Transcript File - AP Computer Science

Agenda
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Remarks about last homeworks
Class methods(static methods)
Class signature
Arrays
Presentations topic
homework
Static methods
• BPJ lesson 20; Barron’s chapter 2
• Static methods are sometimes called class
methods.
• The difference between static and no-static
methods: we are accessing them at the class
level rather than the object level.
• Math class vs Random class
Math.sqrt(4.0);
Random.nextInput(); => wrong
public static void main(String[] args) {
testMethod2.calculateRectanglePerimeter(
4, 13);
//testMethod2.printSmiley();
testMethod2 tM = new testMethod2();
tM.printSmiley();
}
Practice
• Create a class Aclass with two methods, one
with static keyword, and one without. This
class doesn’t need to have main().
• Create another class Bclass with a main() to
call the methods in Aclass. Try to see the
differences about static and non-static
methods calling.
Arrays
• An array is a structure that can store many of
the same kind of data together at once.
• An array has a fixed length and can contain
only as many data items as its length allows:
• Array of size 4
1
2
3
4
Philip Costa Minna Justin
Friends
Arrays
• An Array elements: can be any data type
• Each element has an index value, with 0 being
the index of the first item, 1 the index of the
second item, and so on.
• Declaration:
String[] friends = new String[5]; //5 friends
int[] intArray = new int[20];
char[] charArray = new char[40];
Arrays
• Array Declaration
<type>[] <name>; //declare array
<name> = new <type>[<num>]; //allocate
space for elements
• Array declaration and initialization
String[] friends = {"Kermit", "Lucille", "Sammy", "Roxy"};
• Practice array declaration and
initialization with every primitive
types.
• Example:
int[] intArray = {10, 20, 30};
• Do the exercise on Lesson 18
exercise 1 - 6
Array index
1
2
3
4
Philip Costa Minna Justin
Friends
Array index starts from 0, assume that we want
to create a Friends array like above:
String[] Friends = new String[4];
Friends[0] = “Philip”;
Friends[1] = “Costa”;
Friends[2] = “Minna”;
Friends[3] = “Justin”;
Array Accessing example
doulbe[] sgt = new double[20];
for(int i = 0; i <sgt.length; i++)
sgt[i] = (double) i;
for(int i = 0; i <sgt.length; i++)
System.out.println("sgt element " +sgt[i]);
Class signature
<access _ level> class <name>
{
<variables>
<constructors>
<methods>
}
Classes
• A class is written in a separate file and consists
of a declaration and a body.
• The class declaration includes the access level,
the keyword class, and the class name.
• The class body contains variables, constructors,
and methods.
• Constructors are used to initialize variables in
a class. Variables and methods are called the
members of a class
Homework
Create a StudentRoster application that
prompts the user for the number of
students in the class and then prompts the
user for each student’s name and stores
the names in an array. After all the names
have been entered, the application should
display the title ”Student Roster” and then
list the names in the array.