getBalance();
}
10.1.3 Partially Filled Arrays
• Since an array is fixed in length, and every
element may not be populated with a
value, we need to make sure not to access
an element of an array that does not
contain a valid value
• For an array of object references, this is
even more important from the aspect that
calling a method on a null reference will
throw a runtime exception
10.1.3 Partially Filled Arrays Cont.
• With a partially filled array, keep a
companion variable to track how many
elements are used
public static final int LEN = 100;
private int[] values = new int[LEN]
private valSize = 0; // companion var
10.1.3 Partially Filled Arrays Cont.
public int addElement(int inVal)
{
if (this.valSize >= LEN)
return -1;
this.values[this.valSize] = inVal;
// element index will be returned
// then incremented for next use
return this.valSize++;
}
10.1.4 Common Array Algorithms
•
•
•
•
•
Demonstrated:
Counting Matches
Searching for a Value (linear search)
Finding the Maximum or Minimum
Also (refer to textbook): Filling, Computing
Sum and Average Values, Locating the Position
of an Element, Removing an Element, Inserting
an Element, Copying and Growing an Array,
Printing Element Separators
10.1.4 Common Array Algorithms Cont.
• Counting Matches
double atLeast = 5000.00;
int matches = 0;
for (BankAccount a : accounts)
{
if (a.getBalance() >= atLeast)
matches++;
}
System.out.println(matches);
10.1.4 Common Array Algorithms Cont.
• Searching for a Value (linear search)
int accountNumber = 1001;
for (BankAccount a : accounts)
{
if (a.getAccountNumber() ==
accountNumber) {
System.out.println
(a.getBalance()); break; }
}
10.1.4 Common Array Algorithms Cont.
• Finding the Maximum or Minimum
BankAccount max = accounts.get(0);
for (int i = 1; i < accounts.size(); i++)
{
BankAccount a = account.get(i);
if (a.getBalance() > max.getBalance())
max = a;
}
System.out.println(max.getAccountNumber());
10.1.5 Regression Testing
• A test suite is a set of tests for repeated
testing
• Regression testing involves repeating
previously run tests to ensure that known
failures of prior versions do not appear in
new versions of the software
• Refactoring seeks to improve the internal
design and implementation of code without
affecting its externally visible behavior
10.1.6 Two-Dimensional Arrays
• int[][] table = new int[2][3];
• The array identified by table will have 2 rows
(the row is the FIRST subscript) and 3
columns (the column is the SECOND
subscript).
[0][0]
[0][1]
[0][2]
[1][0]
[1][1]
[1][2]
10.1.6 Two-Dimensional Arrays Cont.
• table[0][0] = 22;
• table[0][1] = 1301;
•. . .
• table[1][2] = 43
// Traversing the 2-D array
for (int i = 0; i < 2; ++i) // rows
for (int j = 0; j < 3; ++j) // cols
System.out.println(table[i][j];
Reference: Big Java 4th Edition by Cay
Horstmann
10.1.1 Wrappers and Auto-boxing (section
7.3 in Big Java)
10.1.2 The Enhanced for Loop (section 7.4
in Big Java)
10.1.3 Partially Filled Arrays (section 7.5 in
Big Java)
10.1.4 Common Array Algorithms (section
7.6 in Big Java)
10.1.5 Regression Testing (section 7.7 in Big
Java)
10.1.6 Two-dimensional Arrays (section 7.8
in Big Java)