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Object-oriented language basics, Part 4
Inheritance: Build objects in layers
Page 7 of 10
Before leaving this topic, let's consider the keyword protected. If you declare a superclass field or method as protected, any method, declared in any class that subclasses the class that declares the protected field or method, can access that
field or method. (You will see an example of protected field and method access when I present packages in a future article.)
Casting objects
Every subclass object is also a superclass object. For example, a Circle object is also a Point object. This statement implies you can assign a subclass object reference to a superclass object reference variable. Consider
the following code fragment:
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More from JavaWorld
Resources
- "Object-Oriented Language Basics," Jeff Friesen (JavaWorld):
- Part 1Learn how to declare classes and create objects (April 2001)
- Part 2Declare and access fields and methods (May 2001)
- Part 3CompositionBuild objects from other objects (June 2001)
- Part 4InheritanceBuild objects in layers (July 2001)
- Part 5The root of all classes (August 2001)
- Part 6Use interfaces for safe multiple inheritance and a great deal more (September 2001)
- Part 7Learn about Java's many shapes and find out how to accommodate generalities in your class hierarchies (October 2001)
- Download this article's source code and resource files
http://www.javaworld.com/javaworld/jw-07-2001/java101/jw-0706-java101.zip - For a glossary specific to this article, homework, and more, see the Java 101 study guide that accompanies this article
http://www.javaworld.com/javaworld/jw-07-2001/jw-0706-java101guide.html - The Java Language Specification, Second Edition, James Gosling, Bill Joy, Guy Steele, and Gilad Bracha (Sun Microsystems, 2000)
http://www.javasoft.com/docs/books/jls/second_edition/html/j.title.doc.html - The Java Tutorial
http://www.javasoft.com/tutorial/ - The Javadoc Tips FAQ
http://java.sun.com/j2se/javadoc/faq.html - Javadoc 1.3 documentation
http://java.sun.com/j2se/1.3/docs/tooldocs/javadoc/index.html - Sun's official Java language definition
http://java.sun.com/docs/overviews/java/java-overview-1.html - Check out past Java 101 articles
http://www.javaworld.com/javaworld/topicalindex/jw-ti-java101.html - For more articles geared toward the Java beginner, browse the Intro Level section of JavaWorld's Topical Index
http://www.javaworld.com/javaworld/topicalindex/jw-ti-introlevel.html - Java experts answer your toughest Java questions in JavaWorld's Java Q&A column
http://www.javaworld.com/javaworld/javaqa/javaqa-index.html - For Tips 'N Tricks see
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