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References as parameters
• Reference variables hold reference values
(addresses).
• Java allows you to use reference values
as parameters and as return values.
• References are still passed by value.
• Any object that has a reference to another
object can use its public instance variables
and methods.
Class Interfaces
and wrapping up other stuff
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Passing a reference
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Returning a reference
Client code that uses the Point class as supplier (from LineSegment class):
Client code that uses the Point class as supplier (from LineSegment class):
public class LineSegment {
private Point p1, p2;
public Point midpoint( ) {
double midX = ( p1.getX( ) + p2.getX( ) ) / 2.0;
double midY = ( p1.getY( ) + p2.getY( ) ) / 2.0;
Point result = new Point( midX, midY );
return result;
}
public void setFirstPoint( Point p ){
p1 = p;
System.out.println( p.getX( ) );
}
// other code
}
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this
Many programmers will use this explicitly within a
class
public class Test{
public int x;
public Test( ) {
this.showSomething( );
}
public void showSomething( ) {
System.out.println( “x value is “ + this.x );
}
}
public class Test{
public int x;
public Test( int x ) {
this.x = x;
}
...
}
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this
• Sometimes an object needs to refer to itself
• Use the keyword this
Example: if an instance variable and parameter
have the same name
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3 Views of a class
3 Views of a class
1) Source code (implementation)
• only readable by programmers, not always
available
• contains ALL implementation details that
describe exact behavior
3) UML class diagram
(abstract)
• smallest – intended as
development aid
• contains no
implementation or
even description
• may or may not show
private class members
• NOTE: this is NOT an
object diagram!
2) Javadocs specification ( abstract )
• generated from javadocs comments within
source code
• does not show implementation details
• usually only shows public ‘interface’
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How to create Javadocs?
Include a Javadoc comment for every class, every
method and every instance variable in your code
•
/**
* This is the typical format of a simple comment.
*/
To save space you can put a comment on one line:
A common mistake is to put an import statement between
the class comment and the class declaration. Avoid this,
as Javadoc will ignore the class comment.
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Point ( )
<query methods>
+ double getX( )
+ double getY( )
<command methods>
+ void setX( double )
+ void setY( double)
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•
Standard and in-line tags - A tag is a special keyword within a doc
comment that Javadoc can process. Javadoc has standard tags, which
appear as @tag...
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How to generate Javadocs?
There are 4 essential tags required for this course:
@author name-text
Adds an "Author" entry with the specified name-text to the
generated docs when the -author option is used.
@version version-text
Adds a "Version" subheading... A doc comment may contain at
most one @version tag. Version normally refers to the version
of the software or its date of creation.
@param parameter-name description
Adds a parameter to the "Parameters" section. The description
may be continued on the next line.
@return description
Adds a "Returns" section with the description text. This text should
describe the return type and permissible range of values.
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<constructors>
A comment is a description possibly followed by tags - The description
begins after the starting delimiter /** and continues until the tag section.
The tag section starts with the first character @ that begins a line (ignoring
leading asterisks, white space and comment separator). The description
cannot continue after the tag section begins. There can be any number of
tags -- some types of tags can be repeated while others cannot. This
@param starts the tag section:
Javadoc @tags
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- double y
/**
* This is a doc comment.
* @param x is a location
*/
/** This comment takes up only one line. */
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- double x
How to create Javadocs?
( from http://java.sun.com/j2se/1.3/docs/tooldocs/win32/javadoc.html#javadoctags )
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Point
<attributes>
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• In DrJava, select ‘Javadoc’ button
• In BlueJ, select 'Generate Documentation'
• Javadocs will be created for each class in
project.
• Javadocs are html files stored in a folder
named ‘doc’.
• Does not include private methods or variables
by default.
EX: Point.java, BankAccount.java
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Importance of Javadocs
Packages
• They are the standard way to provide abstract
(without implementation) description of a class.
• They provide an easy way to understand the
public interface of a class in the face of
complexity...
When dealing with such large numbers of
classes, it is helpful to organize them into
packages.
– uwcse.jar has over 40 classes arranged into 5
packages
– Java 1.4.2 has over 2500 classes arranged into 135
packages
– The only way to survive is to refer to Javadocs
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Helps to avoid name collisions (2 classes
with the same name).
EX: java.awt.Rectangle vs
uwcse.graphics.Rectangle
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import
public Test( ) {
public Test( ) {
java.awt.Color c = new
Color c = new Color( 20, 40, 60 );
java.awt.Color( 20, 40, 60 );
}
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You can either import a single class:
import java.awt.Color;
public class Test{
public class Test {
}
or the entire package:
import java.awt.*;
}
}
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Careful when importing entire packages -naming collisions may result.
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The Java API
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• java.lang also contains the Math class.
• Math contains many mathematical
functions.
– All Math methods are static methods (or class
methods), so there is no need to create an instance
of the Math class. Instead, use the name of the
class as the method qualifier, like this:
– What are Java’s packages?
• Java automatically imports all classes from
java.lang package.
• java.lang contains the String class.
Exercise: find the Javadocs for the String class.
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Math class
• A set of pre-defined classes is called a
library or an API (Application
Programming Interface).
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Import statement
The import statement allows you to refer to classes by
simple names
Example:
import java.awt.Color;
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double y = Math.sqrt( x );
class name, not
an instance name
Exercise: find the Javadocs for the Math class.
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