Download PowerPoint Presentation - CSE 110 01/24/2005

Introduction to Programming
with Java, for Beginners
Intro to Math Class
Concept of static
API Tour
Intro Random Class
Java Library

Java provides a huge library or collection of
useful programs
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A gold mine of well-tested code that can save
you countless hours of development time
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This huge library information is provided in API Application Programming Interface
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The Math Class
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Description provided in Java API
Collection of common math functions (sin, cos, sqrt, etc.).
And two constants: PI and E
> Math.PI
3.141592653589793
> Math.E
2.718281828459045
> Math.sqrt(25)
5.0
> Math.pow(2,10)
1024
> Math.cos(0)
1.0
> Math.cos(2 * Math.PI)
1.0
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How the Math Class Works
public class Math{
public static final double PI =
3.141592653589793;
public static double sin(double d){ .. }
public static double sqrt(double d) { .. }
private Math(){}
..
}
> Math.PI
3.141592653589793
> Math.sqrt(25)
5.0
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What's different about Math Class

It’s different from a typical Java class
It is a “stateless” class
 We only need one Math class (not multiple
instances)
 No need to instantiate it (hence, no public
constructor)
 All of its variables and methods are static
 static means “applies to the class as a
whole” vs. “applies to an individual instance”

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Static Data or Methods
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A class can have data and methods of its own (not part
of the objects)
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For example, Dog class can keep a count of the number
of objects it has created
class Dog{
static int dogCount; //not an instance variable
…
Dog(String dogName, int dogAge){
name = dogName;
age = dogAge;
dogCount++; //short form for dogCount = dogCount +1
}
}
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Static (contd..)
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In Dr Java
>Dog d1 = new Dog(“Fluffy”, 5);
>Dog d2 = new Dog(“Fido”, 6);
>Dog.dogCount
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Dog.dogCount is asking Dog class for the count of
the dogs created
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Since dogCount belongs to class and not the object
Hence use the classname and not Object name to
access variable dogCount
More on static topic next time
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Using Reading Library (API)
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We will use API documentation for Java Version 5
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Find the documentation for the Math class
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If you scroll down the lower left panel and click on
the link labeled Math, the large "main" panel on the
right will display the documentation for the Math
class
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General Class Interface
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A class' public interface
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Notice the four headings in the main panel.
Field Summary
 Has information about public variables
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Constructor and Method summaries
 Describe the public constructors and methods
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Method Detail
 Provides detail on method inputs (parameter(s))and output
(return type) and some extra details
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The collection of all of a class' public fields,
constructors, and methods constitute its "interface",
that is, the public face that it shows the world.

Note that private variables and methods are not a
part of its interface.
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Math Class Description
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Notice the phrase java.lang at the top of the
main panel above the word Math
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This means that the Math class is part of the core
Java language and hence can be used directly
Math Class Interface
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Field Summary: Has two constants PI and E
Constructor Summary: has no public constructor
Methods Summary: many methods all which are
static
Method Details: e.g. sqrt() takes a double and
returns a double
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Packages and import Statements
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If a class is not part of java language i.e. java.lang, you'll see
package name
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What is a package?
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Basically it's a directory that has a collection of related classes
E.g. Random Class description contains: java.util.Random
Indicating that the Random code is stored in a file called
java/util/Random.class somewhere on your machine.
The java/util directory/folder is known as the "util", or utility
package.
Since Random is not part of Java Language we need to tell
Java where to find it by saying
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import java.util.Random;
Another way is to use the asterisk "wildcard character": import
java.util.*;
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Random Class
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A class to create Random numbers
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Constructor Summary shows the objects of this
type can be created
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E.g. Random r = new Random();
Method Summary shows that it can generate
random values of types:
 integers, doubles etc.
 E.g. r.nextInt(6) – Generate a integer numbers
between 0 (inclusive) and 6 (exclusive)
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How do I generate a number between 1 and 6 ?
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